NEET Biology

NEET Biology Notes Molecular Basis Of Inheritance

DNA

• DNA (deoxyribose nucleic acid) is a long polymer of deoxyribonucleotides.
• The length of DNA is usually defined as the number of nucleotides (or a pair of nucleotide referred to as base pairs or bp) present in it.
• This is the characteristic of an organism.
• A few examples are sited in table.

Chemical Structure of DNA Polynucleotide

• The basic unit of DNA is a nucleotide which has three components: a nitrogenous base, a pentose sugar (deoxyribose), and a phosphate group.
• There are two types of nitrogenous bases: Purines (adenine and guanine) and pyrimidines (cytosine and thymine).
• Cytosine is common for both DNA and RNA (ribose nucleic acid), and thymine is present in DNA only.
• Uracil is present in RNA at the place of thymine.
• A nitrogenous base is linked to the pentose sugar through an N-glycosidic linkage to form a nucleoside such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine, and deoxythymidine.
• When a phosphate group is linked to 5′-OH of a nucleoside through phosphoester linkage, a corresponding nucleotide (or deoxynucleotide depending on the type of sugar present) is formed.
• Two nucleotides are linked through 3′-5′ phosphodiester linkages to form a dinucleotide.
• More nucleotides can be joined in such a manner to form a polynucleotide chain.
• The polymer, thus, formed has a free phosphate moiety at the 5′-end of sugar, which is referred to as the 5′-end of polynucleotide chain.

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• Similarly, at the other end of the polymer, the sugar has a free 3′-OH group which is referred to as the 3′-end of polynucleotide chain.
• Backbone in a polynucleotide chain is formed due to sugar and phosphates (phosphodiester bond). Nitrogenous bases linked to the sugar moiety project from the backbone.
• In RNA, every nucleotide residue has an additional -OH group present at the 2′-position in the ribose.
• Also, in RNA, uracil is found at the place of thymine (S-methyluracil, another chemical name for thymine). In 1953, James Watson and Francis Crick, based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, proposed double helix model for the structure of DNA.
• One of the hallmarks of their proposition was “base pairing between the two strands of polynucleotide chain.” However, this proposition was also based on the observation of Erwin Chargaff (that for a double-stranded DNA, the ratios between adenine and thymine and that between guanine and cytosine are constant and equal.
• Chargaff (1950) made observations on the bases and other contents of DNA. These observations or generalizations are known as Chargaff’s rules.
  • Purine and pyrimidine base pairs are present in equal amounts, i.e., adenine (A) + guanine (G) = thymine (T) + cytosine (C).
  • The molar amount of purine (adenine) is always equal to the molar amount of pyrimidine (thy- mine). Similarly, guanine is equalled by cytosine.
  • Deoxyribose (sugar) and phosphate occur in equimolar proportions.
  • The ratio (A+T)/(G+C) is constant for a species. It is called base ratio. It is 1.52 for humans and 0.93 for E. coli.
• Base pairing is a very unique property of polynucleotide chains.
• Both strands of DNA are said to be complementary to each other. Therefore, if the sequence of bases in one strand is known, then the sequence in the other strand can be predicted.
• Thus, if one DNA strand has A, the other will have T; and if one has G, the other will have C.
• Therefore, if the base sequence of one strand is CAT TAG GAC, the base sequence of the other strand will be GTA ATC CTG.
• Hence, the two polynucleotide strands are complementary to one another.
• Also, if each strand from a DNA or parental DNA acts as a template for the synthesis of a new strand, the two double-stranded DNA or daughter DNA produced will be identical to the parental DNA molecule.

Salient Features of DNA Double Helix

• DNA double helix is made of two polynucleotide chains, where the backbone is constituted by sugar- phosphate and the bases project inside.
• The two chains have anti-parallel polarity. It means, if one chain has polarity 5’P→ 3’OH, the other has 3’OH→ 5’P.
• The bases in the two strands are paired through hydrogen bonds (H-bonds), forming base pairs. Ad- enine forms two hydrogen bonds with thymine from the opposite strand and vice versa. Guanine is bonded with cytosine with three H-bonds. As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix.
• The plane of one base pair stacks over the other in double helix. This, in addition to H-bonds, confers stability to the helical structure.
• The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm and there are roughly 10 bp in each turn. Consequently, the distance between the base pairs in a helix is approximately equal to 0.34 nm. It is because of specific base pairing with a purine lying opposite to pyrimidine which makes two chains 2 nm thick.

molecular basis of inheritance

NEET Biology Notes Molecular Basis Of Inheritance Packaging Of DNA Helix

• The average distance between two adjacent base pairs is 0.34 nm (0.34 x 10m or 3.4 A).
• The length of DNA for a human diploid cell is 6.6 x 10° bp x 0.34 x 10 m/bp = 2.2 m.
• This length is far greater than the dimension of a typical nucleus (approximately 10 m).
• The number of base pairs in E. coli is 4.6 × 10°. Total length is 1.36 mm.
• The long-sized DNA is accommodated in a small area (about 1 um in E. coli) only through packing or com- paction.
• DNA is acidic due to the presence of a large number of phosphate groups.
• Compaction occurs by folding and attachment of DNA with basic proteins-polyamine in prokaryotes and histone in eukaryotes.

DNA Packaging in Prokaryotes

• DNA is found in cytoplasm in supercoiled state.
• The coils are maintained by non-histone basic proteins such as polyamines.
• RNA may also be involved. This compact structure of DNA is called nucleoid or genophore.

DNA Packaging in Eukaryotes

• DNA packaging in eukaryotes is carried out with the help of lysine- and arginine-rich basic proteins called histones.
• The unit of compaction is called nucleosome.

• There are five types of histone proteins: H1, H2A, H2B, H3, and H4.
• Four of these occur in pairs to produce histone octamer (two copies of each-H2A, H2B, H3, and H1) called nu-body or core of nucleosome.
• Their positively charged ends are directed outside.
• They attract negatively charged strands of DNA.
• About 200 bp of DNA are wrapped over nu-body to complete about 13 turns.
• This forms a nucleosome of size 110 x 60 Å (11 x 6 nm).
• The DNA present between two adjacent nucleosomes is called linker DNA.
• It is attached to H, histone protein.
• The length of linker DNA varies from species to species.
• Nucleosome chain gives a “beads-on-string” appearance under electron microscope ).

• The nucleosomes further coil to form a solenoid.
• It has diameter of 30 nm as found in chromatin.
• The beads-on-string structure in chromatin is packaged to form chromatin fibers that are further coiled and condensed at the metaphase stage of cell division to form chromosomes.
• Packaging at higher level requires additional set of proteins (acidic). These are collectively referred to as non-histone chromosomal (NHC) proteins.
• NHC proteins are of three types:
  • Scaffold or structural NHC protein
  • Functional NHC protein, e.g., DNA polymerase and RNA polymerase
  • Regulatory NHC protein, e.g., HMG (high mobil- ity group) proteins that control gene expression)
• In a typical nucleus, some regions of chromatin are loosely packed (and stain light) and are referred to as euchromatin.
• The chromatin that is more densely packed and stains dark is referred to as heterochromatin. Specifically, euchromatin is said to be transcriptionally active while heterochromatin is said to be transcriptionally inactive.

Chemical Composition of Chromosome

A chromosome consists of the following chemical compositions:

• DNA: 40%
• RNA: 1.2%
• Histone protein: 50%
• Acidic proteins: 8.5%
• Lipid: Traces
• Ca, Mg2, Fe12: Traces

NEET Biology Notes Molecular Basis Of Inheritance Search For Genetic Material

The following experiments prove that DNA is the genetic material.

Evidence from Bacterial Transformation

• The transformation experiments conducted by Frederick Griffith in 1928 are of great importance in establishing the nature of genetic material.
• He used two strains of bacterium Diplococcus or Streptococcus pneumoniae or Pneumococcus, i.e., S-3 and R-2.

• • Smooth (S) or capsulated type: These have a mucous coat and produce shiny colonies. These bacteria are virulent and cause pneumonia.
  • Rough (R) or non-capsulated type: Mucous coat is absent and these produce rough colonies. These bacteria are non-virulent and do not cause pneumonia.
    The experiment can be described in the following four steps:
  • Smooth-type bacteria were injected into mice. The mice died as a result of pneumonia caused by bacteria.
    S strain Injected into mice → Mice died ii.
  • Rough-type bacteria were injected into mice. The mice lived and pneumonia did not occur.
    R strain Injected into mice →→ Mice lived
  • Smooth-type bacteria, which normally cause disease, were heat-killed and then injected into mice. The mice lived and pneumonia was not caused.
    S strain (heat-killed) → Injected into mice → Mice lived
  • Rough-type bacteria (living) and smooth- type heat-killed bacteria (both known not to cause disease) were injected together into mice. The mice died due to pneumonia and virulent smooth-type living bacteria could also be recovered from their dead bodies.
    S strain (heat-killed) + R strain (living) →Injected into mice → Mice died
• From the fourth step of the experiment, he concluded that some rough-type bacteria (non-virulent) were transformed into smooth-type bacteria (virulent).
• This occurred perhaps due to the absorption of some transforming substance by rough-type bacteria from heat-killed smooth-type bacteria.
• This transforming substance from smooth-type bacteria caused the synthesis of capsule which resulted in the production of pneumonia and the death of mice.
• Therefore, transforming principle appears to control genetic characters (e.g., capsule, as in this case). However, the biochemical nature of genetic material was not defined from his experiments.

Biochemical Characterization of Transforming Principle

• Later, Avery, Macleod, and McCarty (1944) repeated the experiment in vitro to identify the biochemical nature of transforming substance. They proved that this substance is DNA.

• Pneumococcus bacteria cause disease when capsule is present. Capsule production is under genetic control.
• In the experiment, rough-type bacteria (non-capsulated and non-virulent) were grown in a culture medium to which DNA extract from smooth-type bacteria (capsu- lated and virulent) was added.
• Later, the culture showed the presence of smooth-type bacteria also in addition to rough type.
• This is possible only if the DNA of smooth-type bacteria was absorbed by the rough-type bacteria which developed capsule and became virulent.
• This process of transfer of characters of one bacterium to another by taking up DNA from solution is called transformation.
• When DNA extract was treated with DNase (an enzyme that destroys DNA), transformation did not occur.
• Transformation occurred when proteases and RNases were used. This clearly shows that DNA is the genetic material.

Evidence from Experiments with Bacteriophage

• T2 bacteriophage is a virus that infects bacterium E. coli and multiplies inside it.
• T2 phage is made up of DNA and protein coat.
• Thus, it is the most suitable material to determine whether DNA or protein contains information for the production of new virus (phage) particles.
• Hershey and Chase (1952) demonstrated that only the DNA of the phage enters the bacterial cell and, therefore, contains necessary genetic information for the assembly of new phage particle.

• The functions of DNA and proteins could be found out by labeling them with radioactive tracers.
• DNA contains phosphorus but not sulfur.
• Therefore, phage DNA was labeled with p32 by grow- ing bacteria infected with phages in culture medium containing 32p.
• Similarly, the protein of phage contains sulfur but no phosphorus.
• Thus, the phage protein coat was labeled with S35 by growing bacteria infected with phages in another culture medium containing 35S.
• After the formation of labeled phages, three steps were followed:
  • Infection: Both types of labeled phages were al- lowed to infect normally cultured bacteria in sep- arate experiments.
  • Blending: These bacterial cells were agitated in a blender to break the contact between virus and bacteria.
  • Centrifugation: The virus particles were separated from the bacteria by spinning them in a centrifuge.

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• After centrifugation, the bacterial cells showed the presence of radioactive DNA labeled with p32 while radioactive protein labeled with $35 appeared on the outside of bacteria cells (i.e., in the medium).
• Labeled DNA was also found in the next generation of phage.
• This clearly showed that only DNA enters the bacterial host and not the protein.
• DNA, therefore, is the infective part of virus and also carries all genetic information.
• This provided the unequivocal proof that DNA is the genetic material.

Properties of Genetic Material

• Following are the properties and functions which should be fulfilled by a substance if it is to qualify as genetic material.
  • It should be chemically and structurally stable.
  • It should be able to transmit faithfully to the next generation, as Mendelian characters.
  • It should also be capable of undergoing mutations.
  • It should be able to generate its own kind (replication).
• This can be concluded after examining the above written qualities. DNA is more stable and is preferred as genetic material due to the following reasons:
  • Free 2’OH of RNA makes it more labile and easily degradable. Therefore, DNA in comparison is more stable.
  • The presence of thymine at the place of uracil also confers additional stability to DNA.
  • RNA being unstable mutates at a faster rate.

RNA World

• RNA was the first genetic material.
• There are evidences to suggest that essential life processes such as metabolism, translation, and splicing evolved around RNA.
• RNA used to act as a genetic material as well as a catalyst.
• There are some important biochemical reactions in systems that are catalyzed by RNA catalysts and not by proteinaceous enzymes (e.g., splicing).
• RNA being a catalyst was reactive and, hence, unsta- ble.
• Therefore, DNA has evolved from RNA with chemical modifications that make it more stable.
• DNA being double stranded and having complementary strand further resists changes by evolving a process of repair.
• RNA is an adapter, structural molecule, and in some cases catalytic.
• Thus, RNA is a better material for the transmission of information.

NEET Biology Notes Molecular Basis Of Inheritance Replication Of DNA

• The Watson-Crick model of DNA immediately suggested that the two strands of DNA would separate.
• Each separated or parent strand serves as a template (model or guide) for the formation of a new but complementary strand.
• Thus, the new or daughter DNA molecules formed would be made of one old or parental strand and an- other newly formed complementary strand.
• This method of formation of new daughter DNA molecules is called the semi-conservative method of replication.
• The following experiment suggests that DNA replication is semi-conservative.
• Messelson and Stahl (1958) conducted experiment using heavy nitrogen (15N) to determine whether the concept of semi-conservative replication is correct.
• They used cesium chloride (CSCI) gradient centrifugation technique for this purpose.
• A dense solution of CsCl, on centrifugation, forms density-gradient bands of a solution of lower density at the top that increases gradually towards the bottom with highest density.
• If the DNAs of different densities are mixed with CsCl solution, these would separate from one another and would form a definite density band in the gradient along with CsCl solution.
• Meselson and Stahl created DNA molecules of different densities by using normal nitrogen, 14N, and its heavy isotope, ‘N.
• For this purpose, E. coli was grown in 15NH,Cl-containing culture medium for many generations, to make bacterial DNA completely heavy.
• This non-radioactive or heavy DNA (incorporating 15N) had more density than the DNA with normal nitrogen (14N).
• Bacteria were then transferred to the culture medium containing only normal nitrogen (NH,CI). The change in density was observed by taking DNA samples periodically.

• If DNA replicates semi-conservatively, then each heavy (N) DNA strand should separate and each separated strand should acquire a light (N) partner after one round of replication.
• This should be a hybrid DNA made of two strands, i.e., 14N-15N.
• Meselson and Stahl observed that such DNA was actually half-dense indicating the presence of hybrid DNA molecules.
• After the second round of replication, there would be four DNA molecules.
• Of these, two molecules would be hybrid (14N-15N) showing half density as earlier and the remaining two molecules would be made of light strands (14N-14N). Thus, after the second generation, the same half-dense band (4N-15N) was seen but the density of light bands (14N-14N) increased.
• Meselson and Stahl’s work as such provided the confirmation of the Watson-Crick model of DNA and its semi-conservative replication.
• Taylor proved the semi-conservative mode of chromosome replication in eukaryotes using tritiated thymidine in the root of Vicia faba (faba beans).
• Cairns proved the semi-conservative mode of replication in E. coli by using tritiated thymidine (H3-tdR) in the autoradiography experiment.
• He proposed the 6-model for replication in circular DNA.

Mechanism of DNA Replication

DNA replication involves the following four major steps:

• Initiation of DNA replication
• Unwinding of helix
• Formation of primer strand
• Elongation of new strand

Initiation of Replication

• The replication of DNA always begins at a definite site called the origin of replication.
• Prokaryotes have single origin of replication.
• It is called ori-c in E.coli.
• On the other hand, eukaryotes have several thousand origins of replication.

Unwinding of Helix

• DNA replication requires that the double helical parental molecule is unwound so that its internal bases are available to the replication enzymes.
• Unwinding is brought about by enzyme helicase which is ATP-dependent.
• The unwinding of DNA molecule into two strands results in the formation of Y-shaped structure, called replication fork.

• These exposed single strands are stabilized by a protein known as single-strand binding (SSB) protein.
• Due to unwinding, a supercoiling develops on the end of DNA opposite to the replicating fork.
• This tension is released by enzyme topoisomerase.

Formation of Primer Strand

• A new strand is to be synthesized opposite to the parental strands. DNA polymerase 3 is the true replicase in E. coli. It is incapable of initiating DNA synthesis, i.e., it is unable to deposit the first nucleotide in a daughter (new) strand without the primer.)
• Another enzyme, known as primase, synthesizes a short primer strand of RNA.
• The primer strand then serves as a stepping stone (to start error-less replication).
• Once the initiation of DNA synthesis is completed, this primer RNA strand is then removed enzymatically.

Elongation of New Strand

• Once the primer strand is formed, DNA replication occurs in 5′-3′ direction, i.e., during the synthesis of a new strand, deoxyribonucleoside triphosphates (dATP, dGTP, dTTP, dCTP) are added only to the free 3’OH end.
• Thus, the nucleotide at the 3′ end is always the most recently added nucleotide to the chain.
• As DNA replication proceeds on the two parental strands, the synthesis of daughter or new strand occurs continuously along the parent 3’5′ strand.
• It is now known as the leading daughter strand.
• The synthesis of another daughter strand along the other parental strand, however, takes place in the form of short pieces.
• This is called the lagging daughter strand. These short pieces of DNA are known as the Okazaki fragments. These segments are about 1,000-2,000 nucleotides long in prokaryotes.
• Hence, DNA replication is semi-discontinuous. The discontinuous pieces of lagging strand are joined together by the enzyme DNA ligase (after the removal of primer) to form continuous daughter strand.
• Thus, two DNA molecules are now formed from one molecule.
• Each of these daughter DNA molecules is made of two strands, of which one is old (parental) and the other one is new or complementary strand.
• DNA polymerase is the most important enzyme of DNA replication.
• DNA polymerases are of three types in prokaryotes: DNA polymerase I, II, and III.

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• DNA polymerases 2 and 3 have only 3′ 5′ exonuclease activity.
• Kornberg (1956) succeeded in demonstrating the in vitro synthesis of DNA molecule using a single strand of DNA as a template.
• He extracted and purified an enzyme from E. coli which was capable of linking free DNA nucleotides, in the presence of ATP as an energy source, to form complementary strand.
• He called it DNA polymerase. DNA polymerase I is called the Kornberg enzyme.
• In eukaryotes, DNA polymerases are of five types: DNA polymerase a, ẞ, 7, 8, and ɛ.
• In eukaryotes, the replication of DNA takes place at the S-phase of the cell cycle. The replication of DNA and the cell division cycle should be highly coordinated. A failure in cell division after DNA replication” results in polyploidy (a chromosomal anomaly).

NEET Biology Notes Molecular Basis Of Inheritance Structure Of RNA

RNA is present in all living cells. It is laevo rotatory and is responsible for learning and memory. It is found in the cytoplasm as well as the nucleus.

Types of RNA

• In bacteria, there are three major types of RNAs: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).
• All three RNAs are needed to synthesize a protein in a cell.
• The mRNA provides the template, the tRNA brings aminoacids and reads the genetic code, and the rRNAs play structural and catalytic role during translation.
• RNA is generally involved in protein synthesis but in majority of plant viruses, it serves as a genetic material. Therefore, there are two major types of RNAs: genetic RNA and non-genetic RNA.

Genetic RNA

Fraenkel-Conrat showed that RNA present in TMV (tobacco mosaic virus) is a genetic material. Since then, it is established that RNA acts as a genetic material in most plant viruses.

Non-Genetic RNA

Non-genetic RNA is commonly present in cells where DNA is the genetic material. It is synthesized on DNA template. It is of the following three major types:

• mRNA
  • It was reported by Jacob and Monad.
  • It carries genetic information present in DNA and, so, is called working copy.
  • It constitutes about 3-5% of the total RNA pre- sent in the cell.
  • The molecular weight varies from 25,000 to 1,00,000.
  • It is about 300 nucleotide long at minimum.
  • Structure of prokaryotic mRNA: It is polycistronic in nature, i.e., several cistrons (functional part of DNA) form a single mRNA. Thus, each mRNA has a message to produce several polypeptides. Average life is 2 min.
    At the 5′-end near initiation codon, a sequence of fixed bases is found. It is called the Shine Dal- garno (SD) sequence (5’AGGAGGU3′). It helps in the correct binding of ribosomal subunit (30 S) on it.

• • Structure of eukaryotic mRNA: It is monocistronic in nature and has a message to produce on polypeptide only. It is metabolically stable and its life varies from a few hours to days.
  • UTR (untranslated region): These are sequences of RNA before the start or initiation codon and after the stop or termination codon. These are not translated and are transcribed as part of the same transcript as the coding region. Such UTRS pro- vide stability to mRNA and also increase translational efficiency.
• rRNA
  • rRNA was reported by Kuntz. It is the most stable type of RNA and is a constituent of ribosomes. It forms about 80% of the total cellular RNA.
  • In eukaryotes, four types of rRNAs are found: 28 S, 18 S, 5.85 S, and 5 S, whereas in prokaryotes, three types of rRNAs are found: 23 S, 16 S, and 5 S. These are synthesized by genes present on the DNA of several chromosomes found within a region known as nucleolar organizer.
• tRNA
  • The existence of tRNA was postulated by Crick. It is also known as soluble RNA (SRNA). tRNAs are the smallest molecules that carry amino acids to the site of protein synthesis. These constitute about 15% of the total cellular RNA.
  • tRNA acts as an intermediate molecule between the triplet code of mRNA and the amino acid se- quence of polypeptide chain. All tRNAs have almost the same basic structure. There are over 60 types of tRNAs.
  • Structure of tRNA: The cloverleaf model of tRNA is a two-dimensional model suggested by Holley et.al. A tRNA molecule appears like a cloverleaf, being folded with three or more double helical regions (stem), having loops also. The three-dimensional structure of tRNA was proposed to be inverted L-shaped by Kim and Klug.
  • All tRNA molecules commonly have a guanine residue at their 5′ terminal end. At their 3′ end, un- paired CCA sequence is present. Amino acid gets attached at this end only. The number of nucleo- tides varies from 77 (tRNA alanine) to 207 (tRNA1y- rosine).
• There are three loops in tRNA.
  • Amino acyl synthetase binding loop, also called DHU loop.
  • Ribosomal binding loop with seven unpaired bases. It is also called TVC loop.
  • Anti-codon loop with seven unpaired bases. Out of the seven bases in the anti-codon loop, three bases act as anti-codon for a particular triplet codon present on mRNA.

NEET Biology Notes Molecular Basis Of Inheritance Gene Expression

• DNA, being the genetic material, carries all the informations necessary to program the functions of a cell by controlling the synthesis of enzymes or proteins.
• Beadle and Tatum put forward a theory-one gene one enzyme-in support of the earlier hypothesis that enzymes are proteinaceous in nature and each is produced by a single gene.
• They conducted experiments on the nutritional strains of pink mold, Neurospora crassa.
• This fungus grows on simple nutrient medium and has the ability to synthesize all its cellular components. Such an organism is called prototroph.
• An organism that is unable to synthesize a particular cellular metabolite such as an amino acid or a coenzyme is called auxotroph.
• Beadle and Tatum produced arginine (an amino acid) auxotrophs (mutants of Neurospora unable to synthesize arginine) by giving X-rays treatment to the cells. Arginine synthesis passes through the following path.

• They found that any step of this metabolic chain could be blocked by a mutation in a specific enzyme catalyzing the reaction, each enzyme representing a different gene product.
• Thus, Beadle, and Tatum reached a conclusion that each gene functions to produce a single enzyme.
• Some proteins, e.g., hemoglobin and other quaternary proteins, are made up of two or more than two poly- peptide chains. After this, the “one gene one enzyme” theory was modified into “one gene one polypeptide” hypothesis by Yanofsky. Later, Jacobson and Balti- more proposed “one mRNA one polypeptide” hypothesis.
• Gene and protein: A gene expresses itself by protein synthesis.
• When a particular gene is expressed (i.e., controls a function or a reaction), its information is copied into another nucleic acid, i.e., mRNA, which in turn directs the synthesis of specific proteins.
• These concepts form the central dogma of molecular biology. This has been shown by F. Crick in.

• An exception to this one-way flow of information was reported in 1970.
• H. Temin and D. Baltimore independently discovered reverse transcription in some viruses.
• These viruses can code an enzyme, reverse transcriptase, which can code DNA on RNA template.
• This discovery was important in understanding cancer and, hence, these two scientists were awarded Nobel Prize.
• Commoner suggests circular flow of information.

NEET Biology Notes Molecular Basis Of Inheritance Mechanism Of Protein Synthesis

The process of protein synthesis consists of two major steps:

• Transcription or synthesis of mRNA on DNA
• Translation or synthesis of proteins along mRNA

Transcription

• The transfer of genetic information from DNA to mRNA is known as transcription. The segment of DNA that takes part in transcription is called transcription unit. It has three components.
  • A promoter
  • The structural gene
  • A terminator
• Promoter sequences are present upstream (5′ end) of the structural genes of a transcription unit.
• The binding sites for RNA polymerase lie within the promoter sequence.
• Certain short sequences within the promoter sites are conserved. In prokaryotes, at 10 bp upstream from the start point lies a conserved sequence described as -10 sequence TATAAT or the “Pribnow box” and -35 sequence TTGACA or the “recognition sequence.”

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• Structural gene is a part of that DNA strand which has 3′ 5′ polarity, as transcription occurs in 5′ → 3′ direction. This strand of DNA that directs the synthesis of mRNA is called the template strand. The complementary strand is called non-template or coding strand. It is identical in base sequence to RNA transcribed from the gene, only with U in place of T.
• Terminator is present at the 3′ end of coding strand and defines the end of the process of transcription.

Mechanism of Transcription

• RNA polymerase binds to the promoter region of DNA and the process of transcription begins.

• RNA polymerase moves along the DNA helix and unwinds it.
• One of the two strands of DNA serves as a template for RNA synthesis.
• This results in the formation of complementary RNA strand.
• It is formed at a rate of about 40 to 50 bp/s.
• RNA synthesis comes to a stop when RNA polymerase reaches the terminator sequence.
• The transcription enzyme, i.e., RNA polymerase, is only of one type in prokaryotes and can transcribe all types of RNAs.
• RNA polymerase is a holoenzyme that is represented as (αBB’w)σ.
• The molecular weight of holoenzyme is 4,50,000.
• The enzyme without σ subunit is referred to as core enzyme.
• Though the core enzyme is capable of transcribing DNA into RNA, transcription starts non-specifically at any base on DNA.
• It is σ subunit which confers specificity. Rho factor (p) is required for the termination of transcription.
• Sigma factor (o): Binds to the promoter site of DNA and initiates transcription.
• Core complex: It continues the transcription.
• Rho factor (p): It terminates the transcription; its molecular weight is 55,000.

Transcription in Eukaryotes

• In eukaryotes, the promoter site is recognized by the presence of specific nucleotide sequence called TATA box or the Hogness box (7 base pair long-TATATAT or TATAAAT) located 20 bp upstream to the start point.
• Another sequence is the CAAT box present between -70 bp and 80 bp.
• In eukaryotes, the RNA polymerases are of three types: RNA polymerase I, RNA polymerase II, and RNA polymerase III.
• The functions of different RNA polymerases in eukaryotes are as follows:
  • RNA polymerase 1-5.8 S, 18 S, and 28 S rRNA synthesis.
  • RNA polymerase 2-hnRNA (heterogeneous nuclear RNA) and mRNA synthesis.
  • RNA polymerase 3-tRNA, scRNA (small cytoplasmic RNA), 5S rRNA, and snRNA (small nuclear RNA) synthesis.
• The gene in eukaryotes, however, is made of several pieces of base sequence coding for amino acids called exonic DNA, separated by stretches of non-coding sequences, commonly called intronic DNA.
• Thus, the information on the eukaryotic gene for assembling a protein is not continuous but split.
• This discovery is the result of works of Richard J. Roberts and Philip Sharp.
• They shared 1993 Nobel Prize for Physiology and Medicine for the discovery of “split genes” in higher organisms.
• Most eukaryotic genes contain very long base sequences, all of which do not necessarily form mature mRNA.
• The coding DNA sequences of the gene are called exons and the intervening non-coding DNA sequences are called introns.
• All introns have GU at the 5′ end and AG at the 3′ end. Depending on the size of gene, the number and length of exons may vary from a few to more than 50 nucleotides. Exons alternate with stretches of DNA that contain no genetic information introns.
• The nascent RNA synthesized by RNA polymerase II is called hnRNA.
• It contains both unwanted base sequences (transcribed from introns) alternated with useful base sequences (transcribed from exons).
• This primary transcript is converted into functional mRNA after post-transcriptional processing which involves three steps:
  • Modification of 5′ end by capping: Capping at the 5′ end occurs rapidly after the start of transcription.

• • • The guanosine methylated at the seventh position is added at 5′ with the help of enzyme guanyl transferase.
    • Cap is essential for the formation of mRNA-ribosome complex.
    • Translation is not possible if cap is lacking, be- cause cap is identified by 18S rRNA of ribosome unit.
  • Polyadenylation at 3′ end (tailing): Poly (A) is added to the 3′ end of newly formed hnRNA with the help of enzyme poly A polymerase. It adds about 200-300 adenylate residues.
• Splicing of hnRNA (tailoring): In eukaryotes, the coding sequences of RNA (exons) are interrupted by non-coding sequences (introns).
  • snRNA and protein complex called small nuclear ribonucleoprotein (or snRNPs or snurps) play important role in this process.
  • Here, the introns are removed and the exons come in one plane.
  • This process is called splicing through which a mature mRNA is produced.
• Normally, mRNA carries the codons of a single complete protein molecule (monocistronic mRNA) in cukaryotes, but in prokaryotes, it carries codons from several adjacent DNA cistrons and becomes much longer in size (polycistronic mRNA).

Genetic Code

• The term genetic code was coined by Gamow.
• DNA (or RNA) carries all the genetic information.
• It is expressed in the form of proteins.
• Proteins are made up of 20 different types of essential amino acids.
• The information about the number and sequence of these amino acids forming proteins is present in DNA and is passed on to mRNA during transcription.
• Genetic code is an mRNA sequence containing coded information for one amino acid. It consists of three nu- cleotides (triplet).
• Thus, for 20 amino acids, 64 (4 x 4 x 4 or 43 = 64) minimum possible permutations are available.
• This important discovery was the result of experiments by Marshall W. Nirenberg and J. Heinrich Matthaei and later by H.G. Khorana. Nirenberg and Khorana shared the 1968 Nobel Prize with R.W. Holley who gave the details of tRNA structure. Nirenberg and Mathaei used a synthetic poly(U) RNA and deciphered the code by translating this as polyphenylalanine. Hargobind Kho- rana, using synthetic DNA, prepared polynucleotide with known repeating sequence, CUCUCUCUCUCU; it produced only two amino acids-leucine (CUC) and serine (UCU).

Properties of Genetic Code

• Triplet code: Three adjacent nitrogen bases constitute a codon which specifies the placement of one amino acid in a polypeptide.
• Start signal: Polypeptide synthesis is signaled by two initiation codons-AUG or, rarely, GUG. The first or initiating amino acid is methionine. The initiating codon on mRNA for methionine is AUG. Initiating methionine occurs in formylated state in prokaryotes. At other positions, methionine is non-formylated. Both these methionines are carried by different tRNAs. Rarely, GUG also serves as initiation codon. It normally codes for valine but if present at the initiating position, it would code for methionine. So, GUG is an ambiguous codon.
• Stop signal: Polypeptide chain termination is signaled by three termination codons-UAA (ochre), UAG (amber), and UGA (opal). They do not specify any amino acid and were, hence, called nonsense codons. Whenever present in mRNA, these bring about termination of polypeptide chain and, thus, act as stop signals. Codons UAA, UAG, UGA, AUG, and GUG are also called punctuation codons.
• Commaless: The genetic code is continuous and does not possess pauses (meaningless base) after each tri- plet. If a nucleotide is deleted or added, the whole genetic code will read differently. Thus, a polypeptide having 50 amino acids shall be specified by a linear sequence of 150 nucleotides. If a nucleotide is added or deleted in the middle of this sequence, the amino acids before this will be the same but subsequent amino acids will be quite different.
• Universal code: The genetic code is applicable universally, i.e., a codon specifies the same amino acid from a virus to a human being or a tree.
• Non-overlapping code. Each codon is independent and one codon does not overlap with the next one.
• Degeneracy of code: Since there are 64 triplet codons and only 20 amino acids, the incorporation of some amino acids must be influenced by more than one codon. Only tryptophan and methionine are specified by single codons. All other amino acids are specified by 2-6 codons. The latter are called degenerate codons.

Wobble Hypothesis

• A change in nitrogen base at the third position of a codon does not normally cause any change in the expression of the codon because the codon is mostly read by the first two nitrogen bases (wobble hypothesis of Crick).
• The mutation that does not cause any change in the expression of the gene is called silent mutation.
• The position of the third nitrogen base in a codon which does not influence the reading of the codon is termed as wobble position.
• It pairs with the first position in anticodon. It means that the specificity of codon is determined by the first two bases.
• Wobbling helps one tRNA to read more than one codon and, thus, provides economy in the number of tRNA molecules at the time of translation.

Mutations and Genetic Code

• In a hypothetical mRNA, for example, the codons would normally be translated as follows.

• The insertion of single base G between the third and fourth bases produces a completely different protein from the earlier one.

• Similarly, the deletion of single base C at the fourth place produces a new chain of amino acids and, hence, a different protein.

• The insertion or deletion of one or two nitrogenous bases changes the reading frame from the point of insertion or deletion.
• Such mutations are called frame shift mutations.
• However, the insertion or deletion of three or its mul- tiple bases leads to the insertion or deletion of one or multiple codons. Hence, one or multiple amino acids and reading frames remain unaltered from that point onwards.
• This forms the genetic basis of proof that codon is a triplet and that it is read in a contiguous manner.

Translation

• Translation is the mechanism by which triplet base sequences of mRNA molecules are converted into a specific sequence of amino acids in a polypeptide chain. It occurs on ribosomes.
• The major steps are as follows:
  • Activation of amino acids: In the presence of enzyme aminoacyl-tRNA synthetase (E), specific amino acids (AA) bind with ATP.
  • Charging of tRNA: The AA, -AMP-E, com- plex formed in the first step reacts with a specific tRNA. Thus, amino acid is transferred to tRNA. As a result, the enzyme and AMP are liberated.
  • Formation of polypeptide chain: It is completed in three steps.
    • Chain initiation: It requires three initiation factors in prokaryotes-IF3, IF2, and IF.
      Nine initiation factors are required in eukaryotes. These are elF2, eIF3, eIF1, eIF4A, eIF4B eIF4c, eIF4D, eIFs, and eIF6.
      • Binding of mRNA with smaller subunit of ribosomes (30S/40S): IF, is involved in prokaryotes while eIF2 is involved in eukaryotes. It involves interaction be- tween the Shine Delgarno sequence and the 3′ end of 16S rRNA in prokaryotes.
      • Binding of 30S/40S-mRNA complex with tRNA: Non-formylated methionine is attached with tRNA in eukaryotes and formylated methionine in prokaryotes. IF2 is involved in prokaryotes while eIF3 is involved in eukaryotes.
      • Attachment of larger subunit of ribosomes: Initiation factors in eukaryotes are eIF, and eIF4. Initiation factor in prokaryotes is IF.
    • Chain elongation
      • Ribosomes have two sites for binding amino acyl tRNA: (1) amino acyl or A site (acceptor site) and (2) peptidyl site or P site (donor site).
      • A second charged tRNA molecule along with its appropriate amino acid approaches the ribosome at the A site close to the P site.
      • Its anticodon binds to the complementary codon of mRNA chain.
      • A peptide bond is formed between the COOH group of the first amino acid (methionine) and the NH2 group of the second amino acid.
      • The formation of peptide bond requires energy and is catalyzed by enzyme peptidyl transferase. The initiating formyl methionine or methionine tRNA can bind only with the P site.
      • All other newly coming aminoacyl tRNA bind to the A site.
      • The elongation factors required for prokaryotes are EF-Tu and EF-Ts and that required for eukaryotes is eEF.
      • Translocation is the movement of ribo- some on mRNA. It requires EF-G in prokaryotes and eEF, in eukaryotes.

• • • Chain termination: The termination of polypeptide is signalled by one of the three terminal codons in the mRNA. The three terminal codons are UAG (amber), UAA (ochre), and UGA (opal).
      • AGTP dependent factor known as release factor is associated with the termination codon. It is eRF1 in eukaryotes and RF1 ND RF2 in projaryotes.
      • At the time of termination, the terminal codon immediately follows the last amino acid codon. After this, the polypeptide chain, tRNA, and mRNA are released and the sumunits of ribosomes get dissociated.
      • Gene expression is the mechanism at the molecular level by which a gene is able to express itself in the phenotype of an organism.
      • In the translation unit, mRNA has some sequences that are not translated and are referred to as untranslated regions (UTR).
      • The UTRs are present at both 5′ end (before the start codon) and 3′ end (after the stop codon).
      • They are required for efficient translation process.

NEET Biology Notes Molecular Basis Of Inheritance Regulation Of Gene Expression

• Constitutive genes are those genes that are constantly expressing themselves in a cell because their products are required for normal cellular activities. For example, genes for glycolysis and ATPase.
• Non-constitutive genes are not always expressing themselves in a cell. These are called luxury genes. These are switched on or off according to the requirement of cellular activities. For example, genes for nitrate reductase in plants and lactose system in E. coli. This provides maximum functional efficiency to the cell. Such regulated genes, therefore, are required to be switched on and off when a particular function is to begin or stop.
• This regulation can be achieved by any one of the following two processes:
  • Induction: This is a process of gene regulation where the addition of a substrate stimulates or in- duces the synthesis of enzymes needed for its own breakdown.
  • Repression: In this process of gene regulation, the addition of end product stops the synthesis of enzymes needed for its formation. This phenomenon is also known as feedback repression.

Operon

• Francois Jacob and Jacques Monod (1961) proposed a model of gene regulation, known as the operon model.
• Operon is a coordinated group of genes such as structural gene, operator gene, promoter gene, and regulator gene which function or transcribe together and regulate a metabolic pathway as a unit.

Inducible Operon (Lac Operon)

• In E. coli, the breakdown of lactose requires three enzymes.
• These enzymes are synthesized together in a coordinated manner and the unit is known as lac operon.
• Since the addition of lactose itself stimulates the production of required enzymes, it is also known as inducible system.

• Lac operon consists of the following genes:
  • Structural genes: These genes code for the proteins needed by the cell which include enzymes or other proteins having structural functions. In lac operon, there are following three structural genes:
    • lac a-Gene coding for enzyme transacetylase
    • lac y Gene coding for enzyme permease
    • lac z-Gene coding for enzyme ẞ-galactosidase
  • Operator gene (o): It interacts with a protein molecule (the regulator molecule), which pro- motes (induces) or prevents (represses) the transcription of structural genes.
  • Promoter gene (p): This gene is the recognition point where RNA polymerase remains associated.
  • Regulator gene (i): This is generally known as inhibitory gene (1). This gene codes for a protein called the repressor protein. It is an allosteric protein which can exist in two forms. In one form, it is paratactic to an operator and binds with it, and in the other form, it is paratactic to the inducer (such as lactose).
• The operon is switched off and on.
• The transcription or function of lac genes or structural genes depends on the operator gene.
• When the repressor protein produced by inhibitory (i) gene or regulatory gene binds to the operator (o) gene, RNA polymerase gets blocked. There is no transcription and the operon model remains in “switched off” position.
• On the other hand, when an inducer such as lactose is added, the repressor protein (produced by gene i) gets bound to it.
• The repressor, therefore, gets released from the opera- tor.
• RNA polymerase is now permitted to act.
• Hence, the transcription of lac genes now takes place and the operon model is in “switched on” position.
• All three genes are transcribed by RNA polymerase to form a single mRNA strand.
• Each gene segment of mRNA is called cistron and, therefore, the mRNA strand transcribed by more than one gene is known as polycistronic.

Tryptophan Operon-Repressible Operon System

• Operon model can also be explained using feedback repression.

• In tryptophan (trp) operon, three enzymes are necessary for the synthesis of amino acid tryptophan.
• These enzymes are synthesized by the activity of five different genes in a coordinated manner.
• The addition of tryptophan, however, stops the produc- tion of these enzymes.
• Thus, the system is known as repressible system.
• In this system, there are five structural genes: trp A, trp B, trp C, trp D, and trp E. Three enzymes are needed for the synthesis of tryptophan-an amino acid.
• Regulatory gene (R) produces repressor protein which is known as apo-repressor because it does not get bound to the operator directly.
• Hence, the operator gene remains in “switched on” po- sition.
• Tryptophan, when added, binds to the apo-repressor and is called co-repressor.
• This apo-repressor and co-repressor complex (acti- vated repressor) now binds to the operator gene and blocks the function of RNA polymerase.
• Thus, transcription would not occur and tryptophan operon would be in “switched off” position.
• Feedback repression is functional when there is no further need of the end product and, hence, there is no requirement of continuation of this anabolic pathway.
• This operon stops the process.
• In prokaryotes, the control of the rate of transcriptional initiation is the predominant site for the control of gene expression.

Regulation of Gene Expression in Eukaryotes

• In eukaryotes, the regulation of gene expression can be exerted at four levels:
  • Transcriptional level (formation of primary transcript)
  • Processing level (regulation of splicing)
  • Transport of mRNA from nucleus to the cytoplasm
  • Translational level
• In eukaryotes, functionally related genes do not represent an operon but are present on different sites in chromosomes.
• Here, the structural gene is called split gene. It is a mosaic of exons and introns, i.e., base triplet-amino acid matching is not continuous.
• The Britten-Davidson gene battery model is most popular for eukaryotic genes.
• It proposes the occurrence of five types of genes: producer, receptor, integrator, sensor, and enhancer-silencer.
• Exons are the coding part of cistron which forms RNA.
• Introns are the non-translated part of DNA called IVS (intervening sequences) or spacer DNA.
• An intron begins with GU and ends up with an AG.
• However, the entire split gene is transcribed to form a continuous strip of hnRNA.
• This removal of non-coding intronic part and the fusion of exonic coding parts of RNA is called RNA splicing. About 50%-90% of primary transcribed RNA is discarded during processing.

DNA Fingerprinting

• The chemical structure of everyone’s DNA is the same.
• The only difference between people (or any animal) is the order of the base pairs.
• There are so many millions of base pairs in each person’s DNA that every person has a different sequence.
• Using these sequences, everyone can be identified solely by the sequence of their base pairs.
• However, there are so many millions of base pairs due to which the task would be very time consuming.
• Instead, scientists are able to use a shorter method, because of repeating base patterns in DNA (satellite DNA).
• These patterns do not, however, give an individual “fingerprint.” But these are able to determine whether two DNA samples are from the same person, related people, or non-related individuals.

NEET Biology Notes Molecular Basis Of Inheritance VNTRS, RFLP, SSR, AND RAPD

• Each strand of DNA has stretches that contain genetic information responsible for an organism’s development (exons) and stretches that, apparently, supply no relevant genetic information at all (introns).
• Although the introns may seem useless, it has been found that they contain repeated sequences of base pairs.
• These sequences are called variable number tandem repeats (VNTRS).
• VNTRS, also called minisatellites, were discovered by Alec Jeffreys et. al. of UK.
• These consist of hypervariable repeat regions of DNA having a basic repeat sequence of 11-60 bp and flanked on both sides by restriction sites.
• The number of repeats shows a very high degree of polymophism and the size of VNTR varies from 0.1 kb to 20 kb. The length of minisatellites and the position of restriction sites is different for each person.
• Therefore, when the genomes of two people are cut using the same restriction enzyme, the length and number of fragments obtained are different for both.
• This is called restriction fragment length polymorphism (RFLP).
• These fragments, when separated by gel electrophoresis, and obtained on a Southern blot, constitute what is called DNA fingerprint.
• The father of DNA fingerprinting is Alec Jeffreys while the Indian experts Lalji Singh and V.K. Kashyap are known as the father of Indian technique.

Methodology of DNA Fingerprinting

The Southern blot is one way to analyze the genetic patterns which appear in a person’s DNA. It was devised by E. M. Southern (1975) for separating DNA fragments.

• Isolating the desired DNA: It can be done either chemically (by using a detergent to wash the ex- tra material from the DNA) or mechanically (by applying a large amount of pressure in order to “squeeze out” the DNA).
• Cutting the DNA into several pieces of different size: This is done using one or more restriction enzymes.
• Sorting the DNA pieces by size: The process by which size separation is done is called gel electrophoresis. The DNA is poured into a gel, such as agarose, and an electric charge is applied to the gel, with the positive charge at the bottom and the negative charge at the top. Because DNA has a slightly negative charge, the pieces of DNA will be attracted towards the bottom of the gel. The smaller pieces, however, will be able to move more quickly and, thus, further towards the bottom than the larger pieces. The different-sized pieces of DNA will, therefore, be separated by size, with the smaller pieces towards the bottom and the larger pieces towards the top.
• Denaturing the DNA fragments, so that all of the DNA is rendered single-stranded: This can be done either by heating by chemically treating the DNA in the gel using alkali.
• Blotting the DNA: The gel with size-fractionated DNA is applied to a sheet of nitrocellulose paper or nylon membrane, and then baked to permanently attach the DNA to the sheet. The Southern blot is now ready to be analyzed.

In order to analyze a Southern blot, a radioactive genetic probe is used in a hybridization reaction with the DNA in ques- tion. If an X ray is taken of the Southern blot after a radioactive probe has been allowed to bind with the denatured DNA on the paper, only the areas where the radioactive probe binds will show themselves on the film (autoradiography). This allows researchers to identify, in a particular person’s DNA, the occurrence and frequency of the particular genetic pattern contained in the probe.

Practical Applications of DNA Fingerprinting

• Solving cases of disputed paternity and maternity: Because a person inherits his or her VNTRS from his or her parents, VNTR patterns can be used to establish paternity and maternity.
• Criminal identification and forensics: DNA isolated from blood, hair, skin cells, or other genetic evidence left at the scene of crime can be compared, through VNTR patterns, with the DNA of a criminal suspect to determine guilt or innocence.

• Personal identification: The notion of using DNA fingerprints as a sort of genetic bar code to identify individuals has been discussed, but this is not likely to happen anytime in the near future. The technology required to isolate, keep on file, and then analyze millions of very specified VNTR patterns is both expensive and impractical.

NEET Biology Notes Molecular Basis Of Inheritance Genomics

• The application of DNA sequencing and genome mapping to the study, design, and manufacture of biologically important molecules is known as genomics.
• It is comparatively a more recent branch in the field of biology.
• The term genomics was introduced by Thomas Roder- ick.
• In genomics, the function of gene is identified by using the technique of reverse genetics.
• The study of genomics may be classified into three classes:
  • Structural genomics: It involves the mapping and sequencing of genes.
  • Functional genomics: It involves the identification of function of a particular gene.
  • Application genomics: It involves the use of genomics information for crop improvement, etc.
• The complete genomics of Arabidopsis and rice have been worked out. These have 130 million bp and 430 million bp, respectively.

Human Genome Project

• The Human Genome Project (HGP) was the international, collaborative research program whose goal was the complete mapping and understanding of all genes of human beings.
• All genes together are known as genome.
• The HGP as “mega project” was a 13 year project co- ordinated by the US Department of Energy and the Na- tional Institute of Health.
• An International HGP was launched in the year 1990 and completed in the year 2003.
• The International Human Genome Sequencing Consortium published the first draft of the human genome in the journal Nature in February 2001.
• Human genome is said to have approximately 3 × 10 bp, and the cost of sequencing required is US$ 3 per bp. The total estimated cost of the project would be approximately US$ 9 billion. In human ge- nome, 20,000 to 25,000 genes are present; out of them, the smallest gene is testis-determining fac- tor (TDF) gene with 14 bp and the largest gene is Duchenne muscular dystrophy gene with 2400 × 103 bp.

Goals of HGP

Following are the important goals of the HGP:

• Identification of all, approximately, 20,000-25,000 genes in human DNA.
• To determine the sequence of 3 billion chemical base pairs that make up human DNA.
• To store this information in databases.
• To improve tools for data analysis.
• To transfer related technologies to other sectors, such as industries.
• Bioinformatics, i.e., close association of HGP with the rapid development of a new area in biology.
• Sequencing of model organisms-Complete genome sequence of E.coli, S. cerevisiae, C. elegans, D. melanogaster, D. pseudoobscura, Oryza sativa, and Arabidopsis was achieved in April, 2003.
• Address the ethical, legal, and social issues (ELSI) that may arise from the project.

Methodologies

The HGP techniques include the following:

• Sequence tagged site (STS): It is a short DNA segment that occurs only once in a genome and whose ex- act location and order of bases are known. STSS serve as landmarks on the physical map of a genome. These are also called expressed sequence tags (ESTs). Genes that are expressed as RNA are referred to as ESTS.
• Sequencing the whole set of genome that contained all the coding and non-coding sequences and later assigning different regions in the sequence with functions is known as sequence annotation.
• The employment of restriction fragment length polymorphism (RFLP).
• Yeast artificial chromosomes (YACs).
• Bacterial artificial chromosomes (BACs).
• Polymerase chain reaction (PCR).
• Electrophoresis
  • For sequencing, the total DNA from a cell is isolated and converted into random fragments of relatively smaller sizes (recall DNA is a very long polymer, and there are technical limitations in sequencing very long pieces DNA) and cloned in suitable host using specialized vectors.
  • Cloning results in the amplification of each piece of DNA fragment so that it can be subsequently sequenced with ease.
  • The commonly used hosts were bacteria and yeast, and the vectors were called as BAC and YAC, respectively.

• The fragments were sequenced using automated DNA sequencers that worked on the principle of the method developed by Frederick Sanger. These sequences were then arranged based on some overlapping regions present in them. This required the generation of overlapping fragments for sequencing. The alignment of these sequences was humanly not possible. Therefore, specialized computer-based programs were developed. These sequences were subsequently annotated and were assigned to each chromosome. The sequence of chromosome-1 was completed only in May, 2006. (This was the last of the 24 human chromosomes-22 autosomes and X and Y–to be sequenced.)

Salient Features of Human Genome

Some salient observations drawn from the HGP are as follows:

• The human genome contains 3164.7 million nucleotide bases.
• On an average, a gene consists of 3000 bases, but sizes vary greatly, with the largest known human gene being dystrophin at 2.4 million bases.
• The total number of genes is estimated at 30,000- much lower than the previous estimates of 80,000- 1,40,000 genes. Almost all (99.9%) nucleotide bases are exactly the same in all people.
• The functions are unknown for over 50% of discov- ered genes.
• Less than 2% of the genome codes for proteins.
• Repeated sequences make up very large portion of the human genome.
• Repetitive sequences are stretches of DNA sequences that are repeated many times, sometimes hundred to thousand times. They are thought to have no direct coding functions, but they shed light on chromosome structure, dynamics, and evolution.
• Chromosome-1 has the highest number of genes (2968) while Y-chromosome has the fewest (231).
• Scientists have identified about 1.4 million locations where single-base DNA differences (SNPs or single nucleotide polymorphism, pronounced as “snips”) occur in humans. This information promises to revolutionize the process of finding chromosomal locations for disease-associated sequences and tracing human history.

Future Thrust of HGP

Francis Collins, the director of the public funded genome project, predicted the following progress in the HGP:

By 2010

• Scientists will have developed accurate predictive tests for at least a dozen common diseases so that preventive measures may be taken in advance.
• Infertility specialists will be using sophisticated techniques of pre-implantation genetic diagnosis to screen embryos for genetic disorders and designer babies.
• Medicines will be making use of gene therapy.

By 2020

• Doctors will have “designer drugs” to treat almost every disease.
• Doctors will test patient’s genetic make-up before pre- scribing drugs.
• Doctors will be able to change the genetic make-up of a living person by germ-line therapy.
• The treatment of diseases such as cancer, schizophrenia, and depression will have transformed.

By 2030

• Gene-based healthcare will have completely developed.
• Most medical researches will be carried out using computer models rather than the living tissues of animals.
• Average life span will rise up to 90 years.

The Indian Scenario

• The Indian gene center accounts for 160 species out of 2400 species in all of the 12 megagene centers.
• Though the plant genetic resources activities got in- tensified in India in the first half of the century, these received the required impetus after the creation of NBPGR in 1976 which has headquarters at IARI (In- dian Agricultural Research Institute), New Delhi, and several regional sub-centers at different agroclimatic zones such as Jodhpur (arid areas), Trichur (humid tropical zone), and Shillong (eastern sub-tropical/sub- temperate zone).
• On March 31, 1996, NBPGR held 15,4,533 accessions of various agri-horticultural crops.
• National Facility for Plant Tissue Culture Repository (NFPTCR) was established in 1986 by the department of biotechnology at NBPGR for the conservation of germplasm of vegetatively propagated plants.

NEET Biology Notes Molecular Basis Of Inheritance Summary

• Nucleotide monomers constitute a polymer called nucleic acid. It is of two types: RNA and DNA.
• While DNA is the store house of information, RNA helps in the transfer and expression of information.
• As DNA is structurally and chemically more stable, it is a better genetic material, although both DNA and RNA serve as genetic material.
• RNA was the first to evolve, and DNA was derived from it.
• Bases in two DNA strands show hydrogen bonding (A=T, G=C) and follow Chargaff’s rule, so that both the strands are complementary and DNA replication is semi-conservative.
• The segment of DNA that codes for RNA is known as gene. During transcription, one DNA strand acts as template which directs the synthesis of complementary RNA.
• In prokaryotes, transcription and translation are continuous processes. In eukaryotes, the genes are split. Exons are interrupted by introns. Introns are removed and exons are joined to produce functional RNA.
• The mRNA contains genetic code in combination of three (triplet code) to code for an amino acid. This genetic code is read by tRNA which acts as an adapter molecule.
• There is specific tRNA for each amino acid. Each tRNA binds to amino acid at one end and with codons by H-bonding at the other end.
• Translation occurs at ribosome. Here ribozyme (rRNA enzyme) acts as catalyst which helps in peptide bond formation. The process of translation has evolved around RNA, which shows that life began around RNA.
• Since transcription and translation are energetically very expensive, they are tightly regulated. For example, lac operon, which is regulated by the amount of lactose in medium, i.e., the regulation of enzyme synthesis by its substrate.
• HGP is aimed at sequencing every base in human genome. DNA fingerprinting is used for this which is based on the principle of polymorphism in DNA sequence.

NEET Biology Notes Molecular Basis Of Inheritance Assertion-Reasoning Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).
2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
3. If Assertion is true but Reason is false, then mark (3).
4. If both Assertion and Reason are false, then mark (4).

Question 1. Assertion: RNA polymerase is of three types in eukaryotes for the synthesis of all types of RNAs.

Reason: RNA polymerase consists of six types of poly- peptides along with rho factor which is involved in the termination of RNA synthesis.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 2. Assertion: 5S rRNA and surrounding protein complex provides binding site of tRNA.

Reason: tRNA is soluble RNA with unusual bases.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 3. Assertion: Operator gene is functional when it is not blocked by repressor.

Reason: Regulator gene produces active protein only which acts on operon system in E. coli.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 4. Assertion: Peptidyl transfer site is contributed by larger subunit of ribosome.

Reason: The enzyme peptidyl transferase is contributed by both 23S and 16S ribosomal subunits.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 5. Assertion: Teminism is unidirectional flow of information.

Reason: It requires DNA dependent RNA polymerase enzyme.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 6. Assertion: In bacterial translation mechanism, two tRNA are required by methionine.

Reason: AUG codes for methionine and it shows non- ambiguity also.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 7. Assertion: Nutritional mutant strain of pink mold is auxotroph.

Reason: It is not able to prepare its own metabolites from the raw materials obtained from outside.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 8. Assertion: In DNA fingerprinting, hybridization is done with molecular probe.

Reason: Molecular probe is small DNA segment synthesized in laboratory with known sequence that recognizes complementary sequence in RNA.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 9. Assertion: cDNA libraries are important to scientists in human genomics.

Reason: cDNA is synthetic type of DNA generated from mRNA.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 10. Assertion: SNP (pronounced “snips”) are common in human genome.

Reason: It is minute variation that occurs at a frequency of one in every 300 bases.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 11. Assertion: A single strand of mRNA is capable of forming a number of polypeptide chains.

Reason: Termination codons occur in mRNA.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 12. Assertion: Chromosomal aberrations are caused by a break in the chromosome or its chromatid.

Reason: Duplication, deficiency, transversion, and trans- locations are the causes of chromosomal aberrations.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 13. Assertion: The lac operon model is applicable to E. coli.

Reason: E. coli. lacks a definite nucleus.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 14. Assertion: Amber codon is a termination codon.

Reason: If in mRNA, a termination codon is present, protein synthesis stops abruptly whether it is completed or not.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 15. Assertion: Watson and Crick provided experimental proof of the semi-conservative nature of DNA replication.

Reason: RNA polymerase binds nucleotides in replication.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 16. Assertion: The mRNA attaches itself to the ribosome via its 3′ end.

Reason: The mRNA has nucleotide and bases of lagging sequence.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 17. Assertion: Replication and transcription occur in the nucleus but translation occurs in the cytoplasm.

Reason: mRNA is transferred from the nucleus into the cytoplasm where ribosomes and amino acids are available for protein synthesis.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 18. Assertion: Cancer cells are virtually immortal until the body in which they reside dies.

Reason: Cancer is caused by damage to genes regulating the cell division cycle.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

NEET Biology Notes Digestion And Absorption Nutrition

The sum total of the processes starting from taking the food up to its utilization is called nutrition. Depending upon the mode of nutrition, organisms can be classified as autotrophs and heterotrophs. Euglena is mixotrophic (both autotrophic and saprophytic).

Animals Nutrients

• Nutrients may be organic or inorganic in nature.
• The organic constituents of nutrients are carbohydrates, lipids, proteins, and vitamins, and inorganic constituents are minerals and water.
• Carbohydrates, lipids, and proteins are macronutrients or proximate principles of food because these constitute energy sources for the production of heat and different organic functions.
• Minerals, vitamins, and water are micronutrients or protective principles of food because although these do not provide energy, yet their deficiencies are related to specific abnormalities in man.
• About 21 minerals (for example, sodium, potassium, calcium, sulfur, phosphorus, magnesium, and chlorine) or macroelements are known to be essential for human nutrition; they are required as 100 mg per day.
• Trace elements or microelements (for example, iron, iodine, zinc, manganese, cobalt, copper, molybdenum, etc.) are required in very small amounts.
• Altogether 20 vitamins are thought to be required in small amounts in human nutrition.

“digestion and absorption neet notes “

Types of Nutrition in Animals: Based on the methods of procurement or collection of food, heterotrophic nutrition is of three major types:

1. Holozoic Nutrition: When whole plants (or their parts) or whole animals (or their parts) or both are consumed either in solid or liquid form, through the mouth.

It can be classified in the following groups

• Herbivores: Feeding on plants, for example, rabbit, goat, cow, etc.
• Carnivores: Feeding on other animals, for example, tigers, lions, etc.
• Omnivores: Feeding on all types of food, for example, humans, cockroaches, etc.
• Detrivores: Feeding on detritus or organic remains, for example., earthworms, etc.
• Scavengers: Feeding on carrions (carrion eaters), for example., hawks, vultures, etc.
• Insectivores: Eating insects, for example, common bats, etc. Cannibals: Eating other members of one’s own species, for example, many snakes, etc.
• Frugivorcs: Feeding on fruits, for example, parrots, etc. Sanguivores: Taking meals of blood, for example, leech, female mosquito, etc.
• Coprophagous: Eats its own fecal matter, for example, rabbit.

2. Saprozoic Nutrition: When decaying organic materials of plant or animal origin are consumed.

The organisms following such nutrition secrete digestive enzymes directly onto their food and therefore the food is digested outside the body, for example, fungi, bacteria some protozoans, etc.

3. Parasitic Nutrition: When a living organism feeds on another living organism and causes harm to it, for example., tapeworm, malarial parasite, etc.

Read and Learn More NEET Biology Notes

NEET Biology Notes Digestion And Absorption Digestive System Of Mammals

Digestion is the catabolic process in which the non-diffusible complex of organic compounds converts into diffusible simple compounds by hydrolytic enzymes. The digestive system comprises (1) an alimentary canal and (2) acessary digestive glands.

Alimentary Canal: It is a coiled muscular tube about 6-9 m long, extending from mouth to anus. It is divided into three parts:

1. Foregut Or Stomodeum: Ectodermal in origin, it includes a mouth and buccal cavity.
2. Midgut Or Mesenteron: Endodermal in origin, it includes pharynx to rectum.
3. Hindgut Or Proctodeum: Ectodermal in origin, it includes the anal canal and anus.

Path Of Food From Mouth To Anus: Mouth → Buccal cavity → Oropharynx → Oesophagus → Stomach → Small intestine → Large intestine → Anal canal → Anus.

Parts Of The Alimentary Canal

Mouth: The mouth is an opening bounded by upper and lower lips. Lips are attached on the inner side with gums by a thin transparent fold called the labial frenulum of the mesentery.

• The motility of lips is due to orbicular oris muscles.
• Whales and platypuses have immovable lips.
• In men, the middle part of the upper lip is depressed and is called filtrum (a beauty sign).
• The lip of a rabbit has a cleft called a hare lip.

Buccal Cavity

• The buccal cavity is divided into (1) vestibule and (2) oral cavity. The space between lips and teeth is called a vestibule.
• The roof of the oral cavity is made up of a palate. On the anterior side, the hard palate (maxilla, premaxilla, and palatine bone), and on the posterior side, the soft palate is present.
• The mucus epithelium has thick transverse folds called palatine rugae.
• The terminal part of the soft palate hangs in the throat called the uvula.
• On the sides of the uvula, tonsils are present which are made up of lymphatic tissue.
• The floor of the oral cavity is occupied by a muscular tongue.

” class 11th digestion and absorption notes”

Tongue

• The tongue is a voluntary muscular and glandular structure.
• It is attached to the floor of the buccal cavity by a fold called the lingual frenulum of the tongue.
• An inverted V-shaped furrow termed sulcus terminalis divides the upper surface of the tongue into the anterior oral part and posterior pharyngeal part.
• The apex of the sulcus terminalis projects backward and is marked by a small median pit, named foramen cecum.
• Foramen cecum is an embryological remnant and marks the site of the upper end of the thyroglossal duct.
• The upper surface of the oral part of the tongue has small projections called papillae on its surface. These are:

Filiform Papillae: Smallest, most abundant, and have no taste buds.

Fungiform Papillae: These appear as red dots on the tongue and contain taste buds.

Foliate Papillae: Leaf-like, absent in humans.

Circumvallate Papillae: Largest in size, knob-like, and contain taste buds.

Functions Of Tongue

• It helps in the mastication of food.
• It helps in the cleaning of the buccal cavity.
• Helps in speech articulation.
• The sweat glands of dogs are present on the tongue (panting of dog); so it helps in thermoregulation.

Salivary Glands: Four pairs of salivary glands open in the buccal cavity:

Parotid: Largest—present below and in front of ears—Stenson’s duct.

Submaxillary: Medium-sized —present at the angles of the lower jaw—Wharton’s duct.

Sublingual: Smallest—located below the tongue__ Rivinus duct.

Infraorbital: Absent in man, otherwise present below eyes, for example., rabbit.

• Daily secretion of saliva is 1.5 L (pH of saliva is 6.7) and has salivary amylase (ptyalin), maltase, and lysozyme.
• Salivary glands are stimulated to secrete saliva by parasympathetic innervation; on the other hand, sympathetic nerves cause reduced secretion of saliva leading to the drying of the mouth.
• Cl- ions are required for the activation of salivary amylase.
• Viral infection of salivary glands (mainly parotid) causes mumps disease.
• Food mixed with saliva in the buccal cavity is called bolus.

Teeth: Teeth of human beings are of the following types:

1. Thecodont: Part of teeth present in the bony socket, called alveoli.

2. Heterodont: Different sizes and shapes of teeth are called heterodonts.

Teeth present on the upper and lower jaws are

• Incisors: For cutting; have a single root.
• Canines: For tearing; has a single root.
• Premolars: Upper premolars have two roots; lower premolars have one root.
• Molars: For chewing, the upper molars have three roots and the lower molars have two roots.
• Canines: Well-developed in carnivores. In rabbits, canines are absent and this space is called diastema.

3. Diphyodont: Teeth appear twice in life.

• Milk teeth (deciduous teeth/lacteal teeth)
• Permanent teeth (remains throughout life)

Classification Of Teeth According To Position

1. Acrodont: These teeth arise from jaw bones, for example, reptiles, amphibians, and fishes.
2. Pleurodont: Teeth fixed to the lateral surface of the jaw ridge, for example, fangs of snakes.
3. Thecodont: Embedded in sockets, for example, mammals and crocodiles.

Classification Of Teeth According To The Arrangement Of Enamel And Dentine

1. Bunodont: Small cusps, for example, human.
2. Lophodont: Long transverse dentine covered by enamel, for example., elephants.
3. Selenodont: Crescent-shaped cusps, for example, sheep/ cattle.
4. Secodont: Pointed cusps, for example, carnivores.

Structure Of Teeth: A tooth is made up of three parts:

1. Crown: It is the outer part of the tooth, exposed outside gums.
2. Neck: It is the middle part of the tooth which is embedded inside the gums.
3. Root: It is the part of the tooth that is inserted inside the socket of the jaw bone (alveoli).

Dental Formulae:

In a human being, 20 teeth grow twice during a lifetime, i.e., diphyodont \(\frac{2102}{2102}\) (premolars and last molars absent), and 12 teeth appear only once in life, i.e., monophyodont.

Additional Points Regarding Teeth

• Enamel (secreted by ameloblast): The hardest substance of the body—ectodermal in origin.
• Dentine (secreted by odontoblasts): Main part of tooth—mesodermal in origin.
• Caries: Decay of teeth due to degeneration of the enamel and formation of cavities.
• Pyorrhea: Infected gums and tooth sockets.

Pharynx: The pharynx is a common passage in swallowing food and breathing.

• The gullet is the aperture which leads into the esophagus air.
• The glottis is the structure that allows air to enter into the trachea.
• Epiglottis is the structure that prevents entry of food into the windpipe during swallowing in mammals.

Tonsils: The lymphoid tissues of the pharynx are called tonsils.

• Tonsils are of the following types:
• Nasopharyngeal/pharyngeal tonsil/adenoids
• Palatine/faucial tonsils
• Lingual tonsils

These are arranged in a ring-like manner called Waldeyer’s ring.

Oesophagus: It is a long (25cm) and thin tube that pierces the diaphragm and enters the abdominal cavity. Oesophagus is characterized by:

• Absence of visceral peritoneum. Its outermost fibrous (non-coelomic) covering is called tunica adventitia.
• Absence of digestive glands.
• It has mucus-secreting goblet cells.
• Presence of mucous membrane formed of non-keratinized stratified squamous epithelium.
• Presence of voluntary (anterior one-third) and involun¬ tary (posterior two-thirds) muscle fibers.

Stomach: The stomach is oval and pouch-like, divisible into cardiac,fundic, and pyloric parts

digestive system class 11th

• The cardiac sphincter is present at the opening of the esophagus into the cardiac stomach and prevents the regurgitation of food into the esophagus.
• The pyloric part opens into the small intestine and the opening is guarded by the pyloric sphincter.
• The wall of the stomach has three layers of muscles, the outermost longitudinal layer, the middle circular layer, and the innermost oblique layer.
• Mucosa has folds called gastric rugae and cardiac, fundic, and pyloric glands.
• Only fundic glands secrete gastric juice. These contain neck cells (secrete mucus and are present in all three types of glands).
• Oxyntic or parietal cells secrete HCl and Castle’s intrinsic factor for absorption of B₁₂.
• HCl of gastric juice converts Fe³⁺ into Fe²⁺ which makes the absorption of iron possible.
• Non-secretion of HCl (achlorhydria) or gastrectomy can lead to iron deficiency (anemia).
• Peptic cells chief cells or zymogenic cells release large quantities of pepsinogen.

Mucus cells: A large amount of mucus is secreted, Semisolid food, mixed with gastric juices in the stomach, is known as chyme (it is highly acidic).

Compound stomach

• The stomach of ruminants is known as the compound stomach.
• It has four well-defined chambers or compartments, viz., rumen, reticulum, omasum, and abomasum.

• Rumen is the first and the largest chamber mainly meant for the storage of food.
• In camel and deer, omasum is absent, and water cells project from rumen. Digestion of cellulose takes place by fermentation, with the help of symbiont bacteria and protozoans.
• The abomasum is the true stomach, which secretes gastric juices.
• Rabbits eat their own feces (coprophagy) to complete the digestion of cellulose. It is taken as pseudo-rumination.

Small Intestine: It consists of three parts: (1) duodenum, (2) jejunum, and (3) ileum.

• The first part is the duodenum, 25 cm long, C-shaped in humans, and has an opening of the hepatopancreatic duct (bile duct + pancreatic duct).
• A small swelling is present at the opening of the hepatopancreatic duct and is called the ampulla of Vater or hepatopancreatic ampulla and the opening is regulated by the sphincter of Oddi.
• The next part ofsmall intestine is jejunum and ileum.
• The wall of the intestine has thin layers of longitudinal and circular muscles.
• Mucosa has folds plicae circulars (folds of Kerkrings or valvulae conniventes) and villi toward the lumen of the intestine.
• Epithelial cells lining the villi have microvilli which further increase the absorptive area.
• Intestinal glands or crypts of Leiberkuhn have epithelial cells (secrete mucus), path cells (secrete digestive enzymes), and argentaffin cells (probably secrete hormones).

” difference between digestion and absorption”

• In the duodenum, Brunner’s glands are also present (located in the submucosa) which secrete mucus.
• Diffused patches of lymphoid tissues are present throughout the small intestine and are aggregated in the ileum to form Peyer’s patches.
• Intestinal villi are mainly concerned with absorption.
• The main function of intestinal villi is to provide a large surface area for absorption.
• The absorption of digested food mainly occurs in the small intestine mainly the ileum.
• The lymph capillary present in a villus is called a lacteal. It is concerned with the absorption of digested fat.
• The distal end of the ileum is expanded to form a small dilated spherical sac called sacculus rotundus in rabbits. The ileum opens into the cecum through an ileocecal valve.
• Half liquid food mixed with bile, pancreatic juice, and succus entericus in the intestine is called chyle (highly alkaline).

Food + Bile + Pancreatic + Intestinal juice = Chyle

Large Intestine

• It is about 1.5 m long and consists of three parts: (1) caecum, (2) colon, and (3) rectum.
• A blind pouch of the cecum is a vermiform appendix.
• These parts help in the digestion of cellulose in herbivores (rabbit).
• The wall of the colon has sac-like haustra. Histologically, the wall of the colon has three bands of longitudinal muscles called taenia coli.
• Another characteristic of the colon surface is the presence of small fat-filled projections called epiploic appendages.
• The colon part is divisible into ascending, transverse, descending, and sigmoid colon.
• The ascending colon is the smallest and is without mesentery.
• The last part of the rectum is the anal canal having a strong sphincter. It opens outside by the anus.
• In certain conditions (such as persistent constipation), rectal veins can get distended or enlarged due to weakening valves (varicosity). It leads to swollen areas called hemorrhoids (piles).

Histology Of Alimentary Canal: The alimentary canal consists of four basic layers. From the outer surface toward the inner side of the tire lumen (cavity), the layers are as follows:

Visceral Peritoneum (serous membrane or serosa): It is the outermost layer made up of squamous epithelium. It is continuous with the mesentery.

Muscular Coat: It is composed of outer longitudinal and inner circular muscle fibers.

• In the stomach, an additional layer of oblique muscle fibers is found in the circular muscle fibers.
• These muscle fibers are unstriped (smooth). The muscular coat also contains the major nerve supply to the gastrointestinal tract—the mesenteric plexus (plexus of Auerbach), which consists of fibers from both au autonomic divisions and mostly controls the peristaltic movements in the alimentary canal.

Submucosa: It consists of loose connective tissues richly supplied with blood and lymphatic vessels and in some areas with glands.

• Meissner’s plexus (submucosal plexus) is present between the muscular coat and the mucosa which is part of the autonomic nerve supply to the smooth muscles and secretory cells of mucosa glands.
• It controls various secretions of the alimentary canal and movements of the mucosa.

Mucosa (mucous membrane): It is so named because it secretes mucus to lubricate the inner lining of the gut. It is composed of three layers:

• The thin muscularis mucosa lies next to the submucosa. It consists of outer longitudinal and inner circular muscle fibers, both arc unstriped.
• Lamina propria, the middle layer of mucosa, consists of loose connective tissues, blood vessels, glands, and some lymphoid tissues.
• The innermost layer is the epithelium, which forms gastric glands in the stomach, and villi and intestinal glands in the small intestine. In the upper one-third of the esophagus, both Auerbach and Meissner’s plexuses are absent.

Digestive Glands

Liver

• Largest digestive gland. It lies in the upper right side of the abdominal cavity just below the diaphragm.
• The liver is divided into two main lobes: right and left.
• The right lobe is differentiated further into the right lobe proper, a quadrate lobe, and a caudate lobe on the posterior surface.
• The liver is surrounded by Glisson’s capsule’, its trabeculae divide liver lobes into hexagonal lobules.
• Polyhedral hepatocytes arc arranged in cords around a central venule.
• Portal triads contain the hepatic artery, portal venule, bile ductule, and lymphatics.
• Blood sinusoids are present. Kupffer cells are present in sinusoids and are phagocytic in nature.
• Gail’s bladder is situated on the inferior surface of the right lobe. It is 8 cm long and 2 cm wide.
• Bile is secreted by hepatocytes into the bile canaliculi, a series of narrow spaces between adjacent liver cells.

• The canaliculi drain via bile ductules into bile ducts, which run in portal tracts; the bile ducts themselves discharge into the right and left hepatic ducts which unite to form the common hepatic duct at the hilum of the liver.
• Gall bladder has the capacity of storing 30 to 50 mL of bile. It fills and empties via the cystic duct which joins the common hepatic duct to form the bile duct; this in turn empties into the duodenum through the ampulla of Vater (hepatopancreatic ampulla).
• At the point of its entry into the duodenum, the bile duct and adjacent pancreatic duct join each other.
• The sphincter of Boyden surrounds the opening of the bile duct into the pancreatic duct. The sphincter of Oddi surrounds the ampulla of Vater.

Functions Of Liver

1. Secretion of bile: Daily secretes 700-1000 mL. In the gall bladder, electrolytes and water are reabsorbed concentrating bile approximately 10 to 15 times.
   • The bile contains bile pigments bilirubin and biliverdin, bile salts sodium glycocholate, sodium taurocholate, and sodium bicarbonate.
   • Sodium bicarbonate is mainly responsible for the alkaline nature of bile (pH = 8.0).
   • Cholesterol is insoluble in water but its association with bile acid and phospholipid makes it soluble.
   • In the subjects, who regularly produce bile, supersaturated with cholesterol, the cholesterol may be deposited in the gall bladder as gallstones (cholelithiasis).
   • Inflammation of the gall bladder is called cholecystitis.
2. Storage of fat.
3. Urea synthesis.
4. Erythropoiesis (during embryonic period only).
5. Breakdown of RBCs.
6. Iron is stored as ferritin.

Pancreas (Heterocrine Gland)

• A racemosely branched gland situated between the stomach and duodenum.
• The pancreas consists of acini (which secretes digestive enzymes) and islets of Langerhans (which secretes insulin and glucagon hormones).
• The pancreas has two ducts with it. The first is the duct of Santorini which is accessory or non-functional, opening directly into the duodenum and the other is the duct of Wirsung which is functional and combines with the bile duct to form a common hepatopancreatic duct.

Mobility Of Alimentary Canal

• The alimentary canal undergoes regular contraction for proper digestion and absorption of food.
• Food enters into buccal cavity where it is mixed with saliva.
• Food is masticated with the help of teeth. The salivary enzyme ptyalin (salivary amylase) causes chemical breakdown of food (i.e., carbohydrates).
• Smaller food particles are held together by the mucin of saliva forming the food bolus which is then swallowed.
• The food bolus gets swallowed, i.e., enters the esophagus with the coordinated activity of the tongue, soft palate, and pharynx.
• Waves of contraction or peristaltic waves in the esophagus push it downward.
• As the food reaches the end of the esophagus, the cardiac sphincter, regulating the opening of the esophagus into the stomach, relaxes to allow the entry of food into the stomach.
• If the sphincter fails to open up properly, it leads to the accumulation of food in the lower part of the esophagus called cardia achalasia.
• In the stomach, mechanical churning of the food occurs by waves of contractions passing from the cardiac to the pyloric end, and mixing of food with gastric juice also occurs.
• During this activity, cardiac and pyloric sphincter remains closed.
• If the cardiac sphincter remains open, acidic gastric contents may escape into the esophagus, leading to a heart bum condition due to a burning sensation in the esophagus.
• From the stomach, acidic chyme enters the small intestine where digestion is completed followed by the absorption of digested food.
• From the small intestine, the chyle enters the large intestine and is finally egested out.
• Movements in the alimentary canal are caused by myenteric plexus, as well as hormones such as motilin, villikinin, gastrin, etc. It shows peristaltic movements.
• The peristaltic movements involve contraction and relaxation resulting in wave-like movement.
• Contraction is due to the contraction of circular muscles and relaxation of longitudinal muscles. Relaxation is caused by the simultaneous contraction of longitudinal muscles and the relaxation of circular muscles.
• Peristaltic movements start from the esophagus.
• The churning movements of the stomach are also peristaltic movements which become powerful as they proceed toward pylorus.
• In the large intestine, peristaltic movements are moderately weak.

Digestion And Gastrointestinal Secretions

Digestion Of Carbohydrates

• The diet of most of the animals including man consists of carbohydrates.
• Depending upon the complexity, carbohydrates are of three types: polysaccharides, disaccharides, and monosaccharides.
• During the process of digestion, both poly-and disaccharides are broken down into monosaccharides, and in this form, they can be absorbed into the body.
• Some of these complex carbohydrates are starch and cellulose present in cereal grains, potatoes, fruits, and tubers, sucrose present in cane sugar, lactose present in milk, etc.
• Enzymes that act- on carbohydrates are collectively known as carbohydrases. In the mouth cavity, the food is mixed with saliva.
• It contains an enzyme called salivary amylase or ptyalin.
• Salivary amylase acts on starch and converts it into maltose, isomaltose, and small dextrins or limit dextrin’ (disaccharides).

• Chewing and mastication of food increase the action of salivary amylase on starch by increasing the surface area of food on which the enzyme acts.
• Cooking of food also helps the action of salivary amylase by causing breaches in the cellulosic cell walls.
• The amylase can, thus, enter into the food and digest starch.
• About 30 % of the starch present in food is hydrolyzed in the mouth itself.
• The action of salivary amylase continues for sometimes even in the stomach but soon. HCl present in the gastric juice destroys all the enzymes.
• Salivary amylase is absent from the saliva of many mammals such as cows and buffaloes; and predatory carnivores such as lions and tigers. However, pigs have got amylase in their saliva.
• Pancreatic juice and intestinal juice also contain carbohydrate-digesting enzymes.
• Pancreatic juice contains pancreatic amylase that acts on starch to digest it into maltose, isomaltose, and dextrin.
• Intestinal juice contains a number of carbohydrates such as maltase, isomaltase, sucrase, and lactase.
• Maltase and isomaltase act on maltose, isomaltose, and dextrin and convert them into glucose; sucrase acts on sucrose to convert it into glucose and fructose, and lactase acts on lactose to convert it into glucose and galactose.

• Human beings can digest lactose present in the milk. But with advancing age, they also cannot digest milk. This is because less lactase is produced. In them, lactose remains undigested and gets fermented in the intestine producing gases and acids. This results in flatulence, intestinal cramps, and diarrhea. So these persons must consume curd or yogurt (sweetened curd) as lactose is fermented to lactic acid in them.
• Galactosemia: Galactosemia is a metabolic disorder due to the absence of the enzyme uridyl transferase. As a result, galactose will accumulate leading to mental retardation.
• It can be prevented in galactosemic children by giving them a milk-free diet.

Digestion Of Proteins

• Proteins are complex organic compounds made up of single units called amino acids.
• So in the process of digestion, all proteins are broken down into amino acids.
• Enzymes that hydrolyze proteins are collectively known as proteases or peptidases.
• Many of these enzymes are secreted in their inactive font or proenzymes as their active form would hydrolyze cellular and extracellular proteins of the organism itself.
• Inactive enzymes are converted to their active form only at the site of action.
• Saliva as such does not contain any protein-digesting enzyme, but it can denature the uncooked natural proteins, like the ones present in raw egg, unboiled milk, or uncooked germinating seeds.
• The action of gastric juice: The gastric glands of the stomach produce light-colored, thin, and transparent gastric juice.
• It contains water, hydrochloric acid (0.3%), and inactivated enzymes, prorennin and pepsinogen.
• The presence of MCI makes the medium highly acidic (pH = 1 or 2) so that pepsin can act on proteins to convert them into peptones and proteoses. However, there is no pepsin in invertebrates.
• Both prorennin and pepsinogen are converted to their active forms in the presence of HCl.
• Pepsin and rennin can also perform the same function once they are formed. HCl helps to kill bacteria and other harmful organisms that may be present along with the food.
• Rennin acts on the casein protein of milk and converts it into paracasein which in the presence of calcium ions forms calcium paracaseinate (curdling of milk).
• Adult humans do not produce renin.
• The function of renin is then taken over by pepsin and other milk-coagulating enzymes in them. These reactions are summarized below:

• Pepsin can even digest collagens of connective tissue fibers, but it cannot act on the keratins of horns, hair, skin, or nails.
• The action of pancreatic and intestinal juice: Both pancreatic juice and intestinal juice (succus entericus) are poured into the small intestine.
• Pancreatic juice contains trypsinogen, chymotrypsinogen, procarboxypeptidases, lipases, amylases or amylopsin, DNAases, and RNAases.
• All these enzymes of pancreatic juice can act only in the alkaline medium.
• This change in the medium of food, from acidic to alkaline, is done by the bile juice. Therefore, bile juice acts on the food before the action of pancreatic juice. All these actions are given below:

• In predatory animals, trypsin can hydrolyze fibrinogen of blood into fibrin leading to blood coagulation. However, it is unable to bring about coagulation of milk. Also, trypsin cannot hydrolyze keratins.
• Chymotrypsinogen (inactive) is activated to chymotrypsin by trypsin itself. Chymotrypsin is another important milk-coagulating enzyme and can hydrolyze casein into paracasein which then coagulates to form calcium paracaseinate. However, unlike renin, it acts in an alkaline medium. Chymotrypsin can act on other proteins also.
• Carboxypeptidase hydrolyzes the terminal carboxyl group from peptide bonds to release the last amino acid from the peptides thus making the peptide shorter.
• Intestinal juice or succus entericus contains two protein-digesting enzymes aminopeptidases and carboxypeptidases which hydrolyze the terminal amino group from peptide bonds to release the last amino acid from the peptides thus making the peptide chain shorter.
• Dipeptidase acts on dipeptides to release individual amino acids.
• Enterokinase is also released which activates trypsinogen to trypsin.

Digestion Of Fats

• Fat digestion starts only when the food reaches the small intestine.
• It starts with the action of bile juice from the liver.
• Bile juice contains bile salts that break down the bigger molecules of fat globules into smaller droplets by reducing the surface tension of fat droplets. This process is known as emulsification of fats.
• Lipase is the chief enzyme that acts on emulsified fats.
• It is present both in pancreatic juice and intestinal juice.
• Pancreatic lipase (steapsin) is the strongest lipase.
• Lipase converts emulsified fats into diglycerides and monoglycerides releasing fatty acids at each step.
• At the end of digestion, all fats are converted into fatty acids, glycerol, and monoglycerides.

Digestion Of Nucleic Acids

• Nucleic acids are digested in the small intestine with the help of pancreatic and intestinal juices.
• Pancreatic juice contains two nucleases: DNAase and RNAase. Intestinal juice contains nucleotidase and nucleosidase.

NEET Biology Notes Digestion And Absorption

Almost no absorption takes place in the mouth and esophagus. Water, alcohol, simple salts, and glucose are absorbed in the stomach. In the small intestine, absorption of all digested materials takes place by active, passive, and facilitated transport.

• Glucose, sodium, and amino acids are absorbed actively.
• The absorption of glucose or amino acids involves carrier-mediated transport which binds glucose/amino acid at one site and Na⁺ at the other site. Therefore, the movement of glucose/amino acid is coupled to the concentration gradient of Na⁺ (Co transport).
• Na⁺ moves along the concentration gradient while glucose/amino acids are moving against a concentration gradient.
• The rate of absorption of galactose is the highest.
• Fructose is absorbed by facilitated diffusion.
• The products of fat digestion, monoglycerides, fatty acids, and glycerol are first incorporated into water-soluble droplets called micelles (a combination of fatty acids, monoacylglycerols, and bile salts); reconstructed to triglycerides in the absorptive cells; and released into lymph in the form of protein-coated water-soluble fat droplets called chylomicrons.
• In the large intestine, only water is absorbed. Absorption of vitamin B₁₂ (cobalamine) in man requires a glycoprotein, called intrinsic factor (IF) secreted by the parietal cells of the stomach. Failure to absorb cobalamine causes pernicious anemia associated with a failure of RBC maturation (megaloblastic anemia) and neurological abnormalities.

NEET Biology Notes Digestion And Absorption Assimilation Of Food

The absorbed food materials are transported by blood and lymph. Lymph is finally transferred to the blood circulation. The blood transports absorbed food materials to different body cells where food materials become integral components of the living protoplasm and are used for energy, growth, and repair. This is called the assimilation of food.

• Amino acids are not stored but are taken up by the cells in connection with the synthesis of proteins.
• Proteins are used for growth, repair, etc. Excess amino acids can be converted into glucose and then to fat and are thus stored. This is an irreversible reaction.
• Amino acids can also be converted to glucose and used as fuel for the cell.
• During their conversion to glucose, the amino acids are deaminated (removal of amino groups —NH₂).
• The liver is the chief site for deamination, i.e., a process by which the amino group is removed from amino acids resulting in the production of ammonia.
• Ammonia is soon converted into urea, which is filtered from the blood in the kidney.
• The excess of monosaccharides (glucose, fructose, and galactose) is usually stored in the liver and muscle cells in the form of glycogen (glycogenesis).
• Whenever there is a deficiency of glucose in the blood, glycogen is converted into glucose (glycogenolysis).
• Muscle glycogen is utilized during muscle contraction. Glucose is utilized in the production of energy for various body activities.
• A considerable amount of glucose is converted into fat and stored as such.
• The fat is stored in the fat deposits of the body, such as subcutaneous layers, mesenteries, etc.
• The stored fat is a readily available source of fuel for the cells.
• Fat has important insulating properties in connection with the conservation of heat and maintenance of body temperature.
• Fat also plays a protective role as filling or packing material, between and around the organs.
• In the liver, phospholipids are formed which are returned to the blood, to be used by all the cells. In liver cells, fats are converted into amino acids and carbohydrates.
• Vitamins, salts, and water are also useful for various metabolic processes.

NEET Biology Notes Digestion And Absorption Egestion

Peristalsis gradually pushes the slurry of indigestible materials of the small intestine into the large intestine or colon. Approximately, 1500 mL of chyme normally passes through the large intestine each day. The colon absorbs most of the water, electrolytes, and ions from these contents. This is accomplished by the active pumping of sodium and water by osmosis from die chyme.

• The other function of the colon is to help in the excretion of excess salts from the blood.
• The population of Escherichia coli (bacterium), which is a resident species of the colon, lives on this undigested matter. This bacterium, in turn, produces vitamin B₁₂, vitamin K, thiamin, and riboflavin which are absorbed by the wall of the colon.
• Later on, the chyme is slowly solidified into coherent feces, which are about three-fourths water and one-fourth solid matter consisting of about 10-20% inorganic substance 30% dead bacteria, 10-20% fat, 2-3% protein, and 30% undigested roughage and dry constituents of digestive juices.
• Feces are given out through the anus by the process of defecation or egestion.
• The breakdown of bile pigments occurs, forming stercobilin pigment, which provides a brownish color to the feces.
• The foul smell of the fecal matter is due to skatole (3-methyl indole).
• Dark green mucilaginous material in the intestine of the full-term fetus is called meconium (includes residue from the swallowed amniotic fluid by the fetus and the residues of excretory products from intestinal mucosa and glands).

NEET Biology Notes Digestion And Absorption Cellulose Digestion In Ruminants

The minants ingest large quantities of fodder-rich food in cellulose or lignin. Neither substance can be digested directly by mammals, but the cellulose is attacked in the rumen by the enzymes produced by symbiotic bacteria. Sheep and cattle initially swallow food without masticating. In the rumen, food is mixed with water.

• The rumen has millions of bacteria of several species, some of which attack the cellulose walls of the fodder liberating the cell contents of the food.
• These bacteria also break down carbohydrates and proteins into simple substances.
• In the rumen, the main products of fermentation are acetic, propionic, butyric, and other acids, along with small quantities of other substances such as ethanol.
• Besides this, the symbiotic organisms in the rumen synthesize much more riboflavin and pantothenic acid than is normally procured through diet.
• In addition to bacteria, a number of protozoan ciliates have also been observed in the rumen. Some of these can break down cellulose.
• The stored carbohydrates and proteins within these ciliates are believed to serve as reserves for some time when fodder is not available.
• Cellulose breakdown by bacteria also takes place in the stomach of kangaroo (Setonix brachytirus), and sloth (Choloepus). In kangaroos, the stomach is larger, and in the sloths, there is a compound stomach.
• Cellulose-splitting bacteria are present mostly in the cecum and ventral colon of horses and in the cecum of rabbits. The rabbits, like other lagomorphs and many rodents, have developed a habit of ingestion or pseu domination.
• When a rabbit eats fresh food, it directly reaches the cecum, remains there for one or two days undergoing fermentation, and is then expelled as soft feces.
• These are then again eaten and this time they reach the cardiac stomach, but the freshly eaten food goes straight into the cecum as usual.
• After digestion and absorption, the twice-swallowed food is excreted as hard fecal pellets without letting it in through the cecum.

NEET Biology Notes Digestion And Absorption Nutritional Requirements Of Humans

Carbohydrates are used primarily as sources of chemical energy, to be either metabolized immediately as glucose or stored as glycogen. The synthesis of glycogen is called glycogenesis. The liver can store enough glycogen to maintain blood glucose levels for several hours. Under acute starved conditions, the liver cells begin to convert amino acids and glycerol (digestive products of fat molecules) into glucose. Such production of new glucose is known as gluconeogenesis.

• Proteins are used as structural components of tissues as channels, transporters, regulatory molecules, and enzymes.
• Proteins can also be utilized as energy sources when broken down into amino acids.
• Out of the 22 amino acids identified so far as the constituents of proteins, eight (10 in children) cannot be synthesized in the human body.
• These must be provided in the diet from outside and are designated as essential amino acids.
• Lipid (fat) molecules are especially suitable as concentrated energy reserves. The fat cells of adipose tissues are often called the fat depot of the body.
• Triglycerides are used as fuel. The human body is able to synthesize most of the lipids in enough quantity, except three polyunsaturated fats, such as linoleic, linolenic, and arachidic acids.
• These essential fatty acids must be provided to the human body through diets.

NEET Biology Notes Digestion And Absorption Calorific Value

The amount of heat liberated from the complete combination of 1g of food in a bomb calorimeter (a dosed metal chamber filled with O₂) is its gross calorific value GCV) or gross energy value. The actual amount of energy liberated in the human body due to the combustion of 1g of food is the physiologic calorific value (PCV) of food.

• The energy requirements of animals and the energy content of food are expressed in terms of a measure of heat energy because heat is the ultimate form of all energies. This is often referred to as calorie (cal) or joule (J). which is the amount of heat energy required to raise the temperature of 1g of water through 1°C.
• Since this value is a tiny amount of energy, physiologists commonly use kilocalories (kcal) as a unit of measure (1 keal = 1000 eal) or kilojoule (kJ). One kilocalorie is the amount of energy required to raise the temperature of 1 kg of water through 1°C.
• Nutritionists traditionally refer to kcal as Calorie or Joule (always capitalized).

NEET Biology Notes Digestion And Absorption Minerals And Vitamins

Both minerals and vitamins occur as small molecules and mostly do not require digestion. Minerals are ingested as salts dissolved in water or as part of organic compounds (food). Still, a few of the minerals are absorbed with the aid of digestive juices (like bile) and gastric juices. Of the 21 essential minerals required by man, some are important for maintaining fluid balance, whereas others help to regulate metabolism by acting as a component of enzymes.

• Vitamins are essential for normal metabolism, growth, and sound health.
• Humans can synthesize vitamin A (retinol) with the help of plant pigment, carotene, which is available in yellow and green leafy vegetables.
• Vitamin A forms the retinal pigment of human eyes such as rhodopsin of rod cells and iodopsin of core cells.
• Humans can also synthesize vitamin D (calciferol) in their skin in the presence of ultraviolet rays of light. Although most animals can synthesize vitamin C from glucose, humans cannot; hence, they require it in their diet. Vitamin Bl7 is a newly discovered vitamin with anticancerous properties.
• Chemical nature: Vitamins differ greatly in their chemical nature. Generally, they have nothing in common. Some vitamins are alcohol, some arc proteins, some arc quinone, and some are sterol.
• Provitamins: These are compounds that are changed into vitamins in our body. Examples arc:
• Ergosterol present in food is changed in the skin into vitamin D in the presence of sunlight.
• Carotene, a pigment present in carrots is changed in the liver and in the small intestine into vitamin A.
• Vitamers: This term is used for different forms of the same vitamin. Vitamers are also called isotopic vitamins. Examples are:
  • Vitamin A has vitamins A₁ and A₂ (vitamins).
  • Vitamin D, has vitamin C, D₂, and D₃ (vitamins).
• Vitamin poisons: These are compounds that replace vitamins in our body, for example, antibiotics. Sulfa drugs replace vitamin C; tetracycline or ampicillin replace vitamin B complex.

NEET Biology Notes Digestion And Absorption Nutritional Deficiencies And Disorders

Deficiencies of nutrients such as vitamins, minerals, and proteins, in the food are related to specific disorders, diseases, and abnormalities in humans. Impairment of health due to improper intake of food or nutrients results in the effect recognized as malnutrition. Malnutrition is a term that covers problems of both undernutrition and overnutrition. Two types of diseases due to inadequate nutrition are described.

Differences between kwashiorkor and marasmus:

• An individual or a group of individuals may be undernourished due to non-availability of food, and hence, deficiency of minimum required food and nutrients.
• In this situation of undernutrition, the intake of food is too insufficient to meet the needs for metabolic energy. Consequently, the individual shall have to make up the shortfall by metabolizing some molecules of its own body.
• Excess intake of food and nutrients may cause a great deal of harm to the body. The excess nutrients are stored as increased body mass. Such a situation is attributed to overnutrition. Excess intake of saturated fats such as butter, ghee, vegetable oils, red meat, eggs, etc., often leads to hypercholesterolemia, a condition in which blood cholesterol content becomes abnormally high, ultimately leading to cardiac disorder.
• Deposition of cholesterol on the walls of blood vessels stiffens the blood vessels and increases blood pressure. Besides, excessive intake of calories (sugar, honey, ghee, etc.) may produce overweight and obesity (excessive accumulation of fat in tissues), which is the most common form of overnutrition.
• Very high intakes of minerals and fat-soluble vitamins (obtained from food sources alone) can be toxic. This is because they are stored in the body. With the exception of folic acid (women of childbearing age), people who have well-balanced diets that supply enough energy do not usually need to take dietary supplements.
• But if they do decide to take supplements, then they should follow the advice on the label to reduce the risk of an overdose.

NEET Biology Notes Digestion And Absorption Disorders Of The Digestive System

• Inflammation Of The Intestinal Tract: The most common bacterial viral infection may be caused by intestinal parasites such as tapeworms, roundworms, threadworms, hookworms, and pinworms.
• Jaundice: The Liver is affected, and skin and eyes turn yellow due to the deposition of bile pigments.
• Vomiting: This reflex action is controlled by the vomiting center in the medulla. A feeling of nausea precedes vomiting.
• Diarrhea: It reduces the absorption of food, due to abnormal frequency of bowel movement.
• Constipation: Due to irregular bowel movement, feces are retained within the rectum.
• Indigestion: Feeling of fullness as the food is improperly digested. It is due to inadequate enzyme secretion, anxiety, food poisoning, overeating, and spicy food.
• Belching: It occurs usually when the stomach is over-dilated, air rises, and is expelled through the mouth producing a burping sound.
• Obesity: It is a condition when fat storage in the body increases, leading to its abnormal deposition in the subcutaneous layer. It is caused when energy input exceeds energy output in the body. 8.3 calories of energy from surplus food increases 1g of fat deposition in the body causing obesity.
• Flatus: It is an accumulation of gases in the gastrointestinal tract that are expelled through the anus producing a characteristic sound.
• Hepatitis: It is a condition of inflammation of the liver caused by infection of bacteria, viruses, or protozoa. It may cause cirrhosis.
• Colitis: It is a disorder in which inflammation of the colon and rectum, loose motions, bloody feces, and dehydration occur. It is caused by the infection of protozoans such as Entamoeba.
• Appendicitis: The wall of the vermiform appendix ruptures and bacteria are released in the coelomic cavity. Severe infection may cause death. By surgery, the appendix is removed.
• Hernia: It is a protrusion of the intestine into the inguinal canal and may extend into the scrotal sac.
• Mumps: It is a viral infection in parotid salivary glands. The throat region swells and fever develops.
• Nausea: It refers to the discomfort which leads to vomiting. It may be caused by distension of the stomach or of the gastrointestinal tract.
• Vomiting: It is a condition developed by reverse peristalsis caused by harmful substances in the stomach irritation in the pharynx or disturbance in semicircular canals. Food substance comes out through the mouth. Vomiting center is located in the medulla oblongata.
• Tonsillitis: The pharyngeal tonsils (lymphoid tissues) become infected with microorganisms. It causes pain in the throat followed by fever.

Summary Of Digestion:

Gastrointestinal Hormones In Mammals:

Summary Of Chemical Digestion Of Food:

Summary Of Human Vitamins:

NEET Biology Notes Digestion And Absorption Points To Remember

1. Assimilation: Utilization of absorbed material by the cell.
2. The hunger center is in the hypothalamus.
3. The Satiety center is also in the hypothalamus.
4. Heartburn has nothing to do with the heart. It is caused by the regurgitation of acid from the stomach into the esophagus.
5. Splanchnology is the study of the viscera.
6. NIN: National Institute of Nutrition, Hyderabad.
7. Anorexia: Loss of appetite.
8. Spoilt hay of sweet clover Melilotus indica (fodder and green manure) contains a substance called dicumarol. Dicumarol prevents the action of vitamin K as it is antagonistic to it.
9. What destroys the vitamins? Overcooking and excessive boiling, medicines such as aspirin, antacids, and diuretics lead to iron deficiency—anemia.
10. Tea/coffee inhibits the absorption of iron from the diet. Prolonged consumption of tea or coffee after meals can lead to iron deficiency—anemia.
11. In the upper one-third of the esophagus, only skeletal muscles are found.
12. The chief seat of water absorption is the small intestine.
13. The liver produces proteins such as albumin, fibrinogen, and prothrombin, but does not produce globulin.
14. Poison glands of a snake are modified labial glands, homologous to parotid salivary glands.
15. Vomerine teeth of frogs kill the prey.
16. The tongue of a whale is not movable.
17. The gall bladder is absent in adult lamprey (jawless vertebrate), grain-eating birds, rats, whales, all the perissodactyla (odd-toed hoofed mammals such as horses), and some artiodactyla (even-toed hoofed mammals).
18. Alcoholics are short of vitamin C.
19. C-shaped duodenum is a characteristic of man.
20. During a high fever, one does not feel like taking meals because a high temperature shuts off the appetite center.
21. Bile is alkaline in humans, but acidic in cats and dogs.
22. Basal metabolic rate (BMR) is the minimum energy requirement for the maintenance of the body during rest or sleep. For a normal human adult, it is 1600 keal/day.
23. Routine metabolic Rate (RMR): It is the energy requirement of a moderately active person. RMR is 2800 keal/day for adult males and 2200 keal/day for adult females.
24. Entero-hepatic circulation: Of the total bile salts that enter the duodenum. 90-95% are reabsorbed actively from the terminal ileum in the portal vein and return to the liver, to be excreted again This is enterohepatic circulation. Approximately 93% of the cellular material is composed of C, H, and 0, and 2% is composed of N, P, Cl and S, I, F, B, are present in traces.
25. Choloretics are the substances that increase bile secretion from the liver for example, bile salts.
26. Cholagogues are the substances that cause the contraction of the gall bladder.
27. Achalasia cardia condition is characterized by failure of the cardiac sphincter to relax completely on swallowing, causing food accumulation in the esophagus and proximal esophagus to dilate.
28. Achloroydria means a lack of HCl secretion in the stomach. The capacity of the human stomach is 1.5-1.7 L.
29. Elephant tusks are modified incisor teeth.
30. Tusks of walrus are modified canines.
31. The teeth of sloths and armadillos have no enamel.
32. Spiny anteaters, scaly ant eaters, and some whales are toothless.

Comparative Study Between A Rabbit And A Human:

NEET Biology Notes Digestion And Absorption Assertion Reasoning Question And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: Gastrectomy causes iron deficiency anemia.

Reason: Hydrochloric acid secreted by oxyntic cells converts ferric into ferrous and iron is absorbed as ferrous ions.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of tire Assertion.

Question 2. Assertion: Cholagogues are substances that cause the contraction of the gall bladder.

Reason: These substances cause a release of CCK-PZ from the duodenum

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of tire Assertion.

Question 3. Assertion: Aptyalism patients have higher than normal incidences of dental caries.

Reason: Aptyalism is caused by the action of the parasympathetic nervous system.

Answer: 3. If Assertion is true, but Reason is false.

Question 4. Assertion: In humans, the duct of Wirsung from the pancreas combines with the bile duct before opening into the duodenum.

Reason: Blockage in the duct of Wirsung will hamper the endocrine function of the pancreas.

Answer: 3. If Assertion is true, but Reason is false.

Question 5. Assertion: In acute constipation, purgatives containing magnesium salts are generally used.

Reason: The osmotic effect of Mg²⁺ in the intestinal lumen prevents water reabsorption from the intestine. Mg²⁺ increases the solute concentration in the intestinal lumen because Mg²⁺ is absorbed very slowly.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of tire Assertion.

NEET Biology Notes Anatomy Of Flowering Plants Introduction

The branch of botany dealing with the internal organization of plants is called anatomy. N. Grew laid the foundation of plant anatomy. He coined the term tissue. He is known as the father of anatomy.

NEET Biology Notes Anatomy Of Flowering Plants Tissues

A group of similar or dissimilar cells that perform a common function and have a common origin is called tissue. Tissues are classified into two main groups: meristematic and permanent.

NEET Biology Notes Anatomy Of Flowering Plants Meristematic Tissues

1. Mesistematic tissues consist of cells that retain the power of division.
2. The protoplasm within the cell is dense, the vacuole is smaller or absent.
3. These cells are isodiametric without intercellular spaces.
4. The nucleus is bigger in size.
5. These cells have thin cellulosic cell walls.
6. These are metabolically active cells with high surface area per unit volume and nucleocytoplasmic ratio.
7. Ergastic substances are absent.
8. Colorless proplastids are present in the cells.

Classification Of Meristems

On The Basis Of Origin And Development

1. Promeristems (Primordial Meristem): Promeristerms are a group of cells that represent the primary stages of meristematic cells. They are found at the apices of embryonic shoots and roots. They give rise to primary meristems.
2. Primary Meristems: They originate from pro meristems. They are found at shoot and root apices, at the apex of leaves, and in intercalary parts. They give rise to primary permanent tissues.
3. Secondary Meristems: They are not present from the beginning of the formation of an organ but develop at a later stage. They give rise to secondary permanent tissues. They develop from primary permanent tissues, for example, interfascicular cambium, cork cambium, and cambium in dicot roots.

On The Basis Of Position

1. Apical Meristem: These cells or tissues are found at the apices of the stem and root. Due to continuous division, the root and stem increase in length. The apical meristem helps the plants to grow in length.
2. Intercalary Meristem: The tissues are intercalated between permanent tissues. These are actually the parts of the apical meristem that get separated from it during the growth of the stem and root in length. The most characteristic example is the stem of grasses and Equisetum. Intercalary meristem is especially responsible for the increase in the length of the stems of grasses.
3. Lateral Meristem: These meristems are present along the lateral side of the stem and root. They divide in a tangential plane, giving rise to secondary permanent tissues on the inner and outer sides and leading to the increase in thickness or girth of the plant body, for example, intrafascicular cambium, inter¬fascicular cambium, and cork cambium.

Read and Learn More NEET Biology Notes

On The Basis Of Plane Of Cell Division

1. Mass Meristem: The cells divide anticlinally in all planes so that a mass of cells is formed. For example, the formation of spores, cortex, pith, endosperm, etc.
2. Plate Meristem: The cells divide anticlinally in two planes, so the plate-like area is increased. For example, the formation of epidemics and lamina of leaves.
3. Rib Or File Meristem: The cells divide anticlinally in one plane, so a row or column of cells is formed. For example, the formation of lateral roots.

On The Basis Of Function

1. Protoderm: They are the outermost meristematic cells. They form the skin or epidermis of the plant and epidermal tissue system.
2. Procambium: They are the innermost meristematic cells. They form the primary xylem, primary phloem, and cambium.
3. Ground Meristem: They form ground or funda¬mental tissues such as hypodermis, cortex, pith, pericycle, etc.

Shoot Apex Organization: Shoot apex is present immediately above the youngest leaf primordia. It consists of meristematic cells. The lateral branches of the stem and leaves are formed by the activity of shoot apex. Many theories have been put forward to explain shoot apex such as

Apical Cell Theory: It was proposed by Hofmeister and Nageli. According to this theory, a single apical cell leads to the development of the entire plant body. This theory is applicable to algae, as well as to most of the bryophytes and pteridophytes.

Histogen Theory: It was proposed by Hanstein. According to this theory, a shoot apex consists of the following halogens:

1. Dermatogen: The outermost layer, it forms the epidermis (skin) and epidermal tissue system.
2. Periblem: It gives rise to the tissues between the epidermis and the stele.
3. Plerome: It is the innermost layer and the central mass of cells that give rise to the central stele.

Tunica Corpus Theory: It was proposed by Schmidt (1924). It is based on the plane of division of the cells. According to this theory, the shoot apex consists of two distinct layers:

1. Tunica: It is mostly single-layered and forms epider¬mis. The cells of tunica are smaller than the corpus and divide mostly by anticlinal divisions.
2. Corpus: It represents the central core with larger cells. The cells divide periclinally. Sometimes tunica is multi-layered, only the outer layer forms epidermis, and the remaining layers with corpus form the cortex of the shoot.

” anatomy of flowering plants”

Root Apex Organization: The Root apex consists of a mass of meristematic cells. It is not responsible for the formation of lateral roots. Root cap or calyptrogen is present due to which the root meristem becomes subterminal in position. If the root cap is independent in origin, it arises from dermatogen. Regarding the organization of root apex, the following theories have been put forward

Korper-Kappe Theory: It was proposed b.y Schuepp (1917). This theory is comparable with the tunica corpus theory of shoot apex. Korper means body and kappe means cap.

Quiescent Center Theory: It was proposed by Clowes (1956-5 8). According to this theory, the root apex consists of an inverted cup-like structure, the quiescent center. The cells of this region are with very low mitotic activity (quiescent). They have low amounts of RNA, DNA, and protein. They are surrounded by a layer of actively dividing cells which are responsible for the formation of the different structures of roots.

NEET Biology Notes Anatomy Of Flowering Plants Permanent Tissue

Permanent tissues are composed of living or dead cells that are derived from the meristematic tissues but have lost their ability to divide. There are of three types: simple tissues, complex tissues, and secretory tissues.

Simple Tissues: They are made up of a single kind of cells performing similar functions. Simple tissues are mainly of three types: parenchyma, collenchyma, and sclerenchyma.

Parenchyma (Grew): These cells are found almost in all parts of plants such as roots, stems, leaves, fruits, and seeds. These cells are isodiametric, spherical, oval, or polygonal with intercellular spaces. These cells are living and with thin cellulosic cell wall.

Types Of Parenchyma

• Prosenchyma: Elongated parenchyma with tapering ends is called prosenchyma.
• Aerenchyma: The parenchyma which encloses the air cavity is called aerenchyma (hydrophytes)
• Chlorenchyma: The parenchyma containing chloroplasts is called chlorenchyma.
• Idioblast: Sometimes parenchyma stores secretory substances (orgastic substances) such as tannins, resins, and gum. They are called idioblasts.

Storage Parenchyma: Examples are fruits, and endosperm.

Collenchyma (Schleiden): Collenchyma has thickenings on the cell wall and in corners of intercellular spaces. They are not found in the roots and monocots. These cells form hypodermis in the stem and petiole. These are living mechanical tissues with high refractive index. The thickening material in the cell wall contains pectocellulose.

Types Of Collenchyma

• Angular Collenchyma: Angular walls thickened, for example, the stem of a marigold and tomato.
• Lamellate Collenchyma: Tangential walls thickened, for example, the stem of a sunflower.
• Lacunate Collenchyma: Lacunate thickening, intercellular spaces are present, for example, stem of cucurbita.
• Functions Of Collenchyma: They provide mechanical support, flexibility, and elasticity to the organs. Due to the peripheral position in stems they resist the bending and pulling action of wind. Collenchyma is especially useful for young plants and herbaceous organs where these act as important supporting tissues.

Sclerenchyma (Mettenius): Sclerenchyma has thickened secondary walls due to the deposition of lignin. At maturity they become dead. These cells have simple pits. They are of two types: Sclereids and sclerenchymatous fibers.

Sclereids: They may be spherical, oval, or cylindrical. They are lignified, and extremely thick-walled. So the lumen of the cells is almost obliterated. They are found in hard parts of the plant.

Types Of Sclereids

1. Brachysclereids (Stone Cells): Grittiness in fruits is due to stone cells, for example, pear, and sapota.
2. Macrosclereids (Rod Cells): Found in the seed coat of leguminous plants.
3. Osteosclereids (Prop Cells): Found in the leaves and seed coat of many monocots and the sub-epidermis of legume seed coats.
4. Astrosclereids (Star Cells): They are common in the stem and leaves of dicots, for example, tea leaves, petiole of lotus, etc.
5. Trichosclereids (Internal Hair): Long, hair-like branched sclereids. They are common in hydrophytes. These are also present in the aerial roots of Monstera.

Sclei Enchymatous Fibers: They are long and tapering at ends. These are thick-walled cells (lignified). These fully developed fiber cells are always dead. The length of fiber varies from 72 to 550 mm in angiosperms and 1 to 12 mm in gymnosperms.

The fibers are present in the hypodermis of monocot stems, in the pericycle of many dicots, in the secondary wood, and in the vascular bundle sheath in monocot stems, for example, jute, flax, hemp, etc. Living fibers are found in tamarix.

Function Of Sclerenchyma: The main function of sclerenchyma is to provide mechanical strength.

Complex Tissues: A group of more than one type of cells having a common origin and working together as a unit is called complex tissue. These are made up of different types of cells: xylem and phloem.

Xylem (Nageli) Or Hadrome (Haberlandt): Xylem is the chief water-conducting element. It consists of the following types of cells

Tracheids: They are elongated cells with pointed chisel-like ends. Their wall is tough, thickened, lignified, and thickened or may be annular, spiral, reticulate, scalariform, or pitted. Cells are dead at maturity and have bordered pits.

In pteridophytes and gymnosperms, wood mainly consists of tracheids (no vessels). In angiosperms, tracheids are associated with vessels. The main function of tracheids is the conduction of water. Tracheids are the most primitive type of conducting elements in the xylem.

Vessels: Vessels are also elongated and tube-like. They are formed from a row of cells placed end to end. The partition walls are either perforated or disappear altogether resulting in an elongated tube. Walls are thickened, and lignified, and may have annular, spiral, reticulate, or scalariform thickening.

Vessels are dead at maturity and without nuclei. In pteridophytes and gymnosperms, vessels are absent (non-porous wood). Sometimes primitive vessels are present in gnetum and ephedra (Gnetales).

Vessels are characteristic of angiosperms (porous wood). Vesselless angiosperm families are Tetracentraceae, Trochodendraceae, and Winteraceae. The main function is the conduction of water. Vessels are an advanced type of conducting element.

On the basis of distribution and size of vessels, porous wood is of two types:

1. Diffuse Porous Wood (Primitive): Vessels of the same size are uniformly distributed throughout the growth, for example, Pyrus, Betula.
2. Ring Porbus Wood (Advanced): Large vessels are formed in early wood when the need of water is great and small vessels are formed in latewood, for example, Quercus, and morus.

Wood Or Xylem Fiber: These cells are elongated and pointed at both ends. Cell wall is highly lignified having simple pits. The air commonly found in the secondary xylem. They may lie

1. Fiber Tracheids: Fiber such ns tracheids with bordered pits.
2. Libriform Fiber: They have extremely thick walls and simple pits. They provide mechanical support.

Wood Or Xylem Parenchyma: They are living parenchymatous cells associated with xylem. They may occur as axial parenchyma or ray parenchyma. When parenchyma is diffused or not associated with vessels, they are called as apotracheal parenchyma and when parenchyma surrounds or is associated with vessels, it is called para tracheal parenchyma.

On The Basis Of Origin Xylem Is Of Two Types:

1. Primary Xylem: It is derived from procambium during the formation of the primary plant body. It is differentiated into protoxylem (formed first and consists of tracheary elements and xylem parenchyma) and metaxylem (formed later and consists of tracheary elements, xylem parenchyma, and fiber). The cells of metaxylem are bigger in size than the protoxylem.
2. Secondary Xylem: It is formed from cambium during secondary growth. It is well differentiated into two systems:
   • Axial Or Vertical System
     • Tracked elements (tracheids and vessels): For conduction of water
     • Xylem or wood fiber: For support
     • Xylem parenchyma: For storage of food
   • Ray Or Horizontal System: Ray parenchyma: For storage of food.

Phloem (Nageli) Or Bast Or Leptome (Haberlandt): The main function of phloem is the transport organic food materials from leaves to stems and roots in down word direction. Phloem consists of the following types of cells

Sieve Element: The sieve elements in angiosperms are sieve tubes which are cylindrical tube like cells with perforated cross walls called sieve plates. Sieve tubes are associated with companion cells, and they are without nuclei. In pteridophytes and gymnosperms, the sieve elements have sieve plates on their lateral walls and companion cells are absent. They are called as sieve cells.

The walls of sieve tube elements are made up of cellulose and pectic substances. The cytoplasm is confined to a thin peripheral layer. P-proteins are proteinaceous structures present in sieve tubes and are believed to be responsible for

1. The movement of materials and
2. The sealing of pores after wounding.

In old sieve tubes, at the end of the growing season, a callose plug (made of callose carbohydrate) is deposited on the sieve plate which inhibits the activity of the sieve tubes. In the spring season, the callose plug gets dissolved.

Companion Cells: They are elongated living parenchymatous thin-walled cells. They are associated laterally to sieve tubes and have dense cytoplasm and nuclei. Companion cells are absent in pteridophytes and gymnosperms. Both sieve tubes and companion cells are related ontogenetically because both develop from the same mother cell. So these are sister cells.

Phloem Or Bast Fiber: They are absent or fewer in phloem and abundantly found in secondary phloem. They are sclerenchymatous fibers associated with phloem. Phloem fibers of plants such as jute, flax, and hemp are rotted in water and extracted for making ropes and coarse textiles.

Phloem Parenchyma: They are parenchymatous living cells with cellulosic cell walls and nuclei. The main function is the storage of food. They are not found in monocotyledonous plants.

Types Of Phloem

On The Basis Of Position

1. External Phloem: It is of normal type and is present outside the xylem, for example, mostly angiosperms and gymnosperms.
2. Internal Or Intraxylary Phloem: It originates from procambium and is the primary phloem which occurs on the innerside of primary xylem in bilateral bundles, for example, members of Apocynaceae, Asclepiadaceae, Convolvtilaceae, Solanaceae etc.
3. Included Or Interxylary Phloem: It originates from cambium and is a secondary phloem that occurs in groups within the secondary xylem, for example, Leptadaenia, Salvadora, Chenopodiwn, Boerhaavia, Amaranthus, etc.

On The Basis Of Origin:

1. Primary Phloem: It develops from procambium and does not have radial differentiation or rays are absent. It is differentiated into protophloem (consists of sieve elements and parenchyma) and metaphloem (develops after protophloem and consists of sieve elements, parenchyma, and fiber). During primary growth, the protophloem elements are crushed by the surrounding tissues and disappear. This process is known as obliteration.
2. Secondary Phloem: It develops from cambium during secondary growth. It shows radial differentiation. It consists of two distinct systems such as
   • Axial or vertical system
     • Sieve Elements: Sieve tube and companion cells. For conduction of food.
     • Bast fiber: For support.
     • Bast Parenchyma: For storage of blood.
   • Ray Or Horizontal System
     • Ray Parenchyma: It is for the storage of food

Secretory Tissues: Secretory tissues perform special functions in plants. For example, secretion of resins, gums, oil, and later. These tissues are of two types laticiferous and glandular tissues.

Laticiferous Tissues: They contain colorless, milky, or yellow-colored juice called latex. These tissues are of two types

1. Latex Cells: They do not fuse and do not form a network. Plants having such tissues are called simple or non-articulated laticifers, for example, Calotropis (Asclep Fabaceae), Nerium, Vinca (Apocyanaceae), Euphorbia (Euphorbiaceae), Ficus (Moraceae).
2. Latex Vessels: They are formed due to the fusion of cells and form a network-like structure. Plants having such tissues are called compound or articulated laticifers, for example, Argemone, Pabaver (Papaveraccae), Sonchus (Compositae), Hevea, and Manihot (Euphorbiaceae).

” class 11 biology anatomy of flowering plants notes “

Glandular Tissues: They include different types of glands which secrete oils, gums, mucilage, tannins, and resins. They may be

1. External Glands: Present as epidermal outgrowths. They are of many types
   • Glandular Hair: With a stalk and head, for example, tobacoo, Plumbago, Boerhaavia.
   • Stinging Hair: Secrete poisonous substances, for example, Urtica.
   • Nectaries: Secrete sugary substance; maybe extra-floral present on the stem, leaves, etc. Examples: Nepenthes, Catheranthus, or floral, for example, Corchorus, Thea, Polygonum, Jatropha.
   • Digestive Glands: Present in insectivorous plants, for example, Drosera, Nepenthes, etc.
2. Internal Glands
   • Oil Glands: Present in the mesophyll of leaves and cortex of stem fruit, for example, orange, lemon, etc.
   • Mucilage-Secreting Glands: Leaves of piper betel.
   • Gum, tannin, and resin-secreting glands or ducts are present in gymnosperms and angiosperms, for example, Pious resin ducts are schizogenous in origin.

NEET Biology Notes Anatomy Of Flowering Plants Tissue System

The various types of tissues present in the body of a plant perform different functions. Several tissues may collectively perform the same function. A collection of tissues performing the same general function is known as a “tissue system.” According to Sachs (1975), there are three major tissue systems in plants, namely the epidermal tissue system, the ground or fundamental tissue system, and the vascular tissue system.

Epidermal Tissue System: It consists of the epidermis and its associated structure such as hairs, trichomes cuticle, stomata, and bulliform cells. Mostly epidermis is single-layered parenchymatous but multilayered in Ficus, Nerium. The epidermis is mainly protective in nature.

In grasses, motor or bulliform cells are present in the upper epidermis. In grasses and Equisetum, silica is present in the epidermal cells. The epidermal cells containing cystoliths are called lithocysts.

Ground Or Fundamental Tissue System: It extends from the epidermis up to the center excluding vascular tissue. Ground tissues constitute the following parts

1. Cortex: It lies between the epidermis and the pericycle. It is further differentiated into the following parts:
   • Hypodermis: It is collenchymatous in the dicot stem and sclerenchymatous in the monocot stem. It provides strength.
   • General Cortex: It consists of parenchymatous cells. Its main function is the storage of food.
   • Endodermis (Starch Sheath): It is mostly single-layered and is made up of parenchymatous barrel-shaped compactly arranged cells. The inner and radial walls of endodermal cells have Casparian strips. In roots, thick-walled endodermal cells are interrupted by thin-walled cells just outside the protoxylem patches.
     • These thin-walled endodermal cells are called passage cells. The endodermis behaves as water water-tight dam to check the loss of water and air tight dam to check the entry of air in xylem elements.
2. Pericycle: It lies between endodermis and vascular tissue. It is parenchymatous in roots and sclerenchymatous or mixed with parenchyma in the stem. The pericycle cells just opposite the protoxylem are considered as seats for the origin of lateral roots. In dicot roots, the pericycle forms part of the cambium or the whole of the cork cambium.
3. Pith: It occupies the central part in the dicot stem and monocot root. It is mostly made up of parenchymatous cells. In the dicot root, the pith is completely obliterated by the metaxylem elements. In the dicot stem, the pith cells between the vascular bundles become radially elongated and are known as primary medullary rays or pith rays. They help in lateral translocation.

Vascular Tissue System: Vascular bundles found in the steer part constitute the vascular tissue system. Xylem. phloem and cambium are the major part of the vascular bundle. Vascular bundles may be of the following type:

1. Radial: When the xylem and phloem are arranged on different radii alternating with each other, for example, roots.
2. Conjoint: When xylem and phloem combine in the same bundles and are present on the same radius, for example, stem. Conjoint vascular bundles may be of the following types
   • Collateral: The xylem is towards the inner side and the phloem, is towards the outer side.
     • Open: Cambium is present between the xylem and phloem, for example, the dicot stem.
     • Closed: Cambium is absent between the xylem and phloem, for example, monocot stem.
   • Bicollateral: When the xylem has cambium and phloem on both sides, for example, members of Cucurbitaceae, Solanaceae, Apocyanaceae, etc.
3. Concentric: When one vascular tissue surrounds the other. They are of two types
   • Amphicribral Or Hadrocentric: The xylem is surrounded on all sides by phloem, for example, ferns.
   • Amphivasal Or Leptocentric: The phloem is surrounded on all sides by the xylem, for example, Yucca, and Dracaena.

NEET Biology Notes Anatomy Of Flowering Plants Internal Structures Of Dicot And Monocot Plants

Anatomy Of Root: The three zones that can be distinguished in a root are

1. Epidermis (Epiblema/Rhizodermis): It is single-layered (uniseriate) and consists of tightly placed, thin-walled unscrutinized cells. This epidermis layer is called as epiblema, piliferous layer, or rhizodermis. Epiblema in younger roots bears unicellular root hairs (water-absorbing organs).
2. Cortex: It consists of thin-walled parenchymatous cells with intercellular spaces. In most monocots and some dicots, the cortex layer below the epidermis becomes suberized to form protective tissue called exodermis.
   • The cells of the cortex store food material (for example, carrots). The innermost layer of the cortex develops into endodermis. It is made up of closely packed living cells characterized by the presence of band-like thickening; made of lignin and suberin on their radial and transverse walls.
   • These bands or strips are called Casparian bands or strips. Some cells of endodermis lying opposite to the protoxylem remain thin-walled and are called passage cells which allow radial diffusion of water.
3. Vascular Bundles: Vascular bundles are radial and exarch. The center of the monocot root is occupied by parenchymatous cells called piths.

Some basic differences between a monocot and a dicot root are given.

” plant anatomy ncert”

Differences Between Dicot And Monocot Root

Anatomy Of Stem: The Primary structure of the diet stem consists of the following layers

• Epidermis: It is the outermost layer consisting of a single layer of closely arranged cells with cuticles (cutinized). It bears multicellular hairs.
• Cortex: It is differentiated into hypodermis, general cortex, and endodermis. Hypodennis is collenchymatous. The general cortex bundles consist of the phloem and xylem.
• Vascular Bundles: Vascular bundles are conjoint, collateral, or collateral, open and endarch, and are arranged in a ring (eustele).
• Pith: It is the central portion of the stem consisting of parenchymatous cells with narrow, radially elongated parenchymatous cells extending from the pith toward the periphery called medullary rays. The main function of pith is food storage.

The primary structure of a monocot stem consists of the following layers:

• Epidermis: It is the outermost layer and consists of compactly arranged parenchymatous cells which are usually covered with cuticles.
• Hypodennis: The cells of hypodermis are sclerenchymatous providing mechanical strength to the stem.
• Ground tissue: All the tissues internal to hypodermis represent the ground tissue. It is made up of parenchymatous cells rich in food reserves such as starch.
• Vascular bundles: They lie scattered in the ground tissue. Each vascular bundle is surrounded by a two- or three-layered sclercnchymatous sheath called bundle sheath. The vascular bundles are conjoint, collateral, closed, and endarch (atactostelc). Vessels are arranged in a V-shaped manner. Schizolysigenous water cavity or canal arc present below protoxylem.

Shows the transverse section of the dicot stem and monocot stem. The differences between dicot stem and monocot stem are given.

Anatomy Of Leaf: In the cross-section of a dorsiventral leaf (dicot), the following parts can be made out

Epidermis: The upper and lower surfaces are covered by the epidermis. The cells of the epidermis are parenchymatous and are closely packed together without any intercellular spaces. Mostly the stomata are restricted to the lower surface of the leaf such leaf is called hypostomatic. The outer walls of the epidermal cells are thickened and cutinized (cuticle) which prevents the loss of water.

Mesophyll: Between the two epidermal layers, there are numerous chlorenchyma cells that constitute the mesophyll. In dicots, there are two distinct layers of mesophyll, the palisade (the upper layer consisting of closely arranged column-shaped cells containing abundant chloroplasts) and spongy tissue (the lower layer of irregularly shaped cells containing fewer chloroplasts).

Vascular Bundles: Vascular bundles in the leaf are located in the midrib and the veins. Vascular bundles are conjoint, collateral, and closed. Bundles are surrounded by a compact layer of parenchymatous cells which is called bundle sheath. The xylem (protoxylem) is towards the upper epidermis (adaxial) and the phloem is on the lower side (abaxial). Like the dicot leaf, an isobilateral leaf (monocot) can also be differentiated into the following types of tissues

NEET Biology Notes Anatomy Of Flowering Plants Secondary Growth

Secondary growth is the increase in girth thickness or diameter of the axis due to the formation of new tissues as a result of the joint activity of vascular cambium and cork cambium in stellar and extrasolar regions, respectively. It occurs in the roots and stems of gymnosperms and dicots. Secondary growth in the dicot stem is completed in the following steps:

Formation Of Vascular Cambium Ring

1. Intrafascicular Cambium: It is primary in origin, present in between the primary phloem and primary xylem.
2. Interfascicular Cambium: It is a true secondary meristem. It originates from the parenchyma cells of the medullary rays region. It lies in between the vascular bundles.
3. Vascular Cambium Ring: Both intrafascicular and interfascicular cambium join together and form cambium ring.

Cambium Cells Are Of Two Types:

1. Fusiform Initials: They form tracheids, vessels, fibers, and axial parenchyma in the secondary xylem and sieve tubes, companion cells, fibers, and axial parenchyma in the secondary phloem.
2. Ray Initials: These are isodiametric and form ray parenchyma and vascular rays.
3. The Periclinal division of the vascular cambium ring helps in the formation of secondary phloem tout side the vascular cambium) and secondary xylem firmer to vascular cambium). The amount of secondary xylem produced is H 10 times greater than secondary phloem.

Fate Of Primary Phloem And Primary Xylem: The primary phloem is crushed to death, known as obliteration. The primary xylem, being dead and lignified, is replaced in the pith region.

Formation Of Secondary Structures

1. Annual Rings: These are formed by the seasonal activity of vascular cambium. Cambium does not stay active uniformly throughout the year. In spring or summer, cambium is more active and forms large-sized xylem elements (vessels) which constitute spring or early wood.
   • In autumn or winter, cambium stays less active and cuts off small-sized xylem elements (vessels) and constitutes autumn wood or latewood. Both autumn and spring wood constitute a growth or annual ring. In one year, only one growth ring is formed. In successive years, numerous growth rings are formed. Thus, by counting the number of annual rings in the main stem at the base, we can determine the age of a tree. This branch of science is known as dendrochronology.
   • Growth rings are distinct or sharply demarcated in the plants of temperate climates, for example, Shimla, Nainital, Mussoorie, etc., due to the presence of contrasting seasonal variations. Growth rings are not distinct or sharply demarcated in the trees of tropical climates (near the equator) for example, Calcutta, Bombay, and Madras, due to the absence of contrast¬ing seasonal variations.
2. Heart Wood And Sapwood: The young elements of the secondary xylem in the peripheral region constitute sapwood or alburnum. It is light in color and physiologically active. The water conduction takes place through sapwood.
   • Sapwood is converted into heartwood or duramen in the central region. It is darker in color—due to the deposition of tannins, gums, and resins, and physiologically inactive (almost dead). It provides mechanical support only
   • During the conversion of sapwood into heartwood, the most important change is the development of tyloses in the heartwood. Tyloses are balloon-like structures, that develop from xylem parenchyma.
   • These tyloses block the passage of xylem vessels, hence so also called tracheal plug. The heart wood is commercially used as wood. When the plant is made hollow, it does not die because the water conduction takes place through sapwood.
   • The heartwood is well developed in Moms alba (mulberry). The heartwood is absent in Populus and salix plants. The wood of Tectona grandis is termite-resistant. As a tree grows older, the thickness of heartwood increases and sapwood remains the same.
   • Heartwood is much more durable and resistant to microorganisms, insects, pests, etc. than sapwood. The wood of dicot trees is called porous or hardwood because it consists of vessels (pores). The wood of gymnosperms does not contain vessels (pores) and is known as soft or non-porous wood. Such wood consists of 90 to 95% tracheids and 5 to 10% of ray cells. Sapwood will decay faster if exposed freely to the air.
3. Formation Of Cork Cambium: Cork cambium or phellogen develops from the outer layer of the cortex. It produces a secondary cortex or phelloderm on the inner side and cork or phellem on the outer side. The cells of phellem are dead, suberized, and impervious to water.
   • Cork cells are airtight and used as bottle stoppers or corks. The bottle cork is prepared from the cork of Quercus suber (oak tree). The cells of the phelloderm are thin-walled, living, and store food. Phellem, phellogen, and phelloderm are collectively called periderm. The periderm is a secondary protective tissue.
   • Due to the pressure of the secondary xylem, the epidermis ruptures arid cortex is largely lost after two or three years of secondary growth. In the cork layer (bark), the lenticels are present which are meant for gaseous exchange.
   • In cork, lenticels have loosely arranged cells called complementary cells with intercellular spaces. For bottle corks, the cork is processed in such a manner so that lenticels come in the vertical direction.

Bark includes all the dead and living tissues outside the vascular cambium. Bark may be of two types:

” flower anatomy”

• Scaly Bark: When develops in strips, for example, Eucalyptus, Psidium.
• Ring Bark: When develops in the form of a sheet or ring, for example, Betula (bhojpatra). The outermost layer of bark is dead and called as rhytidome. The bark of Betula was used as a substitute of paper in ancient times to write manuscripts.

Secondary Growth In Dicot Root: Vascular bundles in the dicot root are radial, exarch, and mostly triarch. Vascular cambium is formed secondary) from conjunctive parenchyma cells lying just below each phloem strand. Thus, the number of cambium strips formed equals the number of phloem strands. The cells of the pericycle lying outside the protoxylem also become meristematic to form part of strips of cambium.

These cambial strips join the first formed cambium strips to form a complete but wavy ring of vascular cambium. This cambium ring produces a secondary xylem on the inner side and a secondary phloem on the outer side.

In roots, the growth rings are not distinct, because there is no seasonal variation under the soil. From the outer layers of the pericycle arises the phellogen which cuts phellem (cork) or the outer side and secondary cortex or phelloderm toward the inner side.

NEET Biology Notes Anatomy Of Flowering Plants Point To Remember

Examples of dicots with scattered vascular bundles are Podophyllum, Peperomia, Piper, and Popover. Examples of cortical vascular bundles are Nyctanthus, Kalanchoe, and Casuarina.

• Examples of medullary bundles are Mivabilis, Bougainvillea, Amaranthus, and Achyranthus. Examples of polystelic conditions are Primula and Dianthera.
• Anomalous or abnormal secondary growth occurs in Bougainvillea, Boerhaavia, Chenopodium, and Aristolchia.
• Some monocots show abnormal secondary growth by meristematic tissue which develops around vascular bundles, for example, Dracaena, Yucca, Agave, etc.  Virgin cork is the first formed periderm.

Wound Cork: It is the secondary meristem; formed below the injured cell, it forms a cork on the outer side and a callus below which heals the wound.

• Abnormal secondary growth in dicot root occurs in beetroot (Beta vulgaris) and sweet potato (Ipomoea batatas) by the formation of numerous accessory rings of cambium which cut more storage—parenchyma in secondary phloem and less secondary xylem.
• Homoxylous wood is the wood of vessels dicots, for example, Ranales (Winteraceae, Tetracentraceae Trochodendraceae).
• Heteroxylous wood is the wood of vessel-bearing dicots.

The latex of some plants is of great commercial importance such as

1. The source of commercial rubber is the latex of Hevea brasiliensis, Ficus elastica, Ciyptostegia, and Manihot glaziovii.
2. The source of chewing or chuckle gum is the latex of Achras sapota.
3. The source of the enzyme papain is the latex of Carica papaya.
4. The source of alkaloid opium is the latex of Popover somnific (poppy).

Polyderm is a special type of protective tissue that occurs in roots and underground stems of the members of Rosaceae and Myrtaceae. Its outermost layer is dead and suberized.

NEET Biology Notes Anatomy Of Flowering Plants Assertion Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

” anatomy of flower “

Question 1. Assertion: In maize stem, endodermis is present between the general cortex and pericycle.

Reasoning: Eustele is present in maize Stem.

Answer: 4. If both Assertion and Reason are false.

Question 2. Assertion: In the Cucurbita stem, vascular bundles are conjoint, bicollateral, and either open or closed.

Reasoning: The outer and inner cambium are present and only the inner cambium is functional in the Cucurbita stem.

Answer: 4. If both Assertion and Reason are false.

Question 3. Assertion: Fusiform cells are elongated and tapering cells.

Reasoning: These cells form an axial system consisting of vascular rays.

Answer: 3. If Assertion is true, but Reason is false.

Question 4. Assertion: Septa less tracheids are absent in Trochodendron.

Reasoning: Heteroxylous wood is present in Trochodendron.

Answer: 3. If Assertion is true, but Reason is false.

Question 5. Assertion: According to Hanstein, there are three halogens in a monocot root.

Reasoning: In monocot roots, the outermost groups of initials form both root cap and dermatogen.

Answer: 4. If both Assertion and Reason are false.

Question 6. Assertion: The apical meristem is always protected.

Reasoning: A root cap is present above the meristem in roots.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 7. Assertion: The stem in herbaceous plants do not develop cracks during severe wind and is used to bond under these conditions.

Reasoning: Sclerenchyma is peripheral in position and provides flexibility to the herbaceous stems.

Answer: 3. If Assertion is true, but Reason is false.

Question 8. Assertion: The death of a companion cell leads to the death of a sieve cell also.

Reasoning: Both companion and sieve cells are phloem cells.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 9. Assertion: Dicot roots are mostly tetrach.

Reasoning: There occur four phloem bundles forming rays.

Answer: 4. If both Assertion and Reason are false.

Question 10. Assertion: Heartwood is not involved in the conduction function.

Reasoning: Tyloses and depositions of tannins, resins, and gums is common in duramen cells.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 11. Assertion: Vascular cambium appears wavy in dicot roots.

Reasoning: Vascular cambium is formed by conjunctive tissue in dicot roots which is found located inside the xylem and outside phloem strands.

Answer: 3. If Assertion is true, but Reason is false.

Question 12. Assertion: Velamen is hygroscopic in nature and absorbs environmental moisture.

Reasoning: Velamen is common in orchids which are epiphytes.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

NEET Biology Notes For Morphology Of A Flowering Plants

Morphology deals with the study of forms and features of different plant organs such as roots, stems, leaves, flowers, seeds, fruits, etc. The body of a flowering plant can be divided into two fundamental parts:

1. An Underground Root System And
2. An Aboveground Shoot System. The Root System Grows Downwards Into The Soil Anchors The Plant Firmly In The Soil And Absorbs Water And Various Dissolved Minerals From It. The Shoot Supports Foliage Leaves And Helps In The Conduction Of Water And Mineral Substances From The Soil And Food Material.

NEET Biology Notes For Morphology Of A Flowering Plants Life Span Of A Flowering Plant

The Life Span Of A Flowering Plant Can Be Any Of The Following Forms:

Annual: The life cycle is completed in one season or a few weeks to a few months. Examples are wheat, maize, and Euphorbia prostrata.

Biennials: The life cycle is completed in two seasons: vegetative growth occurs in the first season and food is stored and reproduction occurs in the second season, for example, henbane. Radish, carrot, and turnip are biennials in colder areas and annuals in warmer areas.

Perennials: The plant lives for a few to many years and may bear flowers and fruits every year (polycarpic). Bamboo (Bambusatulda) and agave are monocarpic, i.e., flowering occurs only once in life lifetime.

NEET Biology Notes For Morphology Of A Flowering Plants Habit Of Plants

Herb: Stem is soft, less than 2 m in height.

Shrub: Perennial woody stem with medium height, tiunk is absent for example, Capparis, Rosa, etc.

Trees: Woody stems of great height; the stem is called a trunk. An unbranched stem is called caudex or columnar, for example, a palm. An erect stem with swollen nodes is called a culm, for example, bamboo.

Excurrent: The lateral branches of the trunk do not compete with the stem, for example, Finns, Casuarina, and Eucalyptus.

Deliquescent: The main stem or trunk disappears after some time and the crown is dome-shaped, for example, Dalbergia, Ficus benghalensis.

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NEET Biology Notes For Morphology Of A Flowering Plants Root

Roots develop from the radicle of the seed. They are non-green, underground, positively geotropic, and negatively phototropic. Roots usually do not bear buds, but buds are present for vegetative propagation in sweet potato (Ipomoea) and Indian redwood (Dalbergia). They do not bear nodes and internodes. They have unicellular root hair. Lateral roots arise endogenously, i.e., from the pericycle.

Types Of Roots: Roots Are Mainly Of Two Types

1. Tap Root: They develop from seed radicles. The primary root grows and gives rise to secondary and tertiary roots forming a tap root system. For example, dicots.
2. Adventitious Roots: They develop from any part of the plant body other than the radicle. For example, monocots. They are usually shallow surface feeders such as fibrous roots of grasses.

Parts Of Root

Root Cap: At the apex of the root, a smooth cap-gaped structure is present which is called as root cap. It is protective. Multiple root cap is found in trial roots of screwpine (Pandanus). In hydrophytes, the root cap is either absent or replaced by a root pocket, for example, Pistia, Lemna, and Eichhomia.

Zone Of Cell Formation Or Division: The cells of this region are in an active state of division and their number increases continuously. No vacuoles or small vacuoles are present. It extends to a few millimeters.

Zone Of Cell Elongation: Maximum growth in the cells occurs in this zone. A large central vacuole is present.

Zone Of Cell Maturation: The cells are differentiated into permanent tissues depending upon the functions they have to perform. Root hairs are also present in this zone. In hydrophytes, root hairs are absent because they absorb water through the general body surface.

Modifications Of Tap Roots

1. Storage/Fleshy Roots
   1. Fusiform: Example, radish (Raphamts sativus).
   2. Conical: Example, carrot (Daucus carota).
   3. Napiform: Example, turnip (Brassica rapa), beetroot (Beta vulgaris).
   4. Respiratory Root (Pneumatophores): Example, Avicennia. Sonneratia.
2. Respiratory Root (Pneumatophores): Example, Avicennia, Sonneratia.
3. Nodulated Roots: Example, Pisum sativum, Cicer arietinum.

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Modifications Of Adventitious Roots

1. Storage Roots
   1. Tuberous: Example, sweet potato (Ipomoea batatas)
   2. Fasciculated: Example, Asparagus, Dahlia.
   3. Palmate: Example, orchis.
   4. Nodulose: Example, mango ginger (Curcuma amada)
   5. Beaded Or Moniliform: Example, Portulaca, Momordica.
   6. Annulated: Example, ipecac (Psychortai)
2. Adventitious Roots That Provide Extra Support
   1. Prop Roots: Example, old banyan tree (Ficus benghalensis)
   2. Stilt Roots: Example, sugarcane, maize.
   3. Climbing Roots: Example, Pothos, Piper.
   4. Buttress Roots: Example, Bombax.
3. Adventitious Roots With Special Function
   1. Respiratory Roots: Example, Jussiaea.
   2. Assimilatory Roots: Example, Tinospora, Trapa.
   3. Haustoria: Example, Cuscuta.
   4. Hygroscopic Roots: Example, orchids.
   5. Contractile Roots: Example, saffron (Crocus), Freesia.
   6. Root Thorns: Example, Pothos arinatus, Acanthorhiza.
   7. Foliar/Epiphyllous/Leaf Roots: Example, Btyophyllum, Bignonia, Salvinia.

NEET Biology Notes For Morphology Of A Flowering Plants Stem

Stem is formed by the prolongation of the plumule of the embryo. It is positively phototropic negatively geotropic and hydrotropic. It bears nodes and internodes. The leaf-bearing part of the stem is called the shoot. It has buds. A bud is a condensed immature or embryonic shoot having a growing point surrounded by immature leaves. Cabbage is the largest bud.

According To Nature, Buds Can Be:

• Vegetative: Form leafy shoots.
• Floral: Form flowers.
• Mixed: Form both vegetative and floral characters.

According To Position, Buds Can Be Lateral Or Terminal. Lateral Buds Are Of Four Types:

1. Axillary: Present in the axil of a leaf.
2. Accessory: Additional buds occur either on the side or above the axillary bud.
3. Extra-axillary: Developing on the node but outside the leaf base.
4. Adventitious: Formed from places other than nodes. These can be:
   • Foliar: Example, Biyophyllum, Begonia.
   • Radical: Example, Dalbergia, Ipomoea batata (sweet potato).
   • Cauline: Example, jackfruit.

Types And Modifications Of Stem

Aerial Stems (Epiterranean Stem): It may be reduced, erect, or weak.

1. Reduced: The stem is reduced to a disc, for example, radish, carrot, turnip.
2. Erect Stem: The stem is strong and upright, for example, maize, wheat, and mango. An erect stem with swollen nodes is called a culm (for example, bamboo).
3. Weak Stem: These are thin, soft, and weak and need support. These can be upright or prostrate. These are of the following types.
   • Creepers: The stem creeps on earth and the roots arise at the nodes, for example, grasses, strawberries, and Oxalis.
   • Trailers: The stem creeps on the ground but the roots do not arise at the nodes. They may be of the following types:
     • Prostrate Or Procumbent And Diffuse: Example, Evolvulus, Tribulus.
     • Decumbent: Example, Tridax.
     • Diffuse: Example, Boerhaavia.
   • Lianas (Stem Climber): Woody perennial climbers found in tropical rainforests are lianas. They twine themselves around tall trees to secure sunlight, for example, Hiptage, and Bauhinici vahlii (phanera).
   • Climbers: Plants are with long weak stems and have organs of attachment to climb objects. They may be:
     • Rootlet Climber: Example, Tecoma, Pothos, Piper betel (paan).
     • Hook Climber: In Bougainvillea, Duranta.
     • Tendril Climber: Tendrils are thread-like structures that help in climbing plants.
       • Entire Leaf: Leaf tendrillar, for example, Lathyrus sativus.
       • Leaflet: Leaflet tendrillar for example, Pisum.
       • Twiners: The stem body twines around the support without any special organ of attachment, for example, Cuscuta, Dolichos, and Quisqualis.

Sub-aerial Stems

1. Runner: It is an elongated, prostrate, aerial branch with long internodes and roots that strike at nodes, for example, Oxatis, grasses, Hydrocotyl
2. Sucker: It arises from the axillary bud of the underground part of the stem. The branch creeps below the soil surface grows obliquely upward and produces new shoots. Examples, are Mentha, Chrysanthemum, and Rosa.
3. Offset: Short horizontal branch producing a cluster of leaves above and a cluster of roots below, for example, Pistia, Eichhornia.
4. Stolon: It is a subterranean long lateral branch arising from the base of the stem, for example, Colocasia. It first grows obliquely upward and then bends down to touch the ground surface.

“morphology of flowers “

Underground Stems

1. Rhizome: It grows parallel or horizontally to the soil surface. It bears nodes, internodes, buds, and scaly leaves, for example, ginger, banana, turmeric, and ferns. It is of two types:
   • Rootstocks: It is upright or oblique with the tip almost reaching the soil surface, for example, Dryopteris.
   • Straggling: It is horizontal and branched. Branching may be
     • Racemose: The Axis is monopodial, for example, Saccharum, lotus.
     • Uniparous Cymose: Axis is sympodial, for example, Zingiber officinale (ginger), Curcuma domestica (turmeric), and Canna.
2. Tuber: It is the terminal portion of underground stem branches that are swollen on account of accumulation of food, for example, potato, Jerusalem artichoke (Helianthus tuberosus).
3. Corm: It grows vertically to the soil surface. It bears nodes, internodes, buds, and scaly leaves, for example, Colocasia, Gladiolus, Colchicum, Crocus, and Amorphophallus.
4. Bulb: Stem is reduced and disc-shaped. The bud is surrounded by many concentric leaves. The leaf bases are fleshy and edible, for example, onion, lily, and garlic. The bulb may be tunicated or scaly.
   • Tunicated (Layered Or Laminate): The bulb is covered with a dry membranous sheath of scales called a tunic. These bulbs may be again of two types:
     • Simple Tunicated: Onion, Tulipa, and Narcissus.
     • Compound Tunicated Bulb: Garlic.
   • Scaly Or Imbricate Or Naked Bulb: Tunic is absent, for example, lily.

Special Stem Modifications

• Phylloclade: It is a green flattened or rounded succulent stem with leaves either feebly developed or modified into spines, for example, Opuntia, or Casuarina.
• Cladode: Phylloclade with one intemode is called cladode, for example, Asparagus, and Ruscus.
• Thorn: It is the modification of axillary buds, for example, Bougainvillea, Duranta, Carrisa, Alhagi, etc. Thoms of Alhagi possess flowers and in Duranta, thorns bear smalt foliage leaves. Thoms of Carissa are terminal and branched.
• Stem Tendril: Example, Vitis, Passiflora.
• Bulbils: A condensed auxiliary bud (vegetative) is called a bulbil. It helps in vegetative reproduction, for example, Dioscorea, Glabba, Agave, and Oxalis.

Branching Of Stem: Branching is defined as the mode of arrangement of branches on the stem. It is of two types:

1. Lateral Branching: Branches are produced laterally from the main stem. It may be racemose or cymose.
   • Racemose Type: The main stem grows indefi¬nitely by the terminal bud and produces branches later in the acropetal succession, for example, Casuarina, Poly alt hia, etc.
   • Cymose Type: The growth of the main stem is limited and lateral branches produced by the main stem show more vigorous growth. It may be of the following types:
     • Uniparous Cyme: When one lateral branch is produced at a time. It has two distinct types: helicoid (for example, Saraca) and scorpoid (for example, Vine).
     • Biparous Cyme: When two lateral branches develop at a time, for example, Mirabilis, Datura.
     • Multiparous Cyme: When more than two branches develop at a time, for example, Croton, Euphorbia.
2. Dichotomous Branching: When the terminal bud gives out two branches of equal size in a forked manner, for example, Pandanus, or Hyphaene.

NEET Biology Notes For Morphology Of A Flowering Plants Leaf (Phylopopium)

Leaves are lateral, flat, green, and expanded part of plants that arise from nodes on the stem or branches. Usually leaf has a bud in its axil. The chief functions of leaf is photosynthesis and transpiration. All leaves of a plant are collectively called phyllome. The leaves are of the following types:

1. Cotyledonary Leaves: These are embryonic or seed leaves.
2. Cataphylls: These are scale leaves. These may store food also, for example, onions.
3. Hypsophylls or bract leaves.
4. Prophylls: The first formed leaves.
5. Floral Leaves: Include sepals, petals, or perianth.
6. Sporophylls: Bear spores; this is also used for stamens and carpels.
7. Foliage Leaves: The green leaves of the plant are called foliage leaves.

“plus one botany short notes pdf “

Parts Of A Leaf: A leaf consists of the following three parts: leaf base, petiole, and lamina.

1. Leaf Base (Hypopodium): Leaves are attached to the stem by the leaf base. In some plants, the leaf base becomes swollen and is called pulvinus which is responsible for sleep movement, for example, Cassia, Mimosa, Bean. In some plants, the leaf base expands into a sheath (sheathing leaf base), for example, grasses, and bananas (monocots). When the leaf base partially encloses the stem, it is called auriculate or semi-amplexicaul, for example, prickly poppy, Calotropis procera (Madar).
   • If it completely encloses the stem, it is called amplexicaul, for example, Sonchus, Polygonum.
   • In some plants, minute appendages arising from the leaf base are known as stipules. Leaves with stipules are called stipulate (for example, Rosa, Polygonum) and those without stipules are called exstipulate, for example, Ipomoea.
   • Types Of Stipules: Depending upon duration, they can be:
     • Caducous: Fall off before unfolding of leaf, for example, Michelle Champaca.
     • Deciduous: Fall off soon after unfolding of leaves, for example, Cassia tora and Dillenia indica.
     • Persistent: Remain attached to the leaf throughout life, for example, rose, pea, etc. On The Basis Of Structure And Relation To The Leaf, Stipules Are Classified As:
     • Free Lateral: These are free; and present on both sides of the leaf base, for example, Hibiscus rosasinesis.
     • Scaly: Small dry scales present on both sides of the legal base, for example, Desmodium, etc.
     • Intrapetiolar: These are situated between the petiole and axis, for example, Gardenia.
     • Foliaceous: Large, green-leafy structures, two in number, for example, pea (Pisum) and sweet pea (Lathyrus).
     • Tendrillar: One tendri liar stipule lies on each side of the petiole, for example, Smilax.
2. Petiole (Mesopodium): Petiole in Eichhornia becomes spongy and bulbous. In orange (citrus plants), the petiole becomes winged. Petiole is modified into ten¬drils in Clematis. In Australian acacia, the petiole is modified into a leaf-like sickle-shaped phyllode.
3. Lamina (Epipodium): The broad fiat part of the leaf is the lamina (leaf blade).

Types Of Leaf

• Simple Leaf: Leaf which may be entire or incised and the incisions do not touch the midrib, for example, mango, or banyan.
• Compound Leaf: The leaf blade is incised up to the midrib or petiole and thus divides it into two or more leaflets.

They Are Of Two Types:

1. Pinnately Compound Leaves: Rachis bears several lateral leaflets. These may be of the following types:
   • Unipinnate: They are of two types:
     • Paripinnate: Example, Cassia, Sesbania.
     • Imparipinnate: Example, Rosa, Tephrosia, Azadirachta,
   • Bipinnate: Example, Acacia, Mimosa, Delonix.
   • Tripinnate: Example, Morinaa, Melia, Azadirachta.
   • Decompound: Example, Daucus carota (carrot), Parthenium, Coriandrum.
2. Palmately Compound Leaves: It has no rachis and all the leaflets are joined to a common joint at the tip of the petiole. They may be of the following types:
   1. Unifoliate: Example, Citrus.
   2. Bifoliate: Example, Biginonia grandiflora, Princepia. Balanites, Hardwickia.
   3. Trifoliate Or Ternate: Example, Medicago, Aegle, Oxalis, Dolichos.
   4. Quadrifoliate: Example, Marsilea, Paris quadrifolia.
   5. Multifoliate: Example, Cleome, Bombax.

“morphology of flowering plants short notes for neet “

Venation In Leaves: The arrangement of veins on the lamina is called venation. It is of three types: reticulate, parallel, and furcate.

1. Reticulate Venation: The branches of veins form a network, for example, dicots. However, there are some dicots that show parallel venation, for example, Calophyllum, Eryngium, and Corymbium. It Can Be Of Two Types:
   • Pinnate Or Unicostate: for example, mango, banyan, China rose.
   • Palmate Or Multicostate:
     • Convergent, for example, Ztzyphus, Smilax and
     • Divergent, for example, castor (Ricinus), Luffa, and Vitis (grapevine).
2. Parallel Venation: The veins and veinlets remain parallel to each other, for example, in monocots. Some monocots that show reticulate venation are, for example, Smilax, Dioscorea, and Alocasia. This is Two Types:
   • Pinnate Or Unicostate Parallel Venation: Example Banana {Musa paradisiaca), canna.
   • Palmate Or Multicostate:
     • Convergent, bamboo, grass or
     • Divergent, for example, fan palm.
3. Furcate: The veins branch dichotomously but the finer branches do not form reticulum. It is common in ferns (for example, Adiantum). Among higher plants, it is seen in Cirencester.

Phyllotaxy: It is the mode of arrangement of leaves on the stem or its branches. It is of the following types:

1. Alternate: Single leaf arising at each node, for example, mustard.
2. Opposite: Leaves occurring in pairs at the node. They May Be:
   • Decussate: Leaves that stand at a right angle to the next upper or lower pair, for example, Ocinunn sanctum (sacred basil). Zinnia.
   • Superimposed: Leaves that stand parallel to the next upper or lower pair, for example, Psidium (guava), Eugenia jambolana (Jamun).
3. Whorled: Leaves occurring in more than two at each node, for example, Nerium, Alstonia.

Heterophyily: The occurrence of more than one type of leaves on the same plant is known as heterophyily. It is of three types:

1. Developmental Heterophyily: Leaves of different forms and shapes occur at different periods or places on the same plant, for example, mustard, Sonchus, and Eucalyptus.
2. Habitual Heterophyily: Leaves differ in their shape and incisions due to their habit or nature, for example, Artocarpus, Heterophyllus (jack fruit), and Ficus heterophylla.
3. Environmental Heterophyily: This type is found in aquatic plants where the submerged leaves differ from the floating and aerial leaves, for example, Sagittaria, Ranunculus aquatilis, and Limnophilia heterophylla.

Inflorescence: The arrangement of flowers and mode of distribution of flowers on the shoot system of a plant is called inflorescence.

Racemose (Indefinite) Inflorescence: The main axis of inflorescence does not end in a flower but continues to grow. The development of flowers is acropetal. The opening of flowers is centripetal. It is of the following types:

1. Raceme: Peduncle has bisexual and pedicellate flowers arranged acropetally, for example, larkspur, mustard, and radish.
2. Spike: Peduncle has bisexual and sessile flowers, for example, Achyranthes, and Adhathoda.
3. Corymb: The main axis is short. Lower flowers have longer pedicels than upper ones so that all the flowers are brought more or less to the same level, for example, Iberis, and Capsella.
4. Compound Corymb, for example, cauliflower.
5. Umbel: The main axis is reduced very much and all flowers appear to be arising from the same point. At the base of flowers, clusters of bracts form involucre, for example, hydrocodone, onion. Compound umbel, for example, coriander, carrot, Prunus.
6. Spadix: It is a spike with a fleshy axis and has both male and female flowers. It is surrounded by a large colored bract called a spathe, for example, Musa, palm, Colocasia, and Alocasia (characteristic of innocents).
7. Catkin: It is a pendulous spike that bears unisexual flowers, Morns, birch, oak, and Acalypha.
8. Capitulum Or Head: The main axis becomes flat and called a receptacle which bears many sessile and small florets. Peripheral florets called ray florets are pistillate or neuter and zygomorphic whereas disc florets are bisexual and actinomorphic, for example, sunflower, Zinnia, Cosmos (Asteraceae).
9. Panicle: Peduncle branched and branches have pedicellate flowers, for example, gulmohr, and Rhus.
10. Spikelet: It is a small spike. Flowers are produced in the axis of fertile glumes (bract), for example, wheat, and grasses (Poaceae).

“morphology of flowering plants examples list “

Cymose (Definite) Inflorescence: The main axis ends in a flower. The development of flowers is basipetal and the opening of the flowers is centrifugal. It is of the following types:

1. Monochasial Or Uniparous Cyme: It is of two types:
   • Helicoid Cyme: Example, Atropa, datura, Begonia, Heliotropium.
   • Scorpioid Cyme: Example, Solanum nigrum, Ranunculus.
2. Dichasial Or Biparous Cyme: Example, Dianthus, Clerodendron.
3. Polychasial Or Multiparous Cyme: Example, Hamelia, Calotropis.

Special Inflorescence: These are of the following types:

1. Vertici Ilaster: A cluster of sessile or subsessile flowers borne on a dichasial cyme ending in a monochasial cyme (scorpioid) in the form of a condensed whorl on either side of the node. For example, Ocimum (Tulsi), and Salvia (Lamiaceae).
2. Cyathium: It looks like a single flower. In this cup-shaped involucre encloses a single female flower and a number of male flowers. Each male flower is represented by a single stamen, for example, poinsettia (Euphorbia pulcherrima).
3. Hypanthodium: Fleshy receptacle forming a hollow cavity with an apical opening. The flowers are developed on the inner wall of the hollow cavity. The male flowers are situated at the top near the opening. Below them gall flowers are situated which are sterile and at the bottom are situated female flowers with long styles, for example, Ficus (banyan, fig, guar).
4. Coenanthium: In Dorstenia, the receptacle becomes saucer-shaped and its margins arc slightly. The florets are arranged as similar to hypanthodium.

NEET Biology Notes For Morphology Of A Flowering Plants Flower

Flower is defined as a highly condensed and modified reproductive shoot. The following points can be mentioned to justify that Flower is a modified shoot.

1. Calyx, corolla, androecium, and gynoecium represent four whorls of sterile and fertile leaves borne at different nodes. Sometimes intemode between the calyx and corolla becomes elongated and called as anthophore, for example, Silene, and Dianthus.
   • The intemode between corolla and androecium is known as androphore, for example, Passiflora. The intemode between androecium and gynoecium is called as gynophore, for example, Capparis. When androphores and gynophores both are present in the same flower, they are jointly termed as gynandrophores, for example, Gynandropsis, Cleome. The prolongation of the thalamus beyond the carpel is known as carpophore, for example, Coriandrum, and Foeniculum.
2. In Mussaenda, sepals enlarge to form a leafy structure (foliaceous sepals).
3. Sometimes floral bud is transformed into vegetative buds or bulbils, for example, Agave.

Types Of Floral Characters

• Complete Flower: Calyx, corolla, androecium, and gynoecium are present.
• Incomplete Flower: Flower with one of the four whorls missing.
• Bisexual Flower: Both gynoecium and andoecium are present in the same flower.
• Unisexual Flower: Only androecium (staminate flower) or gynoecium (pistillate flower) are present in the flower.
• Monoecious Plant: When both male and female flowers are present on the same plant. Examples, are Cocos, Ricinus, Zea, Colocasia, and Acalypha.
• Dioecious Plant: When male and female flowers are present on separate plants. Examples are mulberry and papaya.
• Polygamous Plant: When unisexual (male or female), bisexual and neuter flowers are present on the same plant. Example, Polygonum, Mango.
• Monocarpic Plant: The plant that produces flowers and fruits only once in life. For example, pea, mustard, or all seasonal plants.
• Polycarpic Plant: The plant which produces flowers and fruits many times in life. Example, mango, and pear (mostly fruit trees).
• Achlamydeous Flower: Flowers are naked without sepals and petals. Example, Pipelaceae.
• Monochlamydeous Flower: Only one whorl is present (perianth). Examples, are Polygonaceae, and Liliaceae.
• Dichiamydeous Flower: Both whorls are present in a flower Example, most of the flowers.
• Hemicyclic Or Spirocyclic Flowers: Some of the floral parts are in circles and some are spirally arranged. Example, Ranunculaceae.
• Cauliflory: Production of flowers on old stems from dormant buds. Examples, are Artocarpus, and Ficus.

Symmetry Of Flower

• Actinomorphic Flower: When a flower can be divided into two equal halves by many vertical sections passing through the center. Example, Cruciferae, Malvaceae.
• Zygomorphic Flower: When a flower can be divided into two equal halves by only one vertical section passing through the center. For example, pea.

“morphology of flowering plants class 11 pdf notes “

Position Of Floral Parts On Thalamus

• Hypogyny: Ovary is at the top and separable from thalamus. Flowers are hypogynous and the ovary is superior. For example, Malva, Brassica.
• Perigyny: Ovary is hail superior, half inferior. For example, rose.
• Epigyny: Calyx and corolla arise from the upper side of the ovary. The ovary is completely surrounded by and fused with the thalamus. The ovary is inferior and the flower is epigynous. Example, Aster, Luffa.

Bracts: Bracts are specialized leaves that arise from the axil of leaves. They are of the following types:

• Petaloid Bracts: Bracts look like petals (brightly colored). Example, Bougainvillea.
• Spathy Bract: This is a large bract enclosing an inflorescence. Examples are bananas, maize, and palms.
• Foliaceous Bracts: Bracts are leaf-like in appearance. Examples, are Adhatoda, and Gynandropsis.
• Involucre: They are green-colored and in one or more whorls a round or below the entire inflorescence. Examples are sunflower and coriander.
• Glumes: These are small dry, scaly bracts found in spikelet of Gramineae. For example, wheat.

Calyx: The lowermost whorl of a flower is called calyx. It is the non-essential whorl and consists of sepals. Sepals may be free (polysepalous) or fused (gamosepalous). Sepals are modified as follows:

• Pappus: Sepals are modified into persistent hairy structures called pappus which help in the dispersal of fruits. Examples are sunflower and Sonchus (Asteraceae).
• Leafy: In Mussaenda, one sepal is modified into a large leaf-like white structure.
• Spinous: In Trapa, the calyx is persistent and modified into two spines.

Corolla: Corolla is the second whorl of flower and consists of a number of petals which are usually brightly colored. The petals may be gamopetalous (fused) or polypetalous (free). Various forms of petals are:

• Cruciform: Four petals arranged like a cross. For example, members of Brassicaceae.
• Papilionaceous: The number of petals is five with the largest petal standard or vexillum enclosing two lateral wings which are free and in turn enclose the innermost keel (united petals). For example, pea.
• Rosaceous: Five or many small-clawed petals and spread regularly outward. For example, rose.
• Caryophyllaceous: Five free long-clawed with limbs spread at right angles to claws. Example, Diantnus.
• Tubular: Petals are like a tube, for example, disc florets of sunflowers.
• Infundibuliform Or Funnel Shaped: Petals are like a funnel. For example, Datura.
• Bilabiate (Two-Lipped): Upper and lower lips are formed by a fusion of petals. Example, Salvia, Ocimum.
• Ligulate Or Strap Shaped: Gamopetalous petals forming tongue-like structure. For example, ray florets of sunflower.
• Campanulate Or Bell-Shaped: Petals like bell. For example, Physalis.
• Rotate Or Wheel Shaped: Example, brinjal.

Aestivation: The arrangement of floral parts in a floral bud is known as aestivation. It may be of the following types:

• Valvate: When sepals or petals lie very close to each other, without overlapping. For example, mustard.
• Twisted or contorted: When one margin of the sepal or petal overlaps the margin of the next and the other margin is overlapped by the third one. For example, China rose.
• Imbricate: When both margins of one of the petals are covered by others and both margins of another one are external, and of the remaining partly internal, partly external. Example, Cassia, Caesalpinia.
• Quincuncial: When two are inner, two are outer, and one is partly outer and partly inner, Example, Ranunculus.
• Vexillary: The posterior one is the largest and almost covers the two lateral petals and the latter in turn nearly overlaps the two anterior petals. For example, pea (Papilionaceae).

Androecium: It is the third and male whorl of flower in which each stamen consists of filament, anther, and connective. When stamens are free, it is called polyandrous, for example, lily, mustard, and radish. A two-lobed anther is called vitreous (for example, pea) and a one-lobed anther is called monoecious (for example, members of Malvaceae). Attachment of filament to the anther is categorized as:

• Adnate: The filament runs along the back to the anther. For example, Michelia (Champa).
• Basifixed: The Anther is fixed to the filament by its base. For example, Datura.
• Dorsifixed: The Anther is fixed to the filament by its back and the other is immobile. For example, passion flower.
• Versatile: Anther is attached to the filament as in dorsifixed but is able to swing freely. For example, wheat and grasses.

Cohesion Of Stamens: The fusion of stamens among themselves is called cohesion. It is of the following types:

• Monadelphous: Stamens may be united by means of their filaments in one bundle. Examples are China rose, lady’s finger, and cotton (Malvaceae).
• Diadelphous: When the filaments are united into two bundles, the anthers remain free. Examples are peas, beans, and gram (Papilionaceae).
• Polyadelphous: When the filaments are united into more than two bundles but anthers are free. Examples are castor (Euphorbiaceae), and lemon (Rutaceae).
• Syngenesious: When anthers are united but the filaments are free. For example, sunflower (Compositae).
• Synandrous: When anthers as well as filaments of stamens are united throughout their whole length. For example, members of Cucurbitaceae.

Adhesion Of Statements: Fusion with other floral parts is called adhesion. It is of the following types:

• Epipetalous: When stamens are united to the petals. For example, China rose, Solatium, sunflower.
• Episepalous: When stamens are united to sepals. Example, Verbena.
• Epiphyllous (Epipetalous): When stamens are united to perianth (tcpal). For example, members of Liliaceae.
• Gynaiidrous: When stamens are attached to the gynoecium (carpel) either throughout their whole length or by their anthers only. For example, Calotropis (forming gynostegium).

Length And Arrangement Of Stamens

• Didynamous: Four stamens, two outer small and two inner long. Examples, are Ocimum, and Salvia (Labiatae).
• Tetradynamous: Six stamens, two outer small and four inner long. Example, mustard, and radish (Brassicaceae).
• Heterostemony: Stamens are of different lengths. Example, Cassia.
• Obdiplostemonous: Two whorls of stamens, outer whorl lying opposite to the petals (antipetalous) and inner whorl lying opposite to sepals (antisepalous). Examples, are Steilaria, Spergnla, and members of Rutaceae.
• Diplostemonous: Two whorls of stamens, outer whorl Tying opposite to sepals (antisepalous) and inner whorl lying opposite to petals (antipetalous). Example, Cassia.

Gynoecium: It is the female part of the flower comprising carpels bearing ovules. It consists of an ovary, style, and stigma. The gynoecium may be monocarpellary or polycarpellary.

Cohesion Of Carpels

• Apocarpous: Carpels are free (no cohesion). Example, Ranunculaceae.
• Syncarnous: Carpels of more than two and fused. For example, most of the plants.
• Number Of Locules: Ovary has locules or chambers and may be unilocular, bilocular, trilocular, tetralocular, or pentalocular (multilocular).

Placentation: The arrangement of ovules on the placenta within the ovary is called placentation. It is of the following types:

• Marginal: Placenta developing along the junction of two margins of the carpel in a one-chambered ovary. It is the characteristic feature of the family Leguminosae. Example, pea, gram.
• Parietal: Ovary is one-chambered and the placentae bearing the ovules develop on the inner wall of the ovary.

The number of placentae corresponds to the number of carpels. Examples are mustard, radish, and cucumber.

• Axile: The ovary is two to many chambered and placenta-bearing ovules develop from the central axis. Examples are tomato, orange, cotton, China rose, and lily.
• Free Central: Ovary is one-chambered and the placenta bearing the ovules develops all around the central axis. Example, Dianthus, Steilaria.
• Basal: The ovary is unilocular and the placenta develops at the base of the ovary on the thalamus and bears a single ovule. Examples are wheat, maize, aster, Zinnia, and sunflower. It is the most advanced arrangement.
• Superficial: Ovary is multilocular with numerous carpels as in the axile type of placentation, but the placenta develops all around the inner surface of the partition wall. For example, water lily. It is the most primitive.

NEET Biology Notes For Morphology Of A Flowering Plants Fruits

A fertilized and ripened ovary is called a fruit. The fruit consists of seed and pericarp (fruit wall). The pericarp develops from the wall of the ovary and is differentiated into epicarp, mesocarp, and endocarp. Seeds develop from ovules. In some plants, the ovary grows into fruit without fertilization. Such fruits are called parthenocarpic fruits. They are seedless, for example, banana, grapes, pineapple, oranges.

The fruit that develops from ovary is called the true fruit. Most of the fruits are true fruits. If any other floral part takes part in fruit formation, it is called false fruit (pseudocarp), for example, apple, or pear.

Types Of Fruits

1. Simple Fruit: These are developed from the ovary of the single flower with or without accessory parts.
   • Dry Indehiscent Fruits: They do not split or burst. Seeds are liberated only by the destruction of the pericarp.
   • Dry Dehiscent Fruits: These fruits burst automatically and discharge their seeds.
   • Dry Schizocarpic Fruits: They are intermediate between dehiscent and indehiscent fruits. One-seeded indehiscent parts are called mericarps while dehiscent one-seeded are termed cocci.
   • Fleshy Or Succulent Fruits: Pericarp and associated structure become fleshy.
2. Aggregate Fruits: These fruits are formed from polycarpel lary, apocaipous ovary. Each carpel develops into a fruitlet and all fruitlets together form an aggregate fruit. An aggregate of simple fruits borne, by a single flower is otherwise known as etaerio.
3. Multiple Or Composite Fruits: These fruits develop from the entire inflorescence. Sorosis develops from the spike, spadix, or catkin inflorescence. Syconus develops from hypanthium inflorescence.

NEET Biology Notes For Morphology Of A Flowering Plants Seed

Morphologically, a ripened ovule is known as a seed. In other words, seed is A mature, integumented megasporangium. Seeds arc characteristic of spermatophytes (gymnosperms and angiosperms).

Parts Of Seed

Seed Coat: The outer, protective covering of the seed is called the seed coat, which develops from integuments of the ovule. In seeds developing from bitegmic ovules, there are two distinct layers in the seed coat. The outer layer is thick, hard, and leathery (developing from the outer integument) called the testa, whereas the inner layer is thick and papery (developing from the inner in-tegument) called legmen. In seeds developing from unitegmic ovules, there is a single-layered seed coat.

Embryo: The embryo is the most important part of the seed which represents a tiny future plant. The embryo has an embryonal axis or main axis called tigellum, to which one or two cotyledons (seed) are attached, depending upon whether the seed is monocot or dicot. The portion of the embryonal axis or flagellum below the point of attachment of cotyledons is called hypocotyl, which bears a radicle or future root at its tip. Similarly, the portion of the embryonal axis or flagellum above the point of attachment of cotyledons is called epicotyl, which bears plumule.

In some seeds (for example, legumes), the reserve food is stored in cotyledons, whereas in others (for example, cereals), there is a special nutritive tissue called endosperm. The seeds having endosperm are called endospermic or albuminous seeds, for example, cereals, castor, etc., whereas seeds in which the endosperm is fully consumed by the embryo and no endosperm is left are called non-endospermic or exalbuminous seeds, for example, gram, pea, cucumber, tamarind, etc. The reserve food materials in seeds may be carbohydrates (for example, wheat, rice) proteins (legumes) fats (castor, peanut, sunflower), etc. All structures inside the seed coat constitute the kernel.

Monocot Embryo: There is a single cotyledon called scutellum, which is attached to the mid-part of the embryonal axis on its lateral side. On the opposite side of the scutellum is present a tongue-shaped flap-like outgrowth called epiblast (for example, wheat) which represents reduced cotyledon. Further, there is a covering or sheath of the radicle called coleorhiza and a sheath of the plumule called coleoptile.

In Castor Seed (Ricinus Communis): There is a specific outgrowth called caruncle or strophiole, present over the hilum. It is formed by the proliferation of cells of the outer integument at the tip. Caruncle is somewhat spongy and helps in the absorption of water during the germination of seed.

Pcrispermic Seeds: Mostly nucellus is consumed after fertilization due to the absorption of food by the endosperm and embryo. The remains are of nucellus in the seed are called perisperm. Such seeds are called pcrispermic seeds, for example, Piper nigrum (black pepper).

Chalazosperimic Seeds: Chalazosperm is perisperm-like tissue in chalazal region. It is a substitute for endosperm, for example, Cynastrum.

Economic Importance Of Papilionaceae

1. Source of pulses (gram, arhar, mung, etc.)
2. Edible oil (soybean, groundnut)
3. Dye (Indigofera)
4. Fodder (sesbania, Trifolium)
5. Ornamentals (lupin, sweet pea)
6. Medicine (mulethi)

Economic Importance Of Solanaceae

1. Source of food (tomato, brinjal, potato)
2. Spice (chili)
3. Medicine (belladonna, ashwagandha)
4. Fumigalory (tobacco)
5. Ornamentals (petunia)

Economic Importance Of Liliaceae

1. Good ornamentals (tulip, Gloriosa)
2. Source of medicine {Aloe) and vegetables (Asparagus)
3. Colchicine (Colchicum autumnale)

NEET Biology Notes For Morphology Of A Flowering Plants  Points To Remember

Style is generally terminal but may be lateral, for example, Graminae, or mango. Gynobasic style arises from the base of the ovary, for example, Labiatae.

• Examples of endospermic dicot seeds are castor, papaya, and cotton.
• Examples of non-endospermic dicot seeds are gram, bean, pea, cucumber, and tamarind.
• Examples of endospermic monocot seeds are maize, rice, and wheat.

Examples Of Non-endospermic Monocot Seeds: are pothos (money plant), Vallisneria, Alisina, and Amotphophallns.

Defense Mechanisms In Plants

1. Thoms: Lemon, pomegranate, Duranta
2. Spines: Agave, Yucca
3. Prickles: Agave, cotton tree, rose
4. Stinging Hair: Laportea, Urtica clioica
5. Glandular Hair: Jatropha, Boerhavia, Tobacco
6. Stiff Hair: Guaphalium
7. Latex: Ficus, Nerium, Euphorbia
8. Alkaloids: Poppy, Datura
9. Geophilous Habit: Ginger, turmeric. Colocasia, onion
10. Myremecophily: Guava, mango, litchi
11. Mimicry: Cladium, Sansevieria

Anemochory is common in orchids and grasses. The jaculator mechanism in Ruellia and the fountain mechanism in Ecballiurn are related to autochory.

NEET Biology Notes For Morphology Of A Flowering Plants Assertion Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1.

Assertion: In head inflorescence, florets are arranged centrifugally.

Reason: There are always two types of florets in head.

Answer: 4. If both Assertion and Reason are false.

Question 2.

Assertion: Staminal tube is present in Malvaceae.

Reason: It is due to a monetary condition.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 3.

Assertion: The nest of Dischidia is a modified structure of the root.

Reason: Nest roots absorb water and food from humus-rich soil collected in the nest.

Answer: 4. If both Assertion and Reason are false.

Question 4.

Assertion: The lower feathery end of the tigellum is known as a radicle.

Reason: Tigellum bears two nodes on which one or two cotyledons develop.

Answer: 4. If both Assertion and Reason are false.

Question 5.

Assertion: There are five alae in the Pisitm sativum flower.

Reason: Both alae are covered by the largest petal.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 6.

Assertion: All floral whorls are supposed to be modified leaves.

Reason: A Flower is considered as a modified shoot bearing floral parts on its nodes.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion

Question 7.

Assertion: Young leaves in Poinsettia are brightly colored to attract pollinators and achieve pollination.

Reason: It is only a bright color that can attract pollinators on all plants.

Answer: 3. If Assertion is true, but Reason is false.

Question 8.

Assertion: Schizocarpic fruits arc intermediate between dehiscent and indehiscent fruits.

Reason: These fruits split into single-seeded parts.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 9.

Assertion: Leaf in Opuntia functions for the storage of sugars.

Reason: Sugar is transported from leaves in Opuntia and gets stored in the stem.

Answer: 4. If both Assertion and Reason are false.

Question 10.

Assertion: Prop roots develop mostly from horizontal branches of main stem.

Reason: Adventitious roots may perform mechanical supporting functions, working as ropes of a tent.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

NEET Biology Notes For Principles Of Inheritance And Variation Inheritance: Heredity And Variations

• Heredity is the transmission of genetic characters from the parents to the offsprings.
• It deals with the phenomenon of “like begets like.” For example, human babies are like human beings in overall characteristics.
• About 200 characters are found to be hereditary in man.
• Variations are common in sexually reproducing organ- isms.
• Asexually reproducing organisms are monoparental and, hence, exhibit no genetic variations.

Pre-Mendelian Ideas About Inheritance or Theories of Blending Inheritance

• The science of genetics arose with the rediscovery of Mendelism in 1900. Early philosophers, thinkers, and workers have presented various theories to explain the phenomenon of inheritance.
• These are called the theories of blending inheritance. Some of these theories are as follows:
  • Moist vapor theory (Pythagoras: 500 BC): Various body parts emit certain vapors, which get aggregated to form a new individual.
  • Reproductive blood theory (Aristotle: 384-322 BC): According to Aristotle, the menstrual fluid and semen are kinds of highly purified blood. Menstrual fluid provides inert substance for embryo formation and semen provides form and shape to embryo.
  • Preformation theory or homunculus theory (J. Swammerdam): According to this theory, the miniature form of individual is already present in the sperm or egg called “homunculus.” Fertilization is required to stimulate its growth.
  • Theory of pangenesis (Darwin, 1868): According to Darwin, each part of body produces minute particles called gemmules or pangenes, which aggregate to form gamete. On fusion, these give rise to a new individual.
  • Theory of epigenesis (K.F. Wolff): According to this idea, neither egg nor sperm had a structural homunculus but the gametes contained undifferentiated living substance capable of forming an organized body after fertilization. This suggested that many new organs and tissues which were originally absent develop structurally de novo due to mysterious vital force.

“principles of inheritance and variation “

• The theory of pangenes was disproved by Weismann.
• A. Weismann proposed his theory of germplasm, ac- cording to which the changes which affect the germplasm are heritable and the changes which affect the somatoplasm are nonheritable.
• Objections to blending inheritance:
  • Unisexual traits
  • Skin color in humans
  • Atavistic character

Read and Learn More NEET Biology Notes

NEET Biology Notes For Principles Of Inheritance And Variation Genetics Terms And Symbols

NEET Biology Notes For Principles Of Inheritance And Variation Mendelian Inheritance

•  Mendel was born on July 22, 1822. He worked on Pisum sativum (garden pea or edible pea) for 7 years by taking 7 pairs of contrasting traits.
• The results were read out in two meetings of the Natu- ral History Society of Brunn in 1865.
• His paper “Experiments on Plant Hybridization” was published in the fourth volume of “Proceedings of Natural Science Society of Brunn” in 1866.
• Mendel was the first to apply statistical analysis and mathematical logic.
• He selected 14 true breeding pea plant varieties.
• He died due to kidney disorder in 1884.
• Mendel selected the characters listed in Table 5.1, in pea plant, for carrying out hybridization experiments.
• Mendel failed to produce the same results in hawk- weed (Hieracium) and beans (Lablab). Detailed inves- tigation by S. Blixt on pea plant led to locate Mendel’s seven characters on four different chromosomes-1, 4, 5, and 7.
• However, Mendel’s work did not receive any recognition, it deserved, till 1900.
• Mendel’s work remained unnoticed and unappreciated for several years due to the following reasons:
  • Communication was not easy in those days and his work could not be widely publicized.
  • His concept of stable, unblending, and discrete units or factors for various traits did not find acceptance from the contemporaries.
  • His approach of using mathematical and statistical analysis to explain biological phenomena was totally new and unacceptable to many biologists of that time.
  • He could not provide any physical proof for the existence of factors. It was the rediscovery of his work by Hugo de Vries (a Dutch), Carl Correns (a German), and Erich von Tschermak (an Austrian botanist), independently in 1900, that brought Mendel to limelight. Correns raised the status of Mendel’s generalizations to laws.
• Selection of pea plant: The main reasons for adopting garden pea (Pisum sativum) for experiments by Men- del were as follows:
  • Pea has many distinct alternative traits (clear con- trasting characters).
  • The lifespan of pea plant is short.
  • Flowers show self (bud) pollination and, so, are true breeding.
  • It is easy to artificially cross-pollinate the pea flowers. The hybrids, thus, produced were fertile.

principles of inheritance and variation class 12

Mendel’s Work and Results

• Mendel made cross between parents having contrasting traits.
• Firstly he made monohybrid cross (cross between parents that differ from each other in one character) followed by dihybrid cross (cross between parents that differ from each other in two characters) and finally trihybrid cross.
• The F, hybrids were self-crossed to give rise to F2 gen- eration.
• Mendel also carried out reciprocal crosses and found that these gave the same result. (Reciprocal cross means opposite cross, i.e., the parent that provides male gamete in one cross provides female gamete in the second experiment and vice versa).
• The result of reciprocal cross proves that both gametes produce the same effect and it does not matter which parent provides male and which one provides female gamete.
• On the basis of his experimental crosses, he formulated four postulates.
  • Postulate 1: According to this postulate, characters are controlled by a pair of unit factors. The two factors are now called alleles or allelomorphic pair.
  • Postulate 2: If two dissimilar unit factors are present in an individual, only one expresses itself. The one which expresses itself is known as the dominant fac- tor, while the second which does not express at all is known as the recessive factor.
  • Postulate 3: According to this postulate, two contrasting alleles responsible for contrasting traits pre- sent in an individual do not get mixed and get separated from each other at the time of gamete formation by F1 hybrid. Due to their recombination, four combinations can be obtained in equal frequency.
• All three postulates are based on Mendel’s monohybrid cross or one-gene interaction.
• Law of dominance and law of segregation can be explained on the basis of monohybrid cross or one-gene interaction.
  • Law of dominance:
    • This law states that when two contrasting alleles for a character come together in an organism, only one is expressed completely and shows visible effect.
    • This allele is called dominant and the other allele of the pair which does not express and remains hidden is called recessive.
    • This law is not universally applicable. Plant height is controlled by two alleles dominant allele (T) and recessive allele (t). These two alleles can be present in three forms.
    • Mendel crossed two pea plants-one homozygous tall (TT) and another homozygous dwarf (tt).
    • He observed that all the F, progeny plants were tall; like one of the parents, none were dwarf.
    • He made similar observations for the other pair of traits and found that F, always resembled only one of the parents, and that the traits of the other parent were not seen in them.

• • Law of segregation or law of purity of gametes
    • This law states that both parental alleles (recessive and dominant) of F, generation separate and are expressed phenotypically in F2 generation. This law is universally applicable.
    • The F2 generation was produced by allowing the F, hybrid to self-pollinate, to find out segregation or separation.
    • It was observed that both dominant and recessive plants appeared in the ratio of 3: 1. Thus, F2 progeny shows both parental forms.
    • On the basis of F2 generation, following observations can be made.
      • An organism generally has two alleles for each character. These alleles may either be similar or dissimilar. An organism with similar alleles of a pair is called pure or true breeding for that character. If the organism contains dissimilar alleles of a pair, the organism is impure or hybrid.
      • An organism receives one of the two alleles from the male gamete and the other from the female gamete. The gametes fuse during fertilization and form a zygote. Zygote develops into an organism.
      • Each gamete (male or female) has only one allele of the pair. Thus, each gamete is pure for a trait. That is why this law is often called the law of purity of gametes.
      • Fusion between male and female gametes to produce a zygote is a random process.
    • Plants obtained in F2 generation show 3 (tall): 1 (dwarf) phenotypic ratio. Of these three tall plants, one is pure or homozygous dominant and the remaining two are heterozygous (tall in this case). There is only one plant that shows recessive character (dwarf in this case). Dwarf is pure or true breeding, being homozygous recessive.
    • Postulate 4
      • This postulate was made on the basis of dihybrid cross or two-genes interaction.
      • He postulated that the inheritance of one character is independent of the inheritance of another character.
    • On the basis of this postulate, Mendel proposed the “law of independent assortment.”
  • Law of independent assortment
    • The law of independent assortment states that when a cross is made between two individuals different from each other in two or more characters, then the inheritance of one character is independent of the inheritance of another character.
    • Because of their independent assortment, besides the parental types, recombinants are also obtained.
    • In dihybrid cross, these combinations are obtained in the ratio of 9:3:3: 1. For example, Mendel crossed homozygous dominant round and yellow seeded plant (RRYY) with homozygous recessive wrinkled and green seeded (rryy) plant.
    • The F, hybrids were all heterozygous, showing yellow and round seeded plants.
    • This law is not universally applicable.

• If the phenotypic ratio of each pair of alleles (e.g., yellow and green color of seed) is considered, it shows 12 (=9+3) yellow seeded plants and 4 (= 3 + 1) green seeded plants.
• This comes to the ratio 3: 1, similar to the one obtained in the F2 generation of monohy- brid cross showing segregation.
• The same is true for another pair of alleles involved, i.e., round and wrinkled seeded plants. So, the results of each character are similar to the monohybrid ratio.

NEET Biology Notes For Principles Of Inheritance And Variation Summarized Account Of Mendel’s Experiments

Back Cross and Test Cross

Back Cross

• F1 hybrids are obtained by crossing two plants of parental generation.
• Mendel devised a cross where the F, hybrid is crossed with any one of the two parents, i.e., homozygous dominant and homozygous recessive.
• Thus, there will be two possibilities:
  • F1 hybrid (Tt) is crossed with homozygous dominant (TT).
  • F1 hybrid (Tt) is crossed with homozygous recessive (tt).
• Both these crosses collectively are called back cross. If F, is crossed with dominant parent, it is called out cross.

Test Cross

• Out of the two types of back crosses, a cross between F, hybrid (Tt) and its homozygous recessive parent (tt) is called test cross.
• This cross is called test cross because it helps to find out whether the given dominant F, phenotype is homozygous or heterozygous.
• A monohybrid test cross between F, tall plant (Tt) and its homozygous recessive parent (tt) will produce 50% heterozygous tall (Tt) and 50% homozygous recessive (tt), i.e., ratio 1: 1, for both phenotype and genotype.

• If a test cross with two characters, i.e., dihybrid test cross, is made, it gives four types of plants in the ratio 1:1:1:1.
• The phenotypes obtained are similar to those found in the F2 generation of dihybrid cross.
• Thus, a dihybrid test cross between F, yellow and round seeded plant (YyRr) and its homozygous recessive green and wrinkled parent (yyrr) will give the following combinations:
  • 1 yellow, round (YyRr); parental combination 25%
  • 1 yellow, wrinkled (Yyrr); recombinants 25%
  • 1 green, round (yyRr); recombirants 25%
  • 1 green, wrinkled (yyrr); parental combination 25%
• If this ratio is obtained, it will be confirmed that F, hybrid with dominant phenotype is in fact heterozygous.
• The parental combinations (50%) are equal to the frequency of recombinants (50%).

Trihybrid Cross

• Mendel crossed two pea plants, which differed in three characters, and observed independent assortment of genes in them.
• He crossed two pea plants pure in three traits viz., height of stem, form of seed, and color of cotyledon of seed.
• The plants crossed were homozygous tall, round, and yellow (TT RR YY) plant and dwarf, wrinkled, and green (tt rr yy) plant.
• All F, individuals produced were tall, round, and yellow (Tt Rr Yy). These are called trihybrids.
• On selfing trihybrids, F2 phenotypic ratio is 27:9:9 :9:3:3:3:1. The ratio for a trihybrid test cross is 1:1:1:1:1:1:1:1.

NEET Biology Notes For Principles Of Inheritance And Variation One-Gene Interaction (With Respect To Post-Mendelian Inheritance)

• Incomplete dominance
  • After Mendelism, a few cases were observed where F, phenotype was intermediate between dominant and recessive phenotypes.
  • The most common example of incomplete dominance is that of flower color in Mirabilis jalapa (Gulbansi or 4’0 clock plant), studied by Carl Correns.
  • Homozygous red (RR) flowered variety was crossed with white (rr) flowered variety.
  • F, offspring had pink flowers.
  • Thus, here one allele is incompletely dominant over the other so that intermediate phenotype is produced by F, hybrid with respect to the parents.
  • This is called incomplete dominance.
  • Incomplete dominance for flower color [red (RR), pink (Rr), white (rr)] is also known to occur in Antirrhinum majus (snapdragon or dog flower).
  • The phenotypic ratio and genotypic ratio in F2 generation are identical in case of incomplete dominance, i.e., 12:1
• Explanation of the concept of dominance
  • Every gene contains information to express a particular trait.
  • Diploid organisms have two copies of each gene. These are called alleles.
  • These two alleles may be identical or non-identical.

• • One of them may be different due to some changes it has undergone which modifies the information that particular allele contains.
  • Theoretically, the modified allele could be responsible for the production of
    • normal/less efficient enzyme or
    • a non-functional enzyme or
    • no enzyme at all.
  • In case (1), the modified allele is equivalent to the unmodified allele, i.e., it will produce the same phenotype/trait.
  • But if the allele produces a non-functional enzyme or no enzyme [cases (2) and (3)], the phenotype may be affected.
  • The unmodified (functioning) allele which represents the original phenotype is the dominant allele and the modified allele is generally the recessive allele.
  • Hence, the recessive trait is due to nonfunctional enzyme or because no enzyme is produced.
  • If the mutated allele forms an altered but functional product, it behaves as incomplete or co- dominant allele.
• Multiple allelism
  • Mendel proposed that each gene has two contrasting forms, i.c., alleles.
  • But there are some genes that have more than two alternative forms (alleles).
  • The presence of more than two alleles for a gene is known as multiple allelism.
  • Multiple alleles are present on the same locus of homologous chromosome.
  • These alleles can be detected only in a population.
  • A well known example to explain multiple alleles in human beings is ABO blood type.
  • Landsteiner discovered ABO system of blood groups. The fourth group, AB, was discovered by de Castello and Steini.
  • Bernstein showed that these groups are controlled by three alleles I^, 13, and 1°/i.
  • These alleles are autosomal and follow the Mendelian pattern of inheritance.
  • Alleles I^ and I produce a slightly different form of sugar while 1o does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three “I” gene alleles.
  • IA and IB are completely dominant over 1°, but when I and I are present together, they both ex- press their own types of sugar, thus, behaving as codominant alleles.

“principles of inheritance “

• • Other examples of multiple alleles are coat color in rabbit, eye color in Drosophila, and self-incompatibility in tobacco. The formula to find the number of genotypes for multiple allelism is (n/2)(n+1), where n is the number of alleles.
• Co-dominance
  • In co-dominance, the genes of an allelomorphic pair are not related as dominant and recessive- both of them express themselves equally in F, hybrids.
  • These follow the law of segregation and F2 progeny exhibits ratio 1 2 1. Heterozygous for sickle-cell anaemia (Hb^Hb), AB, and MN blood groups are examples of co-dominance of alleles.
• Lethal genes or lethality
  • A lethal gene usually results in the death of an individual when present in homozygous condition. The most striking example to explain lethal gene is sickle-cell anaemia (HbsHb).
  • Cuenot (1905) first reported that inheritance in the mouse body color did not agree with Mendelian inheritance, because the dominant allele for yel- low body color is lethal in homozygous condition.
  • The homozygous dominant gene carrying mouse died, proving that dominant gene is lethal in homozygous form.
  • This is called absolute lethality. In plants, it was first reported in Antirrhinum majus by E. Baur, where yellow leaved or golden leaved or aurea plant never breeds true. Thus, the ratio comes out to be 2: 1.

• Pleiotropic genes
  • The ability of a gene to have multiple phenotypic effects (as it influences a number of characters simultaneously) is known as pleiotropy.
  • The gene having multiple phenotypic effects is called pleiotropic gene.
  • It is not essential that the traits are equally influenced. Sometimes, the effect of the gene is more evident in case of one trait (major effect) and less evident in case of others (secondary effect).
  • Occasionally, a number of related changes are caused by a gene. These are together called syn- drome.
  • Some common examples in humans are cystic fibrosis, Marfan syndrome, and phenylketonuria, while in Drosophila, a single gene influences the size of wings, the character of balancers, the po- sition of dorsal bristles, eye color, the shape of spermatheca, fertility, and longevity.
  • In human beings, pleiotropy is exhibited by sickle-cell anaemia in heterozygous condition (Hb*Hbs).
  • In case of pea, the gene which controls starch synthesis also controls the shape of the seed.

NEET Biology Notes For Principles Of Inheritance And Variation Two-Genes Interaction (With Respect To Post-Mendelism)

• Genes usually function or express themselves singly or individually.
• But many cases are known where two genes of the same allelic pair or genes of two or more different allelic pairs influence one another.
• This is called gene interaction.

Non-Allelic Genetic Interactions

• Non-allelic genetic interactions are interactions between genes located on the same chromosome or on different but non-homologous chromosomes controlling a single phenotype to produce a different expression.
• Each interaction is typical in itself and the ratios obtained are different from the Mendelian dihybrid ratios.
• Some of these interactions of genes, which fall under this category and deviate from Mendel’s ratios, are explained here.
• Complementary genes
  • Complementary genes are two genes present on separate loci that interact together to produce a dominant phenotypic character; neither of them if present alone can express itself. It means that these genes are complementary to each other.

• • Bateson and Punnet have demonstrated that in sweet pea (Lathyrus odoratus), the purple color of flowers develops as a result of interaction of two dominant genes, C and P.
  • In the absence of dominant gene C or P or both, the flowers are white.
  • It is believed that gene C produces an enzyme which catalyzes the formation of necessary raw material for the synthesis of pigment anthocyanin and gene P produces an enzyme which transforms the raw material into the pigment.
  • It means the pigment anthocyanin is the product of two biochemical reactions; the end product of one reaction forms the substrate for the other.

• • Therefore, if a plant has ccPP, ccPp, CCpp, or Ccpp genotype, it bears only white flowers. Purple flowers are formed in plants having genotype CCPP, CCPp, CCPP, or CcPp.
  • From the checker board, it is clear that 9: 7 ratio between purple and white is a modification of 9:3:3:1 ratio.
• Duplicate genes
  • If the dominant alleles of two gene loci pro- duce the same phenotype, whether inherited together or separately, the 9:3:3:1 ratio is modified to 15: 1.
  • Example: The capsules of shepherd’s purse (Capsella) occur in two different shapestriangular and top-shaped. When a plant with triangular capsule is crossed with one having top-shaped capsule, in F1, only tri- angular character appears. The F, offspring by self-crossing produces F2 generation with triangular and top-shaped capsules in the ratio of 15: 1.
  • Two independently segregating dominant genes (A and B) have been found to influence the shape of the capsule in the same way. All genotypes having dominant alleles of both or either of these genes (A and B) will produce plants with triangular capsules. Only those with genotype aabb will produce plants with top-shaped capsules.

• Epistasis
  • A gene which masks (hides) the action of an- other gene (non-allelic) is termed as epistatic gene. The process is called epistasis. A gene whose effects are masked is termed as hypostatic gene.
  • Epistasis is of two types:
• Recessive epistasis
  • Here the recessive allele in homozygous condition masks the effect of dominant allele. For example, in mice, the wild body color is known as agouti (grayish); it is controlled by a gene say A which is hypostatic to recessive allele c.

inheritance class 12

• • Dominant allele C in the presence of a gives colored mice.
  • In the presence of dominant allele C, A gives rise to agouti.
  • So, CCaa will be colored and ccAA will be albino.
  • When colored mice (CCaa) are crossed with albino (ccAA), agouti mice (CcAa) appear in F1.
  • cc masks the effect of AA and is, therefore, epistatic.
  • Consequently, ccAA is albino.
  • The ratio 9:3:3: 1 is modified to 9:3:4.
  • The combination ccaa is also albino due to the absence of both the dominant alleles.
• Dominant epistasis
  • In summer squash, or Cucurbita pepo, there are three types of fruit colors-yellow, green, and white.
  • White color is dominant over other colors, while yellow is dominant over green.
  • The gene for white color (W) masks the effects of yellow color gene (Y).
  • So, yellow color is formed only when the dominant epistatic gene is represented by its recessive allele (w). When the hypostatic gene is also recessive (y), the color of the fruit is green.
    White fruit: W-Y-, W-y-
    Yellow fruit: wwY-
    Green fruit: wwyy

• • A cross between a pure breeding white summer squash (WWYY) with a pure breeding green summer squash (wwyy) yields white fruits in the F, generation. Upon selling of F1, the F2 generation comes to have white, yellow, and green fruits, respectively, in the ratio of 12:3: 1.

Polygenic Inheritance or Quantitative Inheritance

• Quantitative inheritance is controlled by two or more genes in which the dominant alleles have cumulative effect, with each dominant allele expressing a part of functional polypeptide and full trait is shown when all dominant alleles are present.
• Genes involved in quantitative inheritance are called polygenes.
• Swedish geneticist, H. Nilsson-Ehle (1908), and East (1910) demonstrated the segregation and assortment of genes controlling quantitative traits. For example, kernel color in wheat and corolla length in tobacco.
• H. Nilsson-Ehle crossed red kerneled variety with white kerneled variety of wheat.
• The grains of F, were uniformly red but intermediate between the red and white of parental generation.
• When the members of F, were self-crossed among themselves, five different phenotypic classes appeared in F2 showing the ratio of 1:4: 6:4: 1.
  • Extreme red-1/16 (as red as to the parent of F1)
  • Deep red (dark red)-4/16
  • Intermediate red-6/16 (similar to F1)
  • Light red-4/16
  • White-1/16 (as white as to the parent of F1)
• Nilsson-Ehle found that the kernel color in wheat is determined by two pairs of genes-AA and BB.
• Genes A and B determine the red color of kernel and are dominant over their recessive alleles. Each gene pair shows Mendelian segregation.
• Heterozygotes for two pairs of genes (AaBb) segregate into 15 red and 1 white kerneled plants.
• But all red kernels do not exhibit the same shade of redness.
• The degree of redness was found to correspond with the number of dominant alleles.

Skin Color in Man

• The presence of melanin pigment in the skin deter- mines the skin color.
• The amount of melanin developing in the individual is determined by three (two also) pairs of genes. These genes are present at three different loci and each dominant gene is responsible for the synthesis of fixed amount of melanin.
• The effect of all the genes is additive and the amount of melanin produced is always proportional to the number of dominant genes.
• Subsequent studies after Davenport have shown that as many as six genes may be involved in controlling the skin color in human beings.
• The effect of all genes is additive. (The character is assumed to be fixed by three pairs of polygenes.)

• • The F1 progeny between an albino and a Negro individual, called mulatto, produces intermediate skin color.
  • In F2 generation, the colored offsprings exhibit different shades in the ratio 1:6:15:20:15:6:1.
  • The frequency distribution for skin color can be rep- resented either as a histogram or in the form of a bell- shaped normal distribution curve.
  • Looking at the histogram, it can be concluded that in polygenic inheritance, the extreme phenotypes are rare and the intermediate ones are more frequent.

Some other examples of quantitative traits are cob length in maize; human intelligence; milk and meat production; height in humans; and size, shape, and number of seeds and fruits in plants.

• Number of phenotypes for polygenes = 2n + 1
• Number of genotypes for polygenes=3″, where n represents pairs of polygenes

NEET Biology Notes For Principles Of Inheritance And Variation Chromosomal Theory Of Inheritance/Parallelism Between Chromosomes And Mendelian Factors

• The chromosomal theory of inheritance was proposed independently by Sutton and Boveri.
• The two workers found a close similarity between the transmission of hereditary traits and the behavior of chromosomes while passing from one generation to the next through the agency of gametes.
• They noted that the behavior of chromosomes is parallel to the behavior of Mendelian factors (genes).
• The salient features of chromosomal theory of inheritance are as follows:
  • Like hereditary traits, chromosomes retain their number, structure, and individuality throughout the life of an organism and from generation to generation. The two neither get lost nor mixed up. They behave as units.
  • Both chromosomes as well as genes occur in pairs in the somatic or diploid cells. The two alleles of a gene pair are located on homologous sites on homologous chromosomes.
  • A gamete contains only one chromosome of a type and only one of the two alleles of a trait.
  • The paired condition of both chromosomes as well as Mendelian factors is restored during fertilization.
  • Homologous chromosomes synapse during meiosis and then separate or segregate independently into different cells. This establishes the quantitative basis for segregation and independent assortment of hereditary factors.
  • Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.
  • Johannsen (1909) coined the term gene for Mendelian factor.
  • Following the synthesis of ideas, the experimental verification of the chromosomal theory of inheritance by T.H. Morgan and his colleagues led to the discovery of the basis for variations that sexual reproduction produced.
  • Thomas Hunt Morgan (1866-1945) is known as the father of experimental genetics. He was awarded the No- bel Prize of physiology in 1933 for his pioneer work in experimental genetics.

Drosophila melanogaster as Material for Experimental Genetics

• Fruit fly Drosophila is a tiny fly of size about 2 mm. It is found over ripe fruits like mango and banana.
• The fly is actually attracted to the yeast cells present on the surface of ripe fruits. Drosophila is more suitable than pea as experimental material because of the following reasons:
  • It can be easily reared and bred under laboratory conditions.
  • The fly has a short life span of about 2 weeks. It can be bred throughout the year so that numerous generations can be obtained in a single year instead of one as in case of pea.
  • A single mating produces hundreds of offsprings. Females are easily distinguishable from males by the larger body size and the presence of ovipositor (egg laying structure).
  • It shows a number of externally visible and easily identifiable contrasting traits.
  • It has a smaller number (four pairs) of morpho- logically distinct chromosomes.
  • Polytene chromosomes occur in the salivary glands of larva. These can be used to study different types of chromosome aberrations.

• • The fly has heteromorphic (XY) sex chromosomes in the male. The transmission of heteromorphic chromosomes can be easily studied from one generation to another.

NEET Biology Notes For Principles Of Inheritance And Variation Linkage (Exception To Law Of Independent Assortment)

• According to Mendel’s law of independent assortment, genes controlling different characters get assorted in- dependent to each other.
• This is correct if the genes are present on two different chromosomes. But if these genes are present on the same chromosome, they may or may not show independent assortment.
• If crossing-over takes place between these two genes, then the genes get segregated and will assort independent to each other. But if there is no crossing-over between these two genes, there is no segregation and, hence, only parental combination will be found in gametes.
• The tendency of some genes to inherit together (en bloc) is known as linkage.
• In 1906, Bateson and Punnet crossed two varieties of Lathyrus odoratus (sweet pea) and observed that the results do not agree with Mendel’s law of independent assortment.
• They formulated the hypothesis of coupling and re- pulsion to explain the unexpected F2 results of dihybrid cross between a homozygous sweet pea having dominant alleles for blue flowers (BB) and long pollen grains (LL) with another homozygous double recessive plant having red flowers and round pollen grains (bbll).
• Test cross ratio of 7:1:1:7 indicated that there was a tendency of the dominant alleles to remain together. Similar was the case with recessive alleles.
• It was called gametic coupling by Bateson and Punnet.
• Two dominant genes from one parent entered the same zygote more frequently than expected.
• The tendency of two dominant genes to remain together in the process of inheritance was called coupling.
• In another cross, they took a sweet pea plant with blue flowers and round pollens (BBII) and other plant with red flowers and long pollens (bbLL) and obtained the ratio of 177 1 by test crossing the F1 generation.

“principles of genetics pdf “

• When two dominant or recessive genes come from different parents, they tend to remain separate. Hence, this ratio was called repulsion ratio.
• T.H. Morgan in 1910 showed that coupling and repulsion are two aspects of the same phenomenon called linkage.
• He suggested that two genes when present on the same chromosome are in coupling phase and when present on two different homologous chromosomes are in re- pulsion phase.
• Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked.
  • At first, he crossed yellow-bodied (y) white-eyed (w) female with brown-bodied (y) red-eyed (w*) male which produced F, with brown-bodied red- eyed female and yellow-bodied white-eyed male. In the F2 generation, obtained by intercrossing of F1 hybrids, the ratio deviated significantly from expected. He found 98.7% to be parental and 1.3% as recombinants.
  • In a second cross between white-eyed miniature- winged female (wwmm) with wild-red-eyed (w’) normal-winged male (m), the F, generation included red-eyed normal-winged female and white-eyed miniature-winged male. After inter- crossing the F, progeny, he found 62.8% parental and 37.2% recombinants.
  • On the basis of results, we can say that the strength of linkage between y and w is higher than that between w and m.
• According to Morgan, the degree or strength of linkage depends upon the distance between the linked genes in the chromosome..
• Linkage, therefore, may be defined as the tendency of two genes of the same chromosome to remain together in the process of inheritance.

Kinds of Linkage

• T.H. Morgan and his coworkers found two types of linkage:
  • Complete linkage
    • It is the linkage of genes on a chromosome which is not altered and is inherited as such from generation to generation without any cross-over.
    • In this type of linkage, the genes are closely associated and tend to remain together. Example: Male Drosophila and female silk- worm (Bombax mort).

• • • 100% parental combinations indicated that the gene for gray body color is completely linked with long wings.
    • In this dihybrid, F2 phenotypic ratio is 3:1 and test cross ratio is 1:1 (like a monohybrid). Another example is the inheritance of red eye and normal wing (PV/PV) with purple eye and vestigial wing character (pv/pv).
  • Incomplete linkage
    • The linked genes do not always stay together because homologous non-sister chromatids may exchange segments of varying length with one another during meiosis.
    • This is known as crossing-over.
    • The linked genes that have chances of separation by crossing-over are called incompletely linked genes and the phenomenon of their in- heritance is called incomplete linkage.
    • It produces both parental and recombinant types in variable ratio.
    • Bateson and Punnet studied Lathyrus odoratus and defined coupling and repulsion of dominant and recessive genes.
    • In the cis arrangement or coupling condition, the incomplete linkage ratio was 7:1:1:7 (14 parental, 2 recombinants).
    • In the trans arrangement or repulsion case, the ratio was 1:7:7:1 (parental 14, recombinants 2).
    • Example: In maize, incomplete linkage was observed by Hutchinson with respect to seed coat color and seed shape. The results showed that parental combinations of alleles (CS/CS and cs/cs) appeared in about 96% cases. The other two were new combinations (Cs/cs and cS/cs) that appeared in about 4% cases. Thus, in about 4% cases, crossing- over occurred between linked genes.

NEET Biology Notes For Principles Of Inheritance And Variation Crossing-Over And Recombination

• Crossing-over is a process that produces new combination of genes by interchanging segments between non-sister chromatids of homologous chromosomes.
• Crossing-over occurs between the homologous chromosomes at four-stranded or tetrad stage during pachytene of prophase 1 of meiosis 1.
• The condition where an individual heterozygous for two pairs of linked genes (AaBb) possesses two dominant genes on one member of the chromosome pair and two recessive genes on the other pair is said to be cis arrangement .

• If an individual heterozygous for two pairs of linked genes (AaBb) possesses one dominant and one recessive allele of each pair of genes on each member of the homologous pair of chromosomes, the arrangement is said to be trans arrangement.

• When two genes are located very close to each other in chromosomes, hardly any crossing-over can be detected.
• The linkage is broken down due to crossing-over.
• Crossing-over will be relatively more frequent if the distance between two genes is more.
• The frequency of crossing-over can be determined cytologically by counting the number of chiasmata.
• The details of crossing-over for two genes A and B and their alleles a and b on homologous chromosomes.

Crossing-Over Occurs at Four-Stranded Stage

• Neurospora (pink mould), an ascomycetous fungus, is used to demonstrate that crossing-over takes place at four-stranded stage.
• It has the following advantages as experimental organism:
  • It is haploid and there is only one allele at each locus. Hence, dominant-recessive relationship does not interfere with observations and analysis.
  • The products of single meiosis can be easily analyzed.
  • The products of meiosis occur in the form of “ordered tetrads,” i.e., the eight ascospores formed are linearly arranged in a sac-like structure called ascus.
• If genes A and B are located on the same chromosome and undergo independent assortment, the genotype of linearly arranged ascospores can be studied.
• If crossing-over takes place at two-strand stage, the ascospores would show Ab, Ab, Ab, Ab, aB, aB, aB, aB (i.e., 4 Ab+4 aB) arrangement [Fig. 5.24(a)]. If crossing-over takes place at four-strand stage, the ascospores would show AB, AB, Ab, Ab, aB aB, ab, ab (i.e., 2AB+2 Ab+2 aB + 2 ab) or 2: 4: 2 arrange- ment.
• Tetrad analysis has demonstrated the presence of such an arrangement and, thus, it is now confirmed that crossing-over occurs at four-stranded stage.

Factors Affecting Crossing-Over

• The distance between the genes is directly proportional to crossing-over.
• Cross-over decreases with age.
• X rays and temperature increase crossing-over.
• Centromere and heterochromatin positions decrease crossing-over.
• One cross-over reduces the frequency of other cross- over in its vicinity. This is called interference.

Chromosomal Mapping

• Crossing-over is important in locating the genes on a chromosome.
• The genes are arranged linearly on the chromosome.
• This sequence and the relative distances between various genes are graphically represented in terms of recombination frequencies or cross-over values (COV).
• This is known as the linkage map of chromosome.
• Distance or cross-over units are called centiMorgan (cM) or map unit.
• The term centiMorgan is used in eukaryotic genetics while the term map unit is used in prokaryotic genetics. Recombination frequency or

• Recombination frequency depends on the distance be- tween the genes.

• If the distance between the genes is less, the chances of crossing-over are less and, hence, recombination frequency is also less, and vice versa. So, recombina- tion frequency is directly proportional to the distance between the genes.
• In any cross, if recombination frequency is 5%, it means the distance between the genes is 5 map units. A.H. Sturtevant suggested that these recombination frequencies can be utilized in predicting the sequence of genes on the chromosome.
• On the basis of recombination frequency, he prepared the first chromosomal map or genetic map for Drosophila.

NEET Biology Notes For Principles Of Inheritance And Variation Sex Determination

Sex Chromosomes and Autosomes

• Sex chromosomes are those chromosomes which determine the sex of the individual in dioecious or unisexual organisms.
• The normal chromosomes, other than sex chromosomes, of an individual are known as autosomes.
• Sex chromosomes may be similar in one sex and dissimilar in the other.
• The two conditions are, respectively, called homomorphic (similar, e.g., XX and ZZ) and heteromorphic (dissimilar, e.g., XY and ZW).
• Individuals having homomorphic sex chromosomes produce only one type of gametes.
• These are, therefore, called homogametic. For exam- ple, male birds, human female, and Drosophila female.
• Individuals having heteromorphic sex chromosomes produce two types of gametes.
• These are termed as heterogametic. For example, female birds, human male, and normal Drosophila male.
• The factors that control the sex of an organism are under genetic control.
• Various mechanisms that lead to sex determination can be classified into the following four categories:
  • Chromosomal mechanism of sex determination
  • Non-allelic genetic sex determination-fertility factor (plasmid) in bacteria
  • Genic balance mechanism or X/A balance
  • Environmental mechanism of sex determination

Chromosomal Mechanism of Sex Determination

• According to this mechanism, there are certain chromosomes known as sex chromosomes, or heterosomes or idiochromosomes, which are responsible for sex de- termination.
• This mechanism may be of the following types:
  • XX-XY type
    • In most insects, plants, and mammals including human beings, females possess two homomorphic (isomorphic) sex chromosomes, i.c., XX.
    • Males possess two heteromorphic sex chromosomes, i.e., XY. The Y-chromosome is often shorter and heterochromatic (made of heterochromatin).
    • Despite the differences in morphology, the Y- chromosome pairs with the X-chromosome at a certain segment during meiosis.
    • It, therefore, carries a segment which is homologous with a segment of X-chromosome.
    • The remaining segment of Y-chromosome is non-homologous and carries only Y-linked or holandric genes, e.g., testis-determining factor (TDF).

• • • Human beings have 22 pairs of autosomes and one pair of sex chromosomes.
    • All the ova (haploid) formed by a female are similar in their chromosome type (22 + X). Therefore, females are homogametic.
    • The male gametes or sperms (haploid) produced by human males are of two types-(22 + X) and (22 + Y).
    • Human males are, therefore, heterogametic.
    • The two sexes produced in the progeny may have 50: 50 ratio.
    • This type of sex-determination was reported in plant Sphaerocarpus for the first time and is also found in plants such as Melandrium and Coccinia.
  • XX-XO type
    • In roundworms, Dioscorea, and some insects (true bugs, grasshoppers, and cockroaches), females have two sex chromosomes, XX, while males have only one sex chromosome, X.
    • There is no second sex chromosome.
    • Therefore, males are designated as XO.
    • Females are homogametic because they pro- duce only one type of eggs.
    • Males are heterogametic with half the male gametes carrying X-chromosome while the other half being devoid of it.
    • The sex ratio produced in the progeny is 1:1.

• • ZW-ZZ type (WZ-WW type)
    • In birds, fishes, silkworm, Fragaria elatior, and some reptiles, both sexes possess two sex chromosomes.
    • Unlike human beings, females contain het- eromorphic sex chromosomes while males have homomorphic sex chromosomes.
    • Because of having heteromorphic sex chromosomes, females are heterogametic and produce two types of eggs (A + Z) and (A + W).
    • The male gametes or sperms are of one type (A + Z). The sex ratio produced in the offspring is 1 : 1.

• • ZO-ZZ type
    • This type of sex determination occurs in butterflies, pigeon, ducks, and moths.
    • It is exactly opposite of the condition found in cockroaches and grasshoppers.
    • Here, females have odd sex chromosomes while males have two homomorphic sex chromosomes.
    • Females are heterogametic.

• • • They produce two types of eggs-with one sex chromosome (A + Z) and without the sex chromosome (A+0).
    • Males are homogametic, forming similar types of sperms (A + Z).
    • The two sexes are obtained in the progeny in the ratio of 1: 1 as both types of eggs are produced in equal ratio.

Arrhenotoky/Haploid-Diploid Mechanism

• This mechanism is found in honey bee. In honey bees, ants, and wasps, the egg, if fertilized, gives rise to female fly.
• The unfertilized egg develops parthenogenetically into male. So, female flies are diploid while male flies are haploid.

Non-Allosomic Genetic Sex Determination

The fertility factor of plasmid in bacteria determines sex.

Genic Balance or X/A Balance Theory of Sex Determination

• The genic balance theory of sex determination was given by C.B. Bridges. According to him, Y-chromosome plays no role in the sex determination of Drosophila; it is the ratio of the number of X-chromosomes to the set of autosomes which determines the sex of fly.
• It was concluded that the X/A ratio greater than 1 expresses superfemaleness, equal to 1 expresses femaleness, below 1 and above 0.5 expresses intersexes, equal to 0.5 expresses maleness, and less than 0.5 ex- presses supermaleness.

• Gynandromorphs: It is a sex mosaic (an individual with one-half of the body male and the other half female).
• Gynandromorphism is common in silk moth and Drosophila. It is developed due to the accidental loss of X-chromosome from a 2A+XX cell during mitosis.
• Gynander: A gynander may be male or female with patches of tissues of other sex on it.

Environmental Mechanism of Sex Determination

• The environmental mechanism of sex determination was observed by F. Baltzer in Bonnelia viridis (marine worm).
• In this organism, the sex is undifferentiated in larva.
• The larva that settle down in mud grow up into mature females, while those that settle down near the proboscis of a female and become parasites develop into males.
• It has been demonstrated that females secrete a certain hormone which induces sex in larva.
• Crepidula and Ophryortocha also show such mechanism.

NEET Biology Notes For Principles Of Inheritance And Variation Sex-Linked Inheritance

• Sex linkage was discovered by Morgan while working on the inheritance of eye color in Drosophila. He made three types of crosses:
  • Cross 1
    • White-eyed male (W) was crossed with red- eyed (W) female.
    • All flies of the F, generation were found to be red eyed.
    • F, flies were allowed to self breed.
    • In the F2 generation, both traits, red eye and white eye, appeared in the ratio of 3: 1 showing that white eye trait is recessive to red eye trait.
    • F1 generation consisted of only red-eyed flies. In F2 generation, all female flies were red eyed, 50% of the male flies were red eyed, and the remaining 50% were white eyed.

• • Cross 2
    • Red-eyed females of F, generation were crossed with white-eyed male.
    • It is similar to the test cross where hybrids are cross-bred with recessive parents.
    • Morgan obtained red- and white-eyed females as well as males in equal proportions-1 red- eyed female 1 white-eyed female: 1 red- eyed male: 1 white-eyed male.
    • The test cross indicated that white eye color was not restricted to the male fly.

• • Cross 3
    • White-eyed females were crossed with red-eyed males. It was the reciprocal of cross 1 and should have given the same result as obtained by Mendel. However, Morgan obtained a surprising result. All males were white eyed while all females were red eyed.

• • • Taking all crosses into consideration, Morgan came to the conclusion that eye color gene is linked to sex and is present on the X-chromosome.
    • The X-chromosome does not pass directly from one parent to the offspring of the same sex but follows a criss-cross inheritance, i.e., it is transferred from one sex to the offspring of the opposite sex.
    • In other words, in criss-cross inheritance, a male transmits his traits to his grandson through his daughter (diagynic), while a female transmits the traits to her granddaughter through her son (diandric).

Sex Linkage in Human Beings

Colorblindness and hemophilia (Bleeder’s disease) are two common examples of sex-linked diseases in human beings.

• Colorblindness
  • This is a human disease which causes the loss of ability to differentiate between red color and green color.
  • The gene for this red-green colorblindness is present on the X-chromosome. Colorblindness is recessive to normal vision.
  • If a colorblind man (XCY) marries a girl with nor- mal vision (XX), the daughters will have normal vision but will be carriers, while sons will also be normal.

• • If a carrier girl (heterozygous for colorblind- ness, XCX) now marries a colorblind man (XY), the offsprings will be 50% females and 50% males.
  • Of the females, 50% will be carriers for color-blindness and the rest 50% will be colorblind.
  • Of the males, 50% will have normal vision and 50% will be colorblind.

• Hemophilia (Bleeder’s disease)
  • A person suffering from this disease cannot synthesize a normal blood protein called antihemophilic globulin (AHG) required for normal blood clotting. Hemophilia-A is more severe.
  • Therefore, even a very small cut may lead to con- tinuous bleeding for a long time.
  • This gene is located on the X-chromosome and is recessive.
  • It remains latent in carrier females.

• • If a normal man marries a girl who is a carrier of hemophilia, the progeny will consist of 50% females and 50% males.
  • Of the females, 50% will be normal and the rest 50% will be hemophilia carriers.
  • Of the males, 50% will be normal and the rest will be hemophiliacs.
  • Hemophilia-B (Christmas disease): In this, plasma thromboplastin is absent. Inheritance is just like hemophilia-A.

NEET Biology Notes For Principles Of Inheritance And Variation Mutation

• Mutation is sudden, inheritable, discontinuous variation due to change in chromosomes and genes.
• Hugo de Vries (1901), one of the discoverers of Mendel’s laws, observed two distinct varieties of Oenothera lamarckiana (evening primrose).
• These differed in the length of stem, flower form, and the color and shape of leaves.
• These mutant varieties are now known to have been produced due to chromosomal aberrations.
• Seth Wright (1791) is considered to be the first to re- cord point or gene mutation.
• He noticed a lamb with unusually short legs.
• This short-legged breed of sheep was known as ancon breed.
• Darwin called this variation as sports.
• Bateson (1894) termed them as discontinuous or saltatory variations.
• The credit for starting the scientific study of mutations goes to Thomas Hunt Morgan (1910).
• He is known for his work on fruit fly, Drosophila melanogaster.
• He found white-eyed mutant of Drosophila. Since then, about 500 mutations have been observed by geneticists around the world.

Types of Mutations

Different classifications of mutations are known, each based on a definite criterion or character.

• On the basis of the agency involved
  • Spontaneous mutations: Such mutations occur at a frequency of 1 x 105 in nature. These are natural mutations and have also been called back- ground mutations.
  • Induced mutations: These have been observed in organisms due to specific factors such as radiations, ultraviolet light, or a variety of chemicals. The agents that induce mutations are called mutagens or mutagenic agents.
• On the basis of the type of cells in which mutations occur
  • Somatic mutations: These mutations occur in the somatic cells, i.e., body cells or the cells other than germinal cells. These mutations do not have any genetic or evolutionary importance. This is because only the derivatives or the daughter cells formed from the mutated cell will show mutation and not the whole organism.
  • Germinal mutations: These mutations occur in the gametes or germ cells and are also known as gametic mutations. Such mutations are heritable and, therefore, are of great evolutionary significance. If the mutations are dominant, these are expressed in the next generation, and if they are recessive, their phenotypic expressions remain suppressed.
• Forward and backward mutations
  • The most common type of mutation is the change from the normal or wild type to a new genotype (recessive or dominant).
  • Such mutations are called forward mutations.
  • An organism which has undergone forward mutation may again develop mutation which restores the original wild-type phenotype.
  • Such reversions are known as backward mutations or reverse mutations.
  • Mutations can occur at any stage during the life cycle of an organism.

Some Other Types of Mutations

• Gene mutation
  • It is alteration in the sequence of nucleotides in nucleic acids or any change in the sequence of triplet bases.
  • If gene mutation arises due to change in single base pair of DNA, it is called point mutation.
  • Gene mutation occurs by the following methods:
    • Frame-shift mutation (gibberish mutation)
      • Deletion: Removal of one or more bases from a nucleotide chain.
      • Insertion or addition: Addition of one or more bases to a nucleotide chain.
    • Substitution: The replacement of one base by another. It is of two types:
      • Transition: When a purine base (A or G) is substituted by another purine base or a pyrimidine base (T or C) is substituted by another pyrimidine base.
      • Transversion: The substitution of a purine base with a pyrimidine base or vice versa.
    • Tautomerization: The purines and pyrimidines in DNA and RNA may exist in several alternate forms or tautomers. Tautomerization occurs through the rearrangement of electrons and protons in a molecule. Tautomers show changed base pairing so as to cause change in sequence, e.g., AT to CG.
      • Nonsense mutation: Such mutation arises when a normal codon-coding for an amino acid-is changed into a chain- terminating codon (UAG, UAA, UGA) resulting in the production of an incomplete polypeptide.
        Nonsense mutations rarely go unnoticed because the incomplete or shorter protein formed is generally inactive.
      • Mis-sense mutation: It involves change in base in a codon, producing a different amino acid at the specific site in a polypeptide. In mis-sense mutation, the change in one amino acid is frequently compatible with some biological activity, e.g., sickle-cell anemia.
• Chromosomal mutation (chromosomal aberrations)
  • A change in chromosome morphology is called chromosomal aberration. Structural changes in chromosomes take place during meiosis. There are four types of chromosomal rearrangements:
    • Deficiency or deletion
      Deficiency occurs due to the loss of a terminal segment of chromosome. Deletion occurs due to the loss of an intercalary part of chromosome, e.g., cri-du-chat syndrome (short arm of chromosome 5 loses a part).
    • Duplication
      It occurs due to the addition of a part of chromosome so that a gene or a set of genes is represented twice, e.g., Barr eye in Drosophila.
    • Translocation
      It involves the shifting of a part of one chromosome to another non-homologous chromosome. So, new recombinant chromosomes are formed, as this induces faulty pairing of chromosomes during meiosis. An important class of translocation having evolutionary significance is known as reciprocal trans- location or segmental interchanges, which involves the mutual exchange of chromo some segments between non-homologous chromosomes, i.e., illegitimate crossing- over. Chronic myelogenous leukemia (CML) occurs due to the translocation of segment of long arm from chromosome 22 to chromosome 9. Chromosome 22 is called Philadelphia chromosome.
    • Inversion
      Inversion is change in the linear order of genes by the rotation of a section of chromosome by 180°. It occurs frequently in Drosophila as a result of X-ray irradiation. It may be of two types:
      • Paracentric: It is inversion without involving centromere. (Inverted segment does not carry centromere.)
      • Pericentric: It is inversion involving centromere.
• Genomatic mutation or numerical changes in chromosome number: It is of two types:
  • Aneuploidy: In aneuploidy, any change in the number of chromosomes in an organism will be different than the multiple of basic set of chromosomes. It results due to the failure of segregation of chromatids during cell division cycle. There will be following two possibilities:
    • Hypoploidy: This arises due to the loss of one or more chromosomes or pairs of chro- mosomes. Thus, the following conditions are likely to be produced:
    • Monosomy (2n-1): It is the result of loss of one chromosome for a homologous pair. Its other variant is double monosomy, i.e., (2n-1-1).
    • Nullisomy (2-2): It is the result of loss of a complete homologous pair of chromosomes.
  • Hyperploidy: This arises due to the addition of one or more chromosomes or pairs of chromosomes. The following conditions are, thus, likely to be produced:
    • Trisomy (2n+1): In this type, a single chromosome is added to the chromo- some set. Trisomics were obtained for the first time in Datura stramonium (Jimson weed) by A.F. Blakeslee and his cowork- ers (1924). In human beings, Mongolism or Down’s syndrome is due to the trisomy of chromosome 21 (2n+1 or 46 + 1), i.e., chromosome 21 is present three times. Others are the Patau syndrome, due to the trisomy of the 13th, and Edward’s syndrome, due to the trisomy of the 18th chromosome.
    • Tetrasomy (2n+2): It is the result of the addition of a complete homologous pair of chromosomes (i.e., two chromosomes).
  • Euploidy: In euploidy, any change in the number of chromosomes is the multiple of the number of chromosomes in a basic set or it occurs due to variation in one or more haploid sets of chromosomes. Accordingly, these may be haploid and polyploid.
    • Haploidy: In haploid, only one set of chromosomes is present. Haploids are better for mutation experimental studies, because all mutations, either dominant or recessive, can express immediately in them (as there is only one allele of each gene present in each cell).
    • Polyploidy: The failure of cytokines after the telophase stage of cell division results in an increase in the whole set of chromosomes in an organism. This phenomenon is known as polyploidy. Polyploids are of particular importance and are, therefore, discussed here. These fall into two major categories:
      • Autopolyploids: These have the same basic set of chromosomes multiplied more than twice, e.g., AAA (autotriploid) and AAAA (autotetraploid). Autopoly- ploids with odd number of chromosomes are seedless but show gigantism (large size) or gigas effect (more yield and more adaptability), but are odd numbered and, so, can only be propagated vegetatively. For example, banana and pineapple. Naturally occurring autotriploids are known in banana, grapes, sugar beet, tomato, and watermelons. Similarly, autotetraploids occur amongst apples, berseem, corn, etc.
      • Autopolyploids can also be produced ar- tificially by treating the seeds, seedlings, or axillary buds with an alkaloid called colchicine. It is extracted from the corm of Colchicum autumnale. The treatment produces doubling of chromosomes.
      • Allopolyploids: These are hybrids whose chromosome sets are derived from two different genomes.
        Let A and B be two different genomes. The two diploid organisms should have AA and BB chromosome sets. The autotetraploid can now be represented as
        AAAA while the allotetraploid is represented as AABB.
        Such polyploids are the result of doubling of chromosomes in F, hybrids derived from two different or related species.
      • The most common example of allopoly- ploidy is Raphanobrassica, developed by Russian geneticist G.O. Karpechenko (1927). A cross was made between Brassica oleracea (2n = 18) and Raphanus sativus (2n = 18). The F, hybrid produced was sterile, because the chromosome sets of both these plants were dissimilar and could not pair dur- ing meiosis. However, some fertile plants with 2n=18A+18 B (18 bivalents) were found amongst these.

Another example is a man-made cereal, Triticale, produced by (a) crossing Triticum durum (2n = 28) with Secale cereale (2n = 14) and then treating the F, hybrid with colchicine to obtain hexaploid triticale and (b) crossing Triticum aestivum (2n = 42) with Secale cereale (2n = 14) and then treating the F, hybrid with colchicine to produce octaploid triticale.

Mutagens

Mutations can be artificially produced by certain agents called mutagens or mutagenic agents. Following are two major types of mutagens:

• Physical mutagens
• Chemical mutagens

Physical Mutagens

• Radiations are the most important physical mutagens.
• H.J. Muller, who used X rays for the first time to increase the rate of mutation in Drosophila, opened an entirely new field in inducing mutations.
• So, Muller is considered as the father of actinobiology.
• The main source of spontaneous mutations is natural radiations coming from the cosmic rays of the sun.
• The spectrum of wavelengths that are shorter (i.e., of higher energy) than the visible light can be subdivided into the following two groups:
  • Ionizing radiations
  • Non-ionizing radiations
    • Physical mutations occur in small amounts in the environment and are known as background radiations.

Following are the biological effects of radiations:

• Effects of ionizing radiations: These radiations include X rays, y rays, a rays, and Brays. Ionizing radiations cause breaks in the chromosome. These cells then show abnor- mal cell divisions. If these include gametes, they may be abnormal and even die prema- turely. Different types of cancers may result due to radiations. The frequency of induced mutations is directly proportional to the doses of radiations.
• Effects of non-ionizing radiations: These radiations have longer wavelengths but carry lower energy. This energy is insufficient to induce ionization. Therefore, non-ionizing radiations such as UV light do not penetrate beyond the human skin. Thymine (pyrimidine) dimer formation is a major mutagenic effect of UV rays that disturbs DNA double helix and, thus, DNA replication.

Chemical Mutagens

• A large number of chemical mutagens are now known. These are more injurious than radiations. The first chemical mutagen used was mustard gas by C. Auer- bach et. al. during World War II.
• Chemical mutagens are placed into two groups:
  • Those which are mutagenic to both replicating and non-replicating DNA such as nitrous acid and
  • Those which are mutagenic only to replicating DNA such as acridine dyes and base analogs.
• Following are the effects of some chemical mutagens:
  • Nitrous acid: It is mutagenic to both replicating and non-replicating DNA. It acts directly by oxidative deamination on A, G, and C bases which contain amino groups. A is deaminated to hypoxanthine which is complementary to cytosine. G is converted to xanthine which pairs with C. Cytosine is converted to U which pairs with A.
  • Acridines: Acridines and proflavins are very powerful mutagens. These can intercalate between DNA bases and interfere with DNA replication, producing insertion or deletion or both of single bases, respectively. This induces frame- shift mutation or gibberish mutation, eg, thalassemia.
  • Base analogs: These have structures similar to the normal bases and are incorporated into DNA only during DNA replication. Base analogs cause mis-pairing and eventually give rise to mutations. Base analogs may be either natural or artificial. Natural base analogs include 5-methyl cytosine, 5-hydroxymethyl cytosine, 6-methyl purine, etc. The most commonly used artificial base ana- logs are 5-bromouracil and 2-aminopurine. 5-Bromouracil is a structural analog of thymine. It gets incorporated into the newly replicated DNA in place of thymine (T). 2-Aminopurine is an artificial base analog of adenine. It acts as a substitute of adenine (A) and can also pair with cytosine (C).

NEET Biology Notes For Principles Of Inheritance And Variation Cytoplasmic Inheritance

• Some self-replicating genes (DNA) are present in the cytoplasm (mitochondrial DNA and chloroplast DNA) also.
• These are called plasmagenes. All plasmagenes to- gether constitute plasmon (like genome).
• The inheritance of characters by plasmagenes is called extranuclear or extrachromosomal inheritance.
• The most important examples of extranuclear inheritance in eukaryotes are maternal inheritance and organelle inheritance.

Maternal Inheritance

• The amount of nuclear hereditary material contributed by the two sexes is almost equal but the cytoplasm in the egg is always much more than that in the sperm.
• So, in extranuclear inheritance, the contribution of fe- male parent is more.
• This is called maternal inheritance.
• The evidence of maternal inheritance is the coiling of shell in snails.

NEET Biology Notes For Principles Of Inheritance And Variation Genetic Disorders

Pedigree Analysis

• A record of inheritance of certain genetic traits for two or more generations presented in the form of a diagram or family tree is called pedigree.
• Parents are shown by horizontal line while their off- springs are connected to it by a vertical line.
• The offsprings are also shown in the form of a horizontal line below the parents and numbered with Arabic numerals.
• Pedigree analysis is the study of pedigree for the transmission of a particular trait and finding the possibility of absence or presence of that trait in homozygous or heterozygous state in a particular individual.
• It is useful for genetic counselors to advise intending couples about the possibility of having children with genetic defects such as hemophilia, colorblindness, alkaptonuria, phenylketonuria, thalassemia, sickle-cell anemia (recessive traits), brachydactyly, myotonic dystrophy, and polydactyly (dominant traits).
• Pedigree analysis indicates that Mendel’s principles are also applicable to human genetics with some modifications found out later such as quantitative inheritance, sex-linked characters, and other linkages.

Symbols Used in Pedigree Analysis

• Proband is the person from which case history starts. If it is male, it is called propositus; if it is female, it is called proposita.

NEET Biology Notes For Principles Of Inheritance And Variation Mendalian Disorders

Sickle-Cell Anemia

• It is an autosomal recessive disorder. In this disorder, the RBCs become sickle shaped under low O2 concentration.
• The affected persons die young.
• Other heterozygous individuals for this trait have normal phenotype and live long.
• The disease is due to the base substitution of the sixth codon in gene coding for the ẞ chain of hemoglobin.
• The middle base of a DNA triplet coding for the amino acid glutamic acid is mutated so that the triplet now codes for valine instead.
• The mutant hemoglobin molecule undergoes polym- erization under low O2 tension causing a change in the shape of RBC from biconcave disc to elongated sickle-like structure .

Thalassemia

Thalassemia is a recessive autosomal disease caused due to the reduced synthesis of a or ẞ polypeptide of hemoglobin. B-thalassemia is a major problem; individuals suffering from major thalassemia often die before ten years of age.

Phenylketonuria

Phenylketonuria is a recessive autosomal disorder (chromosome 12) related to phenylalanine metabolism. This disorder is due to the absence of a liver enzyme called phenylalanine hydroxylase. Due to the lack of this enzyme, phenylalanine follows another pathway and gets converted into phenylpyruvic acid.

This phenyl pyruvic acid upon accumulation in joints causes arthritis; if it hits the brain, it causes mental retardation known as phenyl pyruvic idiocy. Phenylalanine are also excreted through urine because of poor absorption by kidney.

Cystic Fibrosis

Cystic fibrosis is an autosomal recessive disorder common among Caucasian Northern Europeans. Persons suffering from this disease have extremely salty sweat. It is due to decreased Na and CI reabsorption in the ducts.

The disease is due to a gene present on chromosome 7. Due to a defective glycoprotein, thick mucus develops in pancreas and lungs and the formation of fibrous cyst occurs in pancreas.

Huntington’s Chorea

Huntington’s chorea is an autosomal dominant disorder. The gene responsible for this disorder is present on chromosome 4. The disease is characterized by the gradual degradation of brain tissue in the middle age and consequent shrinkage of brain.

Alzheimer’s Disease

Alzheimer’s disease is an autosomal recessive disease that results in mental deterioration (loss of memory, confusion, and anxiety) and, ultimately, the loss of functional capacities. The disease is due to the deposits of ẞ-amyloid, a short protein in brain which results in the degradation of neurons. It involves two defective alleles located on chromosomes 19 and 21. This disease is common in Down’s syndrome.

Myotonic dystrophy is due to a dominant autosomal mutant gene located on the long arm of chromosome 19. Mild myotonia (atrophy and weakness of the musculature of the face and extremities) is most common.

Other Mendelian Disorders

• Alkaptonuria (Garrod, 1908): Due to deficiency of oxidase enzyme
• Albinism (chromosome 11): Absence of tyrosinase
• Tay-Sach’s disease (chromosome 15): Absence of hexosaminidase B
• Gaucher’s disease (chromosomes 1): Due to the inhibition of glucocerebrosidase enzyme action which leads to accumulation of cerebroside

Other Abnormalities due to Autosomal Dominant Gene Mutation

• Polydactyly: Presence of extra fingers and toes
• Brachydactyly: Abnormal short fingers and toes

Abnormalities due to Sex-Linked (X-Linked) Recessive Gene Mutation

• Hemophilia A: Due to lack of antihemophilic globulin
  Hemophilia B: Due to lack of plasma thromboplastin
• Red-green colorblindness: Daltonism
  Protanopia: Red colorblindness
  Tritanopia: Blue colorblindness
  Deuteranopia: Green colorblindness
• Muscular dystrophy: Due to non-synthesis of protein dystrophin; deterioration of muscles at an early stage
• Lesch Nyhan syndrome: Deterioration of nervous system due to HGPRT (hypoxanthin guanine phosphoribosyl transferase) deficiency

NEET Biology Notes For Principles Of Inheritance And Variation Chromosomal Disorders

• Autosomal abnormalities (due to mutation in body chromosome)
  • Down’s syndrome: It occurs due to the trisomy of the 21st chromosome. The affected individual is short-statured with small round head, furrowed tongue, and partially open mouth. The palm is broad with characteristic palm crease and mental retardation. Physical and psychomotor develop- ment is retarded.
  • Edward’s syndrome: It occurs due to the trisomy of the 18th chromosome.
  • Patau’s syndrome: It occurs due to the trisomy of the 13th chromosome.
  • Cri du chat syndrome: It occurs due to deletion in the short arm of the fifth chromosome.
• Allosomal or sex chromosomal disorder
  • Klinefelter’s syndrome: It occurs due to the trisomy of the X-chromosome in male, resulting in a karyotype of 47 (44 + XXY). Individuals have long legs, sparse body hair, small prostrate gland, small testes, reduced mental intelligence, and enlarged breasts (gynaecomastia). Such individuals are sterile.
  • Turner’s syndrome: It is caused due to the absence of one of the X-chromosomes in females, i.e., 45 with chromosome complement 44 + XO. Such females are sterile with undeveloped breasts, short stature, reduced ovaries, and absence of menstrual cycle.
  • Super-female: AA+ XXX, AA + XXXX
  • Jacob’s syndrome or super-male: AA + XYY, also called criminal syndrome

NEET Biology Notes For Principles Of Inheritance And Variation Population Genetics

Hardy Weinberg’s equation is applied to know the distribution of traits and the frequency of autosomal dominant recessive gene distribution in the entire population.

p = Dominant gene/allele

q= Recessive gene/allele

p+q=1

(p+q)2= p2+q2+2pq=1

2/3(p2+2pq) = Frequency of dominant trait

1/3(92) = Frequency of recessive trait

NEET Biology Notes For Principles Of Inheritance And Variation Some Important Definitions

• The subject that deals with the inheritance as well as the variation of characters from parents to offspring is called genetics.
• Inheritance is the process by which characters are passed on from parent to progeny.
• Variation is the degree by which progeny differ from their parents.
• A true breeding line is one that, having undergone con- tinuous selfing, shows the stable trait inheritance and expression for several generations.
• Genes that code for a pair of contrasting traits are known as alleles.
• A cross between F, hybrid (Tt) and its homozygous recessive parent (tt) is called a test cross.
• If F, phenotype does not resemble either of the two parents and is in between the two, it is called incom- plete dominance.
• The presence of more than two alleles for a gene is known as multiple allelism.
• If F, phenotype resembles both the parents, it is called co-dominance.
• If more than one phenotype is influenced by the same gene, it is called pleiotropy.
• If two genes present on different loci produce the same effect when present alone but interact to form a new trait when present together, they are called comple- mentary genes.
• A gene that masks the action of another gene (non- allelic) is termed as an epistatic gene. The process is called epistasis.
• Polygenic inheritance is controlled by two or more genes in which the dominant alleles have cumulative effect, with each dominant allele expressing a part of functional phenotype and full trait is shown when all dominant alleles are present.
• The tendency of some genes to inherit together is called linkage.
• Sex-limited traits are autosomal and found in both sexes but are expressed in one sex only.
• Sex-influenced traits are autosomal and appear more frequently in one sex than in the other.
• Mutation is sudden, inheritable, discontinuous varia- tion due to change in chromosomes and genes.
• If mutation arises due to change in a single base pair of DNA, it is known as point mutation.
• The failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy.
• The failure of cytokinesis after the telophase stage of cell division results in an increase in the whole set of chromosomes in an organism. This phenomenon is known as polyploidy.
• A record of inheritance of certain genetic traits for two or more generations presented in the form of a diagram or family tree is called pedigree.

NEET Biology Notes For Principles Of Inheritance And Variation Formula Chart

NEET Biology Notes For Principles Of Inheritance And Variation Summary

• Genetics is a branch of biology which deals with the principles of inheritance and variation.
• Mendelian inheritance (Mendelism)
  • Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as “factors.”
  • Dominant characters are expressed when factors are in heterozygous condition (law of dominance).
  • The characters never blend in heterozygous condition.
  • Recessive characters are only expressed in homozygous condition.
  • A recessive trait that was not expressed in heterozygous condition may express again when it becomes homozygous. Hence, characters segregate during the formation of gametes (law of segregation).
  • Mendel also studied the inheritance of two characters together and he found that the factors independently assort and combine in all permutations and combinations.
• The factors on chromosomes regulating the characters are called the genotype and the physical expression of the characters is called phenotype.
• Walter Sutton and Theodore Boveri noted that the behavior of chromosomes was parallel to the behavior of genes and used chromosome movement to explain Mendel’s laws.
• Mendel’s law of independent assortment is not true for genes that are located on the same chromosome (i.e., linked genes).
• Closely located genes assorted together, and distantly located genes, due to recombination, assorted independently.
• The frequency of recombination between gene pairs on the same chromosome is a measure of the distance between the genes.
• Mutation is defined as the change in the genetic material. A point mutation is a change of a single base pair in DNA. Some mutations involve changes in the whole set of chromosomes (polyploidy) or change in a subset of chromosome number (aneuploidy).
• Sickle cell anemia is caused due to change of one base in the gene coding for B-chain of hemoglobin.
• Inheritable mutations can be studied by generating a pedigree of a family.
• Down’s syndrome is due to the trisomy of chromo- some 21. In Turner’s syndrome, one X-chromosome is missing and the sex chromosome is XO.
• In Klinefelter’s syndrome, the condition is XXY.

NEET Biology Notes For Principles Of Inheritance And Variation Assertion-Reasoning Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).
2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
3. If Assertion is true but Reason is false, then mark (3).
4. If both Assertion and Reason are false, then mark (4).

Question 1. Assertion: Mendel gave postulates such as “principles of segregation” and “principles of independent assortment” after studying seven pairs of contrasting traits in garden pea.

Reason: He was lucky in selecting seven characters in pea that were located on seven different chromosomes.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 2. Assertion: Test cross is the tool for knowing linkage be- tween genes.

Reason: Monohybrid test cross gives two phenotypes and two genotypes.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 3. Assertion: Marfan syndrome is caused by recessive mu- tant pleiotropic gene.

Reason: Gene mutation leads to more synthesis of fibril- lin proteins.

Answer. 4. If both Assertion and Reason are false, them mark (4).

Question 4. Assertion: In snapdragon, F, plants do not have red or white flowers.

Reason: It is intermediate inheritance with neither of the two alleles of a gene being dominant over each other.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 5. Assertion: en block inheritance of all genes located on the same chromosome may occur in some organisms.

Reason: Dihybrid test cross will have only two pheno- types.

Answer. 4. If both Assertion and Reason are false, them mark (4).

Question 6. Assertion: Morgan’s cross III was conducted in Drosophila to locate genes on chromosome for white eye color.

Reason: The cross was done between red-eyed hybrid female and white-eyed male.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 7. Assertion: Antlers in male deer are sex influenced traits.

Reason: These are controlled by autosomal genes which are influenced by the sex of bearer.

Answer. 4. If both Assertion and Reason are false, them mark (4).

Question 8. Assertion: One drumstick per nucleus is present in the neutrophil of normal female.

Reason: It is absent in the neutrophil of male.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 9. Assertion: Blood group phenotype is controlled by the presence or absence of antigens present on the surface coating of ABC.

Reason: These antigens are of three types and found in the oligosaccharides-rich head regions of a glycophorin.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 10. Assertion: XX-XY type sex determination is found in Coccinia indica.

Reason: Male plant is produced only when Y-chromosome is present irrespective of the number of X-chromosomes.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 11. Assertion: Drosophila melanogaster is widely used in genetic research.

Reason: Drosophila melanogaster is a readily available insect.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 12. Assertion: In humans, the gamete contributed by the male determines whether the child produced will be male or female.

Reason: Sex in humans is a polygenic trait depending upon the cumulative effect of some genes on X-chromosome and some on Y-chromosome.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 13. Assertion: A father may be a hemophilic only if his mother is carrier.

Reason: A father cannot pass on a sex-linked gene to his son.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 14. Assertion: An organism with lethal mutation may not even develop beyond the zygote stage.

Reason: All types of gene mutations are lethal.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 15. Assertion: Cancer cells are virtually immortal until the body in which they reside dies.

Reason: Cancer is caused by damage to genes regulating the cell division cycle.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

NEET Biology Notes For Reproductive Health Introduction

• According to the World Health Organization (WHO), reproductive health means total well-being in all aspects of reproduction, i.e., physical, emotional, social, and behavioral.
• Thus, a society with people who have physically and functionally normal reproductive organs and normal emotional and behavioral interactions among them in all sex-related aspects may be called reproductively healthy.

NEET Biology Notes For Reproductive Health: Problems And Strategies

The problems and strategies of reproductive health in human beings are explained as follows:

• Over-population
  • The main problem of India is its excess population, which is directly connected with reproductive health.
  • To achieve total reproductive health, some plans and programs were started.
  • Family planning program was initiated in 1951 and was periodically assessed.
  • These programs were popularly named Reproductive and Child Healthcare (RCH).
  • The major tasks carried out under these programs are providing facilities and support for building up a reproductive healthy society.
• Awareness about reproduction
  • Audio-visual and print media and government and non-government agencies are doing a good job in creating awareness among people about re- production in humans.
  • Parents, close relatives, friends, and teachers also have a major role in giving the above information.
• Sex education
  Sex education in schools also should be introduced and encouraged to provide right information about myths and misconceptions regarding sex-related aspects.
• Knowledge of growth of reproductive organs and sexually transmitted diseases (STDs)
  Proper information about reproductive organs; adolescence (period of rapid growth between childhood and adulthood); safe and hygienic sexual practices; STDs, e.g., AIDS; etc., would help to lead a reproductive healthy life.
• Birth-control devices and care of mother and child (pre-natal, natal, and post-natal care)
  • Fertile couples and people of marriageable age- group should know about available birth control devices, care of pregnant mothers, post-natal (after birth) care of the mother and child, importance of breast feeding, equal importance for the male and female child, etc.
• Prevention of sex abuse and sex-related crime
  Awareness of problems due to uncontrolled population growth, social evils such as sex abuse and sex-related crimes needs to be created so that people think and take up necessary steps to prevent them and, thereby, build up a reproductively healthy society.
• Information about reproduction-related problems
  • For successful action plans to attain reproductive health, we require good infrastructural facilities, professional expert knowledge, and material support.
  • These are necessary to provide medical help and care for reproduction-related problems such as menstrual problems, infertility, pregnancy, delivery, contraception, abortions, and STDs.
  • Implementation of better techniques and new strategies is also required to provide better care and help to people for reproductive health.
• Amniocentesis Meaning and use
  Amniocentesis is a fetal sex determination and disorder test based on the chromosomal pattern in the amniotic fluid surrounding the developing embryo.
  Procedure
  • Amniotic fluid contains cells from the skin of the fetus and other sources.
  • These cells can be used to determine the sex of the infant, to identify some abnormalities in the number of chromosomes, and to detect certain biochemicals and enzymatic abnormalities.

• • If it is established that the child is likely to suffer from a serious incurable congenital defect, the mother should get the fetus aborted.
  • Misuse of amniocentesis: It is being used to kill normal female fetus. Female feticide is illegal.
• Research in reproductive health area
  • It should be encouraged and supported to find out new methods.
  • “Saheli,” a new oral contraceptive for females, was developed by our scientists at the Central Drug Research Institute (CDRI) in Lucknow, In- dia.
• Medical facilities
  Better awareness about sex-related problems, pre-natal care of mother, medically assisted deliveries, and post-natal care of mother and infant decrease maternal and infant mortality. Small families, better detection and cure of STDs, and increased medical facilities for sex-related problems, etc., indicate improved reproductive health of male and female adults and children.

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NEET Biology Notes For Reproductive Health Measures To Control Over-Population

• Education
  • People, particularly those in the reproductive age- group, should be educated about the advantages of a small family.
  • Mass media and educational institutions can play an important role in this campaign.
  • Posters showing a happy couple and two children with a slogan “Hum Do Humare Do” should be displayed. (Many couples have even adopted “one child norm.”)
• Marriageable age
  Raising the age of marriage is a more effective means to control the population. (Now the marriageable age of females is 18 years and that of males is 21 years.)
• Incentives: Couples with small families should be given incentives.
• Family planning: There are many birth control measures which can check birth rate.

NEET Biology Notes For Reproductive Health Population Explosion And Birth Control

• Rapid increase in population over a relatively short period is called population explosion.
• Census gives information about the number of individuals present in a given region at a given time.

• It means every sixth person in the world is an Indian.
• The time required for a population to double itself is called doubling time.
• The present growth rate of approximately 1.6% per year for India is smaller than the peak of about 2.1% per year during 1965-1970.
• Population growth rate is indicated by (a) the annual average growth rate and (b) the doubling time.
• Growth rate depends on birth (fertility) rate, death (mortality) rate, migration, and age-sex ratio.
• The major reasons for this growth are as follows:
  • A rapid decline in death rate.
  • A decline in maternal mortality rate (MMR).
  • A decline in infant mortality rate (IMR).
  • Increase in the number of people reaching reproducible age.

NEET Biology Notes For Reproductive Health Birth Control

Birth control is the process to prevent conception or pregnancy without interfering with the reproductive health of individuals.

• The characteristics of an ideal contraceptive are that it is (a) user friendly, i.e., comfortable and easy to use, (b) without side-effects, (c) reversible, and (d) completely effective against pregnancy.
• There are several methods of contraception-natural or traditional methods, barriers, IUDs, oral contraceptives, injectables, implants, and surgical methods.
• Couple protection is the process of bringing eligible couples under family planning measures. In India, it is over 55% at present and is voluntary in nature.
• In 2004, there were 60.79 lakh IUD insertions, 48.74 lakh sterilizations, 249.9 lakh condom users, and 87.54 lakh oral pill users.

NEET Biology Notes For Reproductive Health Methods Of Birth Control

• Natural methods
  • These are methods which do not require any device or medicine. So, there are no side-effects; but the chances of failure are very high.
  • Natural methods are of three kinds-safe period, withdrawal, and breast feeding.
    • Safe period (rhythm method)
      • Ovalation occurs roughly about the middle of menstrual cycle.
      • Fertility period is up to 48 h after ovulation, when fertilization can occur.
      • Avoiding sex during the fertility period will naturally prevent conception.
      • Ovulation period can be known by the rise in body temperature by about 1°F; cervical mucus is slippery and can be drawn into a thread (Spinnbarkeit test) when stretched between two fingers.
      • Period prior to ovulation is safe.
      • Period after the fourth day of rise in body temperature (or last positive Spinnbarkeit test) is also considered safe.
      • It is, however, always better to avoid sex from day 10-17 of the menstrual cycle.
    • Withdrawal method (coitus interruptus)
      • The method is based on the withdrawal of penis from the vagina before ejaculation.
      • This method has high failure rate due to pre-ejaculatory release of sperms or failure to withdraw penis from the vagina before ejaculation.
    • Lactational amenorrhea
      • Just after parturition, there is a phase of amenorrhea or absence of menstruation.
      • It is also the phase of intense lactation.
      • Breast feeding the child fully prevents conception.
      • The method is, however, effective only up to a maximum period of 6 months.
• Barrier methods
  • These are mechanical devices which prevent the deposition of sperms into vagina and their pas sage into the uterus.
  • Further, they can be self-inserted by the user in complete privacy.
  • The common barrier methods are condoms, diaphragm, fem shield, and cervical cap.
    • Condom
      • It is a tubular latex sheath which is rolled over the male copulatory organ during sex.
      • The common brand provided by family welfare services is Nirodh.
      • It also provides protection against STDs including AIDS.

• • • Fem shield (female condom)
      • The device is a polyurethane pouch with a ring at either end.
      • The inner ring is smaller and present at the inner closed end.
      • The device covers the external genitalia as well as the lines the vagina.
      • Fem shield provides protection from STDs also.

• • • Diaphragm
      It is a tubular rubber sheath with a flexible metal or spring ring at the margin which is fitted inside the vagina.
    • Cervical cap
      • It is a rubber nipple which is fitted over the cervix and is designed to remain there by suction.
      • The device prevents the entry of sperms into the uterus.

• • • Vault cap
      It is a hemispheric dome-like rubber or plastic cap with a thick rim which is meant for fitting over the vaginal vault over the cervix.
• Chemical methods
  • These are contraceptives which contain spermicidal chemicals.
  • Chemical contraceptives are available in the form of creams (e.g., delfen), jelly (e.g., perceptin, volpar paste), foam tablets (e.g., aerosol foam, chlorimin T or contab), etc.
  • These commonly contain lactic acid, boric acid, citric acid, zinc sulfate, and potassium permanganate.
  • The contraceptives are introduced in the vagina prior to sex.
  • Sponge (today) is a foam suppository or tablet containing nonoxynol-9 as spermicide. It kills the sperm by disrupting the membrane. It is moistened before use to activate the spermicide. The device also absorbs the male ejaculate.
• Intra-uterine devices or IUDs (intra-uterine contraceptive devices or IUCDs)
  • These devices are inserted by doctors or expert nurses in the uterus through vagina. IUDs affect the motility of sperms within the uterus. IUDs can be
    • Non-medicated (e.g., Lippes loop)
    • Copper releasing (e.g., CuT, Cu7, and Multi- load 375): The copper ions suppress the motility and fertilization capacity of sperms.

• • • Hormone releasing (e.g., Progestasert and LNG-20)
      • Hormone releasing IUDs, in addition, make the uterus unsuitable for implantation and the cervix hostile to sperms.
      • IUDs are ideal contraceptives for females who want to delay pregnancy and/or space children. It is one of the most widely accepted methods of contraception in India.
• Oral contraceptives (oral pills)
  • Oral contraceptives are preparations containing either progestin (progestogen or progesterone) alone or a combination of progestogen and oestrogen (estrogen).
  • The pills are taken orally for 21 days in a menstrual cycle starting from the 5th day and ending on the 25th day.
  • However, it is advisable to restart the course after a gap of 7 days irrespective of the onset or non-set of menstruation during the pill-free days.
  • When a pill is missed, it should be taken when- ever one remembers, sometimes two at a time.

• • This helps in keeping the hormonal level optimum for contraception.
  • Hormonal pills act in the following four ways:
    • Inhibition of ovulation.
    • Alternation in uterine endometrium to make it unsuitable for implantation.
    • Changes in cervical mucus impairing its ability to allow the passage and transport of sperms.
    • Inhibition of motility and secretory activity of fallopian tubes.
    • Oral pills are of two types: combined and minipills.
    • Combined pills contain both oestrogen and progestin.
    • These are synthetic products.
    • Oestrogen is an ovulatory that inhibits FSH production. Progestin is anovulatory that inhibits LH production.
    • It protects the endometrial lining from the adverse effect of oestrogen.
    • The hormone also changes cervical mucus.
    • The most commonly used progestin is levonorgestrel or desogestrel.
    • The most common oestrogen is ethinyl oestradiol or menstranol.
    • In monophasic combined pills, both oestrogen and progestin are present in nearly the same amount, e.g., Mala D and Mala L.
    • In multiphasic combined pills, oestrogen is maintained at the same level throughout the 21-day course (0.03 mg) but the amount of progestin is increased (0.05 mg for the first six days, 0.075 mg for the next five days, and 0.125 mg for the last ten days), e.g., triquilar and orthonovum.
    • Minipills are progestin pills only (with no estrogen). They are taken daily without break.
    • Saheli-a non-steroidal preparation-is taken once a week after an initial intake of twice a week dose for 3 months.
• Injectable contraceptives (Depo-Provara)
  • Two types of progestin preparations are used singly: Depot-medroxy progesterone acetate (DMPA) 150 mg every 3 months or 300 mg every 6 months and norethisterone enanthate (NET EN) 200 mg every 2 months.
  • Cyclofem and mesigna are combined injectable contraceptives which are given once every month.
  • They contain progestin preparation as well as oestradiol.
• Implants
  • These are hormone containing devices which are implanted subdermally for providing long-term contraception.
  • Norplant is progestin only. The device comes with six small permeable capsules (34 mm × 2.4 mm each) with about 36 mg levonorgestrel.
  • These are inserted under the skin in a fan-shaped manner inside the upper arm or forearm through a small incision.

• • Suturing is not required. Norplant remains effective for about 5 years.
  • Implanon is a single rod-like device which is implanted through a wide bored needle. It contains 3-keto desogestrel. It remains functional for three years.

• Emergency contraception
  • It is the treatment for unprotected sex, sexual assault, missed pills, and other reasons which have a risk of pregnancy.
  • The drugs used for treating emergency contraceptions are called morning-after pills.
  • These are available in India under the Family Welfare Program since 2002-2003.
  • Two oral tablets to start and two tablets after 12 h provide relief.
  • Other morning-after pills are noral, norgynon, and ovidon.
  • An antiprogesterone pill (mifepristone) is a single-pill treatment.
  • Insertion of IUD within 5 days of unprotected sex prevents implantation.
• Surgical methods of family planning
  • Surgical methods are permanent methods of family planning where there is no need of replacement or augmentation but the reversibility is poor.
  • These are also called the terminal methods of family planning.
  • The methods are operative procedures which block the passage of semen in males and that of ova in females.
  • The techniques are also called sterilization procedures.
  • These are called vasectomy in males and tubectomy in females.
    • Vasectomy (L. vas-vessel, ektome-excision)
      • It is a surgical method of sterilization of males.
      • Vasa deferentia are blocked by cutting and occluding them so that sperms are
    • Conventional vasectomy (scalpel surgery):
      • Under local anesthesia, transverse 1 cm incision is made through the skin of the scrotum with the help of the scalpel over the area of vasa deferentia.

• • • • Each vas deferens is exposed and cut.
      • The two ends are separated and tied.
      • A gap of 1-4 cm is must between the two ends; otherwise reunion can occur.
    • No-scalpel vasectomy
      • Here, instead of scalpel, dissecting forceps and ringed forceps are required.
      • The skin is punctured and the vas deferens is taken out.
      • It is occluded by removal of 1-2 cm followed by ligation of ends.
      • Occlusion can also be achieved by heat and clips.
      • Vasectomy is a reversible procedure as the cut ends can be joined together to open to sperm passage.
    • Tubectomy (L. tubes-pipe, ektome-excision)
      • It is a surgical procedure of female sterilization where a portion of both the fallopian tubes is excised or ligated to block the passage of ovum through them. Tubectomy is performed by conventional transabdominal surgery, conventional laparotomy, and minilaparotomy.
      • In surgical procedures, the fallopian tubes are cut and the cut ends are tied to prevent reunion.
      • The procedure is reversible as the cut ends can be rejoined.
      • In laparoscopic procedure, sterlization is achieved by loop development and constricting the basal region of loop with the help of elastic ring either through a small incision in the abdomen or through the vagina.

Medical Termination of Pregnancy

• Medical termination of pregnancy (MTP) is voluntary or intentional abortion induced and performed to end pregnancy before the completion of full term.
• Worldwide, nearly 20% of the total pregnancies get aborted.
• The number of MTPs is 40-50 million/year.
• Therefore, MTPs play a significant role in the containment of population though these are not performed for this purpose.
• These are mainly meant for removing unsustainable pregnancies.
• Many countries do not have a law about MTPs because the latter involve emotional, ethical, religious, and social issues.
• However, in India, there is a proper act-Medical Termination of Pregnancy Act, 1971.
• It is mainly meant for preventing unnatural maternal deaths due to unsafe abortions (which is 8.9% of the total maternal deaths).
• The act has been amended in 2002.
• Under this act, the termination of pregnancy can be done up to 20 weeks, if pregnancy is likely to produce a congenitally malformed child, is a result of rape and contraceptive failure, or is likely to harm the mother.
• MTP is safe if it is performed up to 12 weeks (first trimester) of pregnancy.
• Misoprostol (prostaglandin) along with mifepristone (antiprogesterone) is an effective combination.
• Vacuum aspiration and surgical procedures are adopted thereafter.
• Abortions in the second trimester are risky.
• These are generally performed after testing the sex of the baby through amniocentesis or sonography.
• This has resulted in large-scale female feticide and complications due to unsafe abortions in the hands of untrained persons.
• To prevent such happening, the government has enacted a law-Prenatal Diagnostic Techniques (Regulation and Prevention of Misuse) Act, 1994-with amendments in 2003.
• It prohibits preconception and prenatal sex determination.

STDs

• The general term STD is applied to any of the large group of diseases that can be spread by sexual contact.
• The group includes conditions traditionally specified as venereal diseases (VD), such as chlamydia, gonorrhea, syphilis, and genital herpes.
• AIDS and hepatitis, which are STDs, can be contracted in other ways also.

NEET Biology Notes For Reproductive Health Infertility

• Infertility (L. in-not, fertilis-fruitful) is failure to conceive even after 1-2 years of regular unprotected sex.
• The term is not a synonym of sterility which means complete inability to produce an offspring.
• Infertility can best be defined as relative sterility.
• It is of two types: primary and secondary.
• Primary infertility is the infertility found in patients who have never conceived.
• Secondary infertility is the infertility found in patients who have previously conceived.
• Infertility is caused by defects found in males, females, and in both of them.

Infertility in Males

• The semen of a fertile male is 2.5-5 ml per ejaculation with a sperm count of over 200-300 million, mostly motile, having proper fructose content and fluidity which is deposited high in the vagina.
• Any defect in the sperm count, sperm structure, and sperm motility of seminal fluid leads to infertility.
• Low sperm count is called oligospermia while the near absence of sperms is known as azoospermia.
• Low sperm motility is called asthenozoospermia while defective sperm morphology is termed as teratozoospermia.

Infertility in Females

• A fertile woman is the one who regularly ovulates once every cycle and passes the egg down the reproductive tract which develops conditions for the smooth pas- sage of sperms and the implantation of fertilized egg.
• The various causes of infertility in females are as follows:
  • Anovulation (nonovulation) and oligoovulation (deficient ovulation).
  • Inadequate growth and functioning of corpus luteum.
  • Ovum not liberated but remains trapped inside the follicle due to hyperprolactinaemia.
  • Chances of failure of fallopian tube to pick up ovum.
  • Noncanalization of uterus.
  • Defective uterine endometrium.
  • Fibroid uterus.
  • Defects in cervix.
  • Defective vaginal growth.

NEET Biology Notes For Reproductive Health Assisted Reproductive Technologies

• More than two decades ago, in an experimental procedure called in vitro fertilization (IVF), doctors joined a woman’s egg and a man’s sperm in a glass dish on the laboratory table.
• For the first time, fertilization happened outside a woman’s body. Nine months later, the first test-tube baby was born.
• Today, assisted reproductive technology (ART) refers not only to IVF but also to several variations tailored to patient’s unique conditions.
• These procedures are usually paired with more conventional therapies, such as fertility drugs, to increase success rates.
• Almost one out of every three cycles of ART results in the birth of a baby.
• But ART procedures are invasive and expensive.
• Though no long-term health effects have been linked to children born using ART procedures, most doctors recommend reserving ART as the last resort for having a baby.
• Following is the list of important techniques which can benefit infertile couples.
  • IVF and ET
    IVF is the fertilization outside the body, in almost similar conditions as that in the body, which is followed by embryo transfer (ET). In this method, popularly known as test-tube baby method, ova from the wife/donor (female) and sperms from the husband/donor (male) are collected and induced to form zygote under simulated conditions in the laboratory. The zygote or early embryos are then transferred into the fallopian tube or uterus to complete their further development.
  • ZIFT
    In ZIFT (zygote intra-fallopian transfer), the zygote, formed in vitro, or early embryo up to 8 blastomeres, is transferred into the fallopian tube.
  • IUT
    In IUT (intra-uterine transfer), the embryos with more than 8 blastomeres are transferred into the uterus for further development. The embryos formed by in vivo fertilization can also be used for transfer to assist the females who cannot conceive.
  • GIFT
    GIFT (gamete intra-fallopian transfer) is the transfer of an ovum collected from a donor into the fallopian tube of the recipient who can pro- vide suitable environment for fertilization and further development.
  • ICSI
    ICSI (intra-cytoplasmic sperm injection) is an- other specialized procedure to form an embryo in the laboratory. In this method, a sperm is directly injected into the ovum.
  • AI
    Al (artificial insemination) is used for the cases of infertility which is either due to the inability of the male partner to inseminate the female or due to very low sperm count in the ejaculate.
    In this technique, the semen collected either from the husband or a healthy donor is artificially introduced either into the vagina or the uterus (IUI- intra-uterine insemination) of the female.
    ART requires extremely high precision handling by specialized professionals and expensive instrumentation. The infertility facilities are presently available only in a few centers in the country.
    Obviously, their benefits are affordable only to a limited number of people. Emotional, religious, and social factors are also involved in the adoption of these methods.
  • Adoption
    Our law also permits legal adoption. Adoption can benefit not only the people who are looking for parenthood but also many orphaned and destitute children in India, who would probably not survive till maturity, unless taken care of.
  • Surrogacy or use of a gestational carrier
    In this method, another woman carries embryo or a donor embryo. It is termed as surrogacy.

NEET Biology Notes For Reproductive Health Assertion-Reasoning Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).
2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
3. If Assertion is true but Reason is false, then mark (3).
4. If both Assertion and Reason are false, then mark (4).

Question 79. Assertion: Population of India crossed 1 billion in May 2000.

Reason: It is the result of rapid decline in death rate, maternal mortality rate (MMR), and infant mortality rate (IMR) as well as an increase in the number of people in reproducible age.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 80. Assertion: Intra-uterine devices (IUDs) are very effec- tive contraceptive method.

Reason: IUDs do not allow sperms to enter uterus.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 81. Assertion: Surgical method blocks gamete transport and thereby prevents conception.

Reason: Surgical method used in males for this purpose is called vasectomy.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 82. Assertion: Saheli-the new oral contraceptive for fe- males contains a non-steroidal preparation.

Reason: It is “once-a-week” pill with very few side- effects and high contraceptive value.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 83. Assertion: In test-tube baby program, ova from wife/donor (female) and sperms from husband/donor (male) are collected and are induced to form zygote under simulated conditions in laboratory.

Reason: Embryos with more than 8 blastomeres are then transferred to fallopian tube (ZIFT) to complete its further development.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 84. Assertion: Surgical methods of contraception are prac-ticed to space two successive conceptions.

Reason: During surgical methods, ovaries from females or testes from males are removed.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 85. Assertion: Natural methods are based on menstrual cycle and the life of sperms.

Reason: Natural methods often fail to contracept.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 86. Assertion: Sexually transmitted diseases get transmitted from the infected to the normal person, only during sexual contact.

Reason: All sexually transmitted diseases can be cured by antibiotics.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 87. Assertion: In 1900, world population was 2000 million.

Reason: Indian population crossed 2000 million mark in May 2000.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 88. Assertion: Marriageable age of Indian females and males is 18 years and 21 years, respectively.

Reason: Under normal conditions, a girl child will re- lease around 450 ova in her lifetime.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

NEET Biology Notes For Human Reproduction Introduction

• Human beings are unisexual.
• The growth, maintenance, and functions of gonads are regulated by gonadotropins secreted from the anterior lobe of pituitary gland.
• The organs that neither produce gametes nor secrete sex hormones but perform important functions in reproduction are called secondary sex organs.
• These include prostate, seminal vesicles, vas deferens, and penis in males; and fallopian tubes, uterus, vagina, and mammary glands in females.
• The characters that distinguish a male from a female externally are called accessory or external sex characters or secondary sex characters.

human reproduction

NEET Biology Notes For Human Reproduction Male Reproductive System

• The male reproductive system of mammals consists of a pair of testes, several accessory glands, a duct system, and a mating organ called the penis. Testis is the primary male sex organ. It produces spermatozoa and secretes the male sex hormone testosterone.
• A human testis measures about 5 cm, 3 cm, and 2.5 cm, respectively, in length, thickness, and width. It is covered by thick, fibrous, connective tissue called tunica albuginea.
• In a man, both testes normally remain suspended in a pouch called scrotum outside the abdominal cavity. This keeps the testes at a low temperature than the body temperature (about 2°C less). This is essential for the maintenance and normal functioning of the spermatogenic tissue of testes.
• Testes descend in the scrotal sac when fetus is about 7 months old. This occurs under the influence of follicle-stimulating hormone (FSH) and testosterone.
• If testes fail to descend, then the condition is called cryptorchidism. It leads to sterility. Scrotum remains connected with the abdomen or pelvic cavity by the inguinal canal. Blood vessels, nerves, and conducting tubes pass through the inguinal canal.
• Cremaster muscles and connective tissue form spermatic cord and surround all structures passing through the inguinal canal. Cremaster muscles and dartos muscles of the scrotal sac help in the positioning of testes.
• When the outside temperature is low, these contract to move the testes close to the abdominal cavity/pelvic cavity. When the outside temperature is high, these relax moving the testes away from the body.
• In some seasonally breeding mammals, testes descend into the scrotum during the breeding season but ascend back into the abdomen in the non-breeding season. For example, rats and bats.

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NEET Biology Notes For Human Reproduction Anatomy Of Testis

• Each testis contains numerous tiny, highly convoluted tubules, called seminiferous tubules. These constitute the spermatogenic tissue of the testis.
• Cells lining these tubules give rise to spermatozoa which are released into the lumen of the tubule.
• In between spermatogenic cells, Sertoli or sustentacular or nurse cells are present which provide nourishment to the developing spermatozoa and regulate spermatogenesis by releasing inhibin to check FSH overactivity.
• The other functions of Sertoli cells include the following:
• • Providing nourishment to the developing spermatozoa.

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• • Absorbing the parts being shed by the developing spermatozoa.
  • Releasing anti-Müllerian factor (AMF) to prevent the development of Müllerian duct/oviduct.
  • Releasing androgen binding protein (ABP).
• Groups of polyhedral cells called interstitial cells, or the Leydig cells, are located in the connective tissue around the seminiferous tubules.
• They constitute the endocrine tissue of the testis. The Leydig cells secrete testosterone into the blood.
• Seminiferous tubules unite to form several straight tubules called tubuli recti which open into irregular cavities in the posterior part of the testis.
• Rete testis is a highly anastomosing labyrinth of cuboidal epithelium lined channels.
• Several tubes called vasa efferentia arise from it and conduct spermatozoa out from the testis. (Seminiferous tubule to vasa efferentia form intratesticular genital duct system).

• The extratesticular duct system consists of tubes which conduct sperms from the testes to the outside.
• It starts with vasa efferentia, which arise from each testis, and becomes confluent to form a folded and coiled tube called epididymis behind each testis.
• The epididymis consists of three parts: (a) Caput, (b) corpus, and (c) cauda. It stores the sperms temporarily.
• From cauda epididymis, a partially coiled tube called vas deferens ascends into the abdomen through the inguinal canal, passes over the urinary bladder, and receives the duct from the seminal vesicle behind the urinary bladder to from an ejaculatory duct.

• Before entering prostate, the ductus deferens dilates to form ampulla. The final portion of ampulla passes through the prostate to open into the urethra shortly after its origin from the urinary bladder.
• Urethra receives the ducts of the prostate and Cowper’s glands, passes through the penis, and opens to the outside.

NEET Biology Notes For Human Reproduction Male External Genital Organ

Penis

• Penis is the copulatory organ of man. It is a cylindrical and erectile, pendulous organ suspended from the pubic region in front of scrotum.
• It remains small and limp (or flaccid) but on sexual arousal, it becomes long, hard, and erect- ready for copulation (or coitus or intercourse). Erect human penis, on an average, is about 15 cm long.

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• The penial mass is itself encased in a fibrous sheath, called tunica albuginea. The interior of penis is mostly formed of three cylindrical cords of spongy, erectile (cavernous) tissue.
• Two of these cords are thicker and situated parallely on the right and left sides, forming the thick part of penis that remains in the front when the penis is limp, but becomes superio-posterior when it is erect.
• These two cords are called corpora cavernosa.
• The fibers of tunica albuginea surround both cords jointly and form a separate sheath around each cord. Some fibers form a partition, called septum penis, between these cords.
• The third, smaller cord forms that part of penis which remains inferio-anterior in erect penis. Urethra runs through this cord. Hence, this cord is called corpus urethrae or spongiosum.
• The extended part of corpus spongiosum is enlarged, forming a bulged, conical structure called glans penis. The surface of glans is formed of a thin, smooth, shiny, and hairless skin.
• The base line of glans is referred to as the neck of the penis. The loose skin of penis folds over and is retractile on glans. It is called foreskin or prepuce.
• At the tip of glans penis is a slit-like external urethral orifice or meatus by which urethra opens out and discharges urine or semen.
• Preputial glands, present in the skin of the penis neck, secrete a white sebaceous substance, called smegma. Microbial infection in smegma causes irritation.

Glands

• Seminal vesicles: These are paired, tubular, coiled glands situated behind the urinary bladder. They secrete viscous fluid that constitutes the main part of the ejaculate. Seminal fluid contains fructose, citric acid, inositol, and prostaglandins.
• Prostate gland: It is a chestnut-shaped gland and is a collection of 30-40 tubuloalveolar glands. It lies at the base of the bladder and surrounds the base of the ure- thra. It contributes an alkaline substance to the seminal fluid.
• The substance of prostate helps the sperms to become active and counteract any adverse effects of urine on sperms. The prostatic fluid provides a characteristic odor to the seminal fluid. Prostate gland secretes citrate ion, calcium, phosphate ion, and profibrinolysin.
• Bulbourethral glands or Cowper’s glands: The two bulbourethral glands are pea-sized structures lying adjacent to the urethra at the base of penis. These secrete a viscous lubricant.
• Duct system, accessory glands, and penis are secondary male sex organs. Their growth, maintenance, and functions are promoted by testosterone secreted by the Leydig cells.
• On the other hand, the growth, maintenance, and functions of seminiferous tubules and the Leydig cells are regulated, respectively, by FSH and interstitial cell stimulating hormone (ICSH) of anterior pituitary.

Semen

• Semen is a mixture of sperms and the secretions of seminiferous tubules, seminal vesicles, prostate gland, and bulbourethral glands.
• The average volume of semen in an ejaculation is 2.5-5 ml, with a sperm count (concentration) of 200-300 million. For normal fertility, at least 60% sperms must have normal shape and size, and at least 40% of them must show vigorous motility. When the sperm count falls below 20 million/ml, the male is likely to be infertile.
• Semen has a slightly alkaline pH of 7.2-7.7. The prostatic secretion gives semen a milky appearance, whereas fluids from the seminal vesicles and bulbourethral glands give it a sticky consistency.
• Semen provides transportation medium and nutrients to sperms. It neutralizes the hostile acidic environment of the male urethra and the female vagina.

NEET Biology Notes For Human Reproduction Female Reproductive System

• The female reproductive system consists of a pair of ovaries, a duct system consisting of a pair of fallopian tubes (oviduct)-uterus, cervix, and vagina. The pair of mammary glands is accessory genital glands.

Ovaries

• Ovary is the primary female sex organ. It produces ova and secretes female sex hormones, estrogen, and progesterone that are responsible for the development of secondary female sex characters and regulate cyclic changes in uterine endometrium.
• Human ovaries are small, almond-like flat bodies about 3 cm in diameter.

Location

Ovaries are located near kidneys and remain attached to the lower abdominal cavity through mesovarium.

Structure

The free surface of the ovary is covered by a germinal epithelium made of a single layer of cubical cells. This epithelium is continuous with the mesothelium, called peritoneum. The epithelium encloses the ovarian stroma. The stroma is divided into two zones a peripheral cortex and an inner medulla. The cortex is covered by a connective tissue, called tunica albuginea, and it contains numerous spherical or oval, sac-like masses of cells, known as ovarian follicles. The medulla consists of loose connective tissue, elastic fibers, blood vessels, and smooth muscle fibers.

Ovarian Follicle

• Ovarian follicle carries a large, centrally placed ovum, surrounded by several layers of granular cells (follicular granulosa or discus proligerus or cumulus oophorus). It is suspended in a small cavity-the antrum.
• Antrum is filled with liquid folliculi. The secondary oocyte in the tertiary follicle forms a new membrane called zona pellucida. The follicle bulges on the surface of the ovary. Such a follicle is called mature Graafian follicle (after De Graaf, who reported them in 1672).

Corpus Luteum

• The ovum is shed from the ovary by rupturing the follicle. The release of ovum is called ovulation and occurs nearly 14 days before the onset of the next menstrual cycle.
• After the extrusion of the ovum, the Graafian follicle transforms into corpus luteum. Corpus luteum is filled with a yellow pigment, called lutein.
• Corpus luteum grows for a few days and if the ovum is fertilized and implantation occurs, then it continues to grow. But if the ovum is not fertilized, then corpus luteum persists only for about 14 days.
• It secretes progesterone. At the end of its functional life, it degenerates and gets converted to a mass of fibrous tissue, called corpusalbicans (white body). It remains as a scar in the ovary throughout the life of a female.

Fallopian Tubes (Oviducts)

• Oviducts are a pair of long (about 10 cm), ciliated, muscular, tubular structures that extend from ovaries to uterus. Each is suspended by mesosalpinx. Each fallopian tube is differentiated into four parts:
• • Infundibulum: The part of oviduct closer to the ovary is the funnel shaped infundibulum. Its edges possess finger-like projections called fimbriae. Fimbriae help in the collection of the ovum after ovulation. Infundibulum opens in the abdominal cavity by an aperture called osteum.
  • Ampulla: The infundibulum leads to a wider part of the oviduct called ampulla.
  • Isthmus: It is the middle, narrow, ciliated part of the oviduct.
  • Uterine part: It is the inner, narrow part which opens in the upper part of uterus. It is involved in the conduction of ovum or zygote towards the uterus by peristalsis and ciliary action. (Fertilization occurs at the junction of ampulla and isthmus.)
• Uterus: It is a large hollow, muscular, highly vascular, and pear-shaped structure present in the pelvis between the bladder and rectum. It is suspended by a mesentery-mesometrium. It is formed of three parts.
  • Fundus: It is the upper dome-shaped part above the opening of fallopian tubes.
  • Body: It is the middle and main part of the uterus.
  • Cervix: It is the lower, narrow part which opens in the body of the uterus by internal os and in vagina below by external os. It is formed of the most powerful sphincter muscle in the body.

Its wall is formed of outer peritoneal layer (called perimetrium); middle muscular myometrium made of smooth muscle fibers, and inner, highly vascular and glandular endometrium.

It is the site of fetal growth during pregnancy. It also takes part in placenta formation and helps in pushing the baby out during parturition.

Vagina

• Vagina is a long (7.5 cm), fibro-muscular tube. It extends backward in the front of rectum and cervix to the vestibule. It is a vascular tube internally lined by mucus membrane and is raised into transverse folds called vaginal rugae.
• In a virgin female, vaginal orifice is closed by a membranous diaphragm called hymen which becomes centrally perforated at puberty for the discharge of menstrual flow (or menses).
• Vagina acts both as copulation canal (as it receives sperms from penis during copulation) and as birth canal during parturition.

NEET Biology Notes For Human Reproduction Female External Genital Organ

• Vulva: It is the external genitalia of females. It has a depression-the vestibule-in front of anus. A vestibule has two apertures-upper external urethral orifice of urethra and lower vaginal orifice of vagina.
• Mons pubis: It is a fleshy and fatty tissue covered by skin and pubic hair.
• Labia majora: It is a pair of fleshy folds which extend from mons pubis and surround the vaginal opening.
• Labia minora: It is another pair of tissue folds below the labia majora.
• Both labia majora and labia minora are provided with sebaceous glands.
• Hymen: It is a membrane that partially covers the vaginal opening. It gets torn during the first coitus.
• Clitoris: It is a tiny, erectile, finger-like structure present at the upper junction of the two labia minora above the urethral opening. The fold of skin that covers the clitoris is called prepuce. Clitoris is homologous to penis (as both are supported by corpora cavernosa).

NEET Biology Notes For Human Reproduction Glands

Vestibular Glands

Vestibular glands are of two types-greater and lesser.

• Greater vestibular or Bartholin’s glands are a pair of small reddish-yellow glands on each side of vaginal orifice. These secrete alkaline secretion for lubrication and neutralizing urinary acidity.
• Lesser vestibular or paraurethral or Skene’s glands are small mucus glands present between urethral and vaginal orifices.

Mammary Gland

• Each mammary gland consists of 15-25 lobules of the compound tubuloalveolar type. These lobules secrete milk to nourish the newborn babies.
• Each lobe is separated from the others by dense connective and adipose tissue and represents a gland. From each lobe, excretory lactiferous ducts emerge independently in the nipple, which has 15-25 openings, each about 0.5 mm in diameter.

• However, the histological structure of mammary glands varies, depending on sex, age, and physiological state.

Path of Milk Ejection

Mammary alveolus → Mammary duct→ Ampulla → Lactiferous duct → Nipple

NEET Biology Notes For Human Reproduction Events In Human Reproduction

Gametogenesis→ Insemination → Fertilization → Implantation → Gestation → Parturition

NEET Biology Notes For Human Reproduction Formation Of Gametes

• Sexual reproduction requires the fusion of two haploid gametes to form a diploid individual. These haploid cells are produced through gametogenesis.
• As there are two types of gametes-spermatozoa and ova gametogenesis can be studied under two broad headings, namely spermatogenesis and oogenesis.
• Spermatogenesis is the formation of spermatozoa, whereas oogenesis is the formation of ova. Both spermatozoa and ova originate from primordial germ cells (PGCs), which are extragonadal in origin.
• In humans, the PGCs originate during early embryonic development from the extra-embryonic mesoderm. Eventually, they migrate to the yolk sac endoderm and, ultimately, to the gonads of the developing embryo, where they undergo further development.

NEET Biology Notes For Human Reproduction Spermatogenesis

• Spermatozoa are produced in the seminiferous tubules of testes. Spermatogenesis is the process of maturation of reproductive cells in the testes.
• Spermatogenesis includes two stages: (a) formation of spermatids and (b) metamorphosis of sperma- tids. Spermatids are formed in three phases, namely (a) phase of multiplication (mitosis), (b) phase of growth, and (c) phase of maturation (meiosis).
• During the phase of multiplication, the primordial germ cells divide repeatedly by mitosis to form diploid spermatogonia.

• During the phase of growth, spermatogonium enlarges in size to form primary spermatocyte and prepares to undergo maturation division.
• During the phase of maturation, the primary spermatocyte undergoes meiosis 1 giving rise to two haploid (n) secondary spermatocytes. The secondary spermatocytes undergo meiosis 2 resulting in the formation of four spermatids.
• The transformation of spermatid to sperms is termed as spermiogenesis. A spermatid is non-motile. It has organelles such as mitochondria, Golgi bodies, centrioles, and nucleus.
• During spermiogenesis, the weight of gamete is reduced along with the development of locomotory structures. The nucleus becomes compact forming the major part of head of spermatozoa.
• The Golgi complex of spermatid gives rise to acrosome. Acrosome forms a cap in front of the nucleus containing lytic agent. It dissolves egg membranes during fertilization.
• The acrosome of mammalian sperm produces sperm lysins. The two centrioles of spermatids become arranged one after the other behind the nucleus.
• The anterior one is known as proximal centriole. It is usually located on the neck of spermatozoan. During fertilization, it is introduced into the egg and is required for the first cleavage.

• The posterior centriole is known as distal centriole. It gives rise to the axial filament of the sperm. Mitochondria from different parts of spermatid get arranged in the middle piece around the axial filament. Mitochondria in the middle piece provide energy to the sperm for locomotion.
• A typical mammalian sperm is flagellated, consisting of four parts, namely head, neck, middle piece, and tail.
• The human sperm was first seen by Hamm and Leeu- wenhoek. Tail-less (non-flagellate), “amoeboid” sperm is found in the roundworm Ascaris.

Hormonal Control of Spermatogenesis

• Spermatogenesis is under the control of endocrine hormones. Hypothalamus produces gonadotropin releasing hormone (GnRH).
• It acts on anterior pituitary to produce gonadotropins, ICSH, and FSH. ICSH acts on the interstitial or Leydig’s cells which produce testosterone.
• Testosterone is essential for the formation of sperms, at least the spermiogenesis part by the Sertoli cells. Under the influence of FSH, the Sertoli cells develop ABP.
• The latter helps in concentrating testosterone in the seminiferous tubules.

• Excess of testosterone inhibits LH/ICSH (luteinizing hormone/interstitial cell stimulating hormone) by ante- rior pituitary and GnRH production by hypothalamus.
• The Sertoli cells also produce a glycoprotein called inhibin. Inhibin suppresses FSH synthesis by anterior pituitary and GnRH synthesis by hypothalamus.
• Thus, the normal release of testosterone is under negative feedback control.

NEET Biology Notes For Human Reproduction Oogenesis

• Oogenesis is the process of maturation of reproductive cells in ovary. Oogenesis starts before birth. In 25-weeks-old female fetus, all oogonia are produced.
• Oogenesis is basically similar to spermatogenesis. It includes the phase of multiplication, the phase of growth, and the phase of maturation.
• During the phase of multiplication, the primordial cells in the ovary divide mitotically to form oogonia (egg mother cell). Each oogonium divides mitotically to form two primary oocytes.
• Primary oocytes undergo growth. The growth phase during oogenesis is comparatively longer.
• Primary oocytes begin the first step of meiosis 1 and proceed up to diakinesis.
• These oocytes resume their development at puberty. The primary oocyte (2n) completes meiosis 1 producing two haploid cells (n)-the larger one is secondary oocyte and the smaller one is first polar body.
• The secondary oocyte starts meiosis 2 and proceeds up to metaphase 2 only. Further development will start only after the arrival of spermatozoa.
• The entry of sperm restarts the cell cycle by breaking down MPF (M-phase promoting factor) and turning on APC (anaphase promoting complex). The completion of meiosis 2 results in the formation of functional egg or ovum and a second polar body.

“reproduction in humans “

Hormonal Control of Oogenesis

• In response to the production of GnRH, anterior pituitary secretes two hormones-FSH and LH .
• FSH stimulates follicular growth and maturation of oocyte. Granulosa cells of developing ovarian follicle produce estrogen.
• In the presence of high titer of both estrogen and LH, ovulation occurs. High concentration of estrogen inhibits secretion of both FSH and GnRH.

• This is negative feedback control. LH helps in converting ruptured Graafian follicle into corpus luteum.
• The latter secretes progesterone which prepares the uterus to receive fertilized ovum. High concentration of progesterone inhibits further release of LH from anterior pituitary and that of GnRH from hypothalamus.

NEET Biology Notes For Human Reproduction Menstrual Cycle

• Menstrual cycle is the cyclic changes in the reproductive tract of primate (man, monkey, and apes) females. Menstruation is the periodic shedding of the endometrium of uterus with bleeding. In healthy women, menstruation occurs at intervals of about 28-29 days.
• Menarche is the starting of menstruation in girls. It occurs at about 13 years of age. Menstrual cycle consists of menstrual phase, proliferative phase (follicular phase), and secretory phase (luteal phase).
• The proliferative phase (5th to 14th day of menstrual cycle) consists of the growth of the endometrium of uterus, the fallopian tube, and the vagina. In ovary, a Graafian follicle secretes estrogen during this phase.
• Estrogen is the hormone active during the prolifera- tive phase. Ovum is released from the follicle near the end of the proliferative phase, i.e., 14th day or midway during menstrual cycle.
• Ovulation occurs under the influence of LH from pituitary. The subsequent 14 days in which corpus luteum is active make up the secretory phase.
• Progesterone secreted by corpus luteum is active during the secretory phase. Uterine endometrium and glands grow further during this phase.
• At the end of the secretory phase, corpus luteum degenerates into corpus albicans in the ovary, progesterone secretion falls, the overgrown uterine endometrium breaks down, and mensturation takes place.
• Menstrual cycle is controlled by FSH, LH, estrogen, and progesterone. Menstrual cycle and menstruation remain suspended during pregnancy and lactation. Menopause (climacteric) is the period of life when menstruation naturally stops.
• Menopause occurs in females at the age of around 45-50 years. The ability to reproduce is lost in females after menopause.

NEET Biology Notes For Human Reproduction Estrous Cycle

• The estrous cycle consists of cyclic changes in the female reproductive system of non-primate mammals. There is no menstruation at the end of estrous cycle. The estrogen level in blood increases resulting in strong sex urge in the female. This is called the “period of heat.”
• The estrous cycles run only during breeding season these remain suspended in females during non-breed- ing season. The suspension of estrous cycles is called the state of anestrum.
• Animals that have only a single estrous during the breeding season are called monoestrous. For example, dog, fox, deer, bat, etc.
• Animals that have recurrence of estrous cycle during the breeding season are called polyestrous. For example, mouse, squirrel, cow, sheep, pig, horse, etc.

NEET Biology Notes For Human Reproduction Events In Mammalian Reproduction

Fertilization

• Ovum is released in the secondary oocyte stage (arrested in metaphase 2). Due to ciliary current produced by fimbriae of oviduct, ovum is drawn in through ostium.
• It reaches ampulla the site of fertilization by the ciliary action of ciliated columnar epithelium lining of oviduct.

• A human sperm can live for many weeks in the male genital duct. Once ejaculated, the sperm can live alive only for 24-48 h outside the body. Sperms move in the liquid medium secreted by the female genital tract (1.5-3 mm/min).
• Prostaglandins of semen help in the movement of sper- matozoa and finally reach the ampulla portion of the oviduct.
• Capacitation of sperm occurs in the female genital system due to the following factors:
  • Removal of membrane cholesterol present over acrosome, weakening the membrane cover.
  • Dilution of decapacitation factors.
  • Entry of Ca2⁺ into sperms causing rapid whiplash motion of the tail.
• Fusion of gametes/syngamy: The various steps are as follows:
  • Acrosomal reaction: The sperms adhere to the surface of egg covers (agglutination). The acro- some starts releasing its hydrolytic enzymes (sperm lysins).
    It includes the following:

• • • Hyaluronidase: It dissolves the hyaluronic acid responsible for the cementing of follicle cells or granulosa cells.
    • Corona digesting enzyme (CDE): It dissolves corona radiata.
    • Zona lysin/acrosin: It digests zona pellucida. It involves zona pellucida compatibility reaction determined by the protein fertilizin over zona pellucida and antifertilizin in case of sperm.
  • The contact of acrosome stimulates the development of an outgrowth by the oocyte called the fertilization cone or the cone of reception.
  • As the sperm head gets in contact with the fertilization cone, it causes the opening up of Na* channel to cause the depolarization of membrane (fast block to check polyspermy) and Ca2⁺ wave inside the egg.
  • Sperm and egg membranes dissolve. Male pronucleus and proximal centriole of sperm enter the cytoplasm of egg and the rest part is left out.
  • Ca2⁺ wave causes the extrusion of cortical granules (cortical reaction) and the zona reaction makes zona pellucida impervious to the second sperm by destroying sperm receptors.
  • Cortical reaction and zona reaction constitute slow block to check polyspermy.
  • The entry of sperm causes the breakdown of metaphase promoting factor (MPF) and turns on anaphase promoting complex (APC). This results in oocyte completing its meiosis 2.
  • Male and female pronuclei approach each other and, finally, mixing up of paternal and maternal chromosomes (amphimixis) occurs resulting in the formation of zygote/synkaryon.

Embryonic Development

Embryonic development includes cleavage, blastulation, implantation, gastrulation, and organogenesis.

Cleavage

• The first cleavage is completed after 30 h of fertiliza- tion. Cleavage furrow passes from the animal vegetal axis as well as the center of zygote (meridional cleavage).
• It divides the zygote into two blastomeres (holoblastic cleavage). The second cleavage is completed after 60 h of fertilization.
• It is also meridional but at right angle to the first one. It is completed earlier in one of the two blastomeres, resulting in transient three-celled stage.
• The third cleavage is horizontal forming 8 blastomeres. It is slightly unequal. Thereafter the rate and pattern of cleavage are not specific.

Morula

• Cleavage results in solid ball of celled morula with 16 cells (occasionally 32 cells). Zona pellucida is still present as the outer cover. Morula undergoes compaction.
• The outer/peripheral cells are small/flat with tight junction while the inner cell mass is slightly large round and with gap junction.
• Morula descends slowly towards the uterus in 4-6 days and corona radiata detaches during this period.

Blastulation or Blastocyst Formation

• Endometrium secretes a nutrient fluid and its mucosal cells become enlarged with stored nutrients. As the morula enters uterus, it obtains enriched supply of nu- trients.
• The outer peripheral cells enlarge and flatten further. They form trophoblast or trophoectoderm. Trophoblast secretes a fluid into the interior. It creates a cavity called blastocoel.
• The inner cell mass now comes to lie on one side as embryonal knob.
• With the formation of blastocoel, morula is converted into blastula which is called blastocyst in mammals because of different nature of surface layer and eccentric inner cell mass.

• Due to the pressure of growing blastocyst, a slit is produced in zona pellucida. The growing blastocyst comes out. At times, it gets broken into two parts which then give rise to identical twins.
• Trophoblast cells in contact with the embryonal knob are called the cells of Rauber. The area of embryonal knob represents animal pole.
• The opposite side is embryonal pole. Soon embryonal knob shows rearrangement to form embryonal disc. The cells of trophoblast layer divide periclinally.
• This makes trophoblast two layered-outer syncyto trophoblast and inner cytotrophoblast. The two layers later form the chorion, amnion, and fetal part of placenta.

Implantation

• Implantation is the embedding of blastocyst into the endometrium of uterus.
• Blastocyst comes in contact with endometrium in the region of embryonal knob or embryonic disc. It adheres to the same.
• The surface cells of trophoblast secrete lytic enzymes which cause the corrosion of endometrial lining.

• They also give rise to finger-like outgrowths called villi. Villi not only help in fixation but also in the absorption of nourishment.
• Implantation causes nutrient enrichment, enlargement of cells, and formation of uterine part of placenta called decidua (L. deciduus-falling off).
• Decidua has three regions:
  • Decidua basalis (basal decidua, tunica serotina): It is the part of decidua underlying the chorionic villi and overlying the myometrium.
  • Decidua capsularis (decidua reflexa): It lies between the embryo and the lumen of uterus.
  • Decidua parietalis (decidua vera): It is the part of decidua that lines the uterus at a place other than the site of attachment of embryo.
• Trophoblast secretes a hormone called human cho- rionic gonadotropin (hCG). The detection of hCG in urine is the basis of pregnancy/Gravindex test.
• hCG maintains the corpus luteum beyond its normal life. It continues to secrete progesterone which pre- vents menstruation and maintains the uterine lining in nutrient-rich state.
• Progesterone induces cervical glands to secrete viscous mucus for filling the cervical canal to form a protective plug.
• Progesterone is also called pregnancy hormone as it is essential for the maintenance of pregnancy. The hormone is secreted by placenta as well.

Gastrulation

• Gastrulation is characterized by the movement of cells in small masses or sheets so as to form primary germinal layers. There are three primary germinal layers: (a) endoderm , (b) ectoderm, and (c) mesoderm.
• The cell movements that occur during gastrulation are called morphogenetic movements since they lead to the initiation of morphogenesis. The product of gastrulation is called gastrula.

Formation of Primary Germinal Layers

• The cells of inner cell mass or embryonal knob get rearranged to form a flat embryonic or germinal disc. The latter differentiates into two layers-outer epiblast of larger columnar cells and inner hypoblast of smaller cuboidal cells.
• Gastrulation begins with the formation of primitive streak on the surface of the epiblast.
• Cross-section through the cranial region of the streak after 15 days showing the invagination of epiblast cells. The first cells to move inward displace the hypoblast to create a definitive endoderm. Primitive node

• Once the definitive endoderm is established, inwardly moving epiblast forms mesoderm.
• Cells remaining in the epiblast form ectoderm. Thus, epiblast is the source of all germ layers in the embryo.

NEET Biology Notes For Human Reproduction Placenta

• Placenta is an organ that connects the fetus and uterine wall.
• It is contributed by both maternal as well as fetal part although there is no blending of the maternal and fetal blood supplies. Placenta acts as an ultrafilter. Soluble inorganic and organic materials; nutrients; hormones; and antibodies against diphtheria, small pox, scarlet fever, measles, etc., can pass from the mother to the fetus.

• Placenta acts as an endocrine gland and synthesizes large quantities of proteins and some hormones, such as hCG, chorionic thyrotropin, chorionic corticotropin, chorionic somatomammotropin, estrogens, and pro- gesterone.
• hCG stimulates corpus luteum to secrete progesterone until the end of pregnancy. In addition, it secretes re- laxin that facilitates parturition by softening the con- nective tissue of symphysis pubica.
• The metabolic activity of the placenta is almost as great as that of the fetus itself. The umbilical cord con- nects the fetus to the placenta.
• During the first trimester (first 3 months) of pregnancy, the basic structure of the baby is formed.
• This involves cell division, cell migration, and the differentiation of cells into many types found in the baby. During this period, the developing baby-called fetus is very sensitive to anything that interferes with the steps involved.
• Virus infection of the mother, e.g., by rubella (German measles) virus or exposure to certain chemicals, may cause malformations in the developing embryo. Such agents are called teratogens (monster forming).
• After 3 months, all the systems of the baby have been formed, at least in a rudimentary form.
• From then, the development of fetus is primarily a matter of growth and minor structural modifications.
• The fetus is less susceptible to teratogens.

NEET Biology Notes For Human Reproduction Parturition

• The gestation period of a human is about 38 weeks/ 266 days followed by birth. The process of giving birth to a baby is called parturition.
• It starts with the rise in estrogen/progesterone ratio and increase in the level of oxytocin secretion by both— mother and fetus.
• It includes three stages.
  • Dilation stage:
    • Uterine contraction starts from the top and occurs at long intervals (once every 30 min). This forces the baby to push its head against the cervix.
    • As a result, the cervix gets dilated with va- gina also showing similar dilation. The di- lation of cervix increases the stimulus for oxytocin secretion, further increasing the strength and frequency of contractions (1- 3 every minute).
    • With continued powerful contractions, the amnion ruptures and the amniotic fluid flows out through vagina.
  • Expulsion stage
    • With further increase in the intensity of uterine and abdominal contraction, the baby comes out through the cervix and vagina with head coming out first.
    • It may take 20-60 min. The umbilical cord is cut. The infant’s lungs expand and breathing begins. This requires a major switch-over in the circulatory system.
    • Blood flow through the umbilical cord ductus arteriosus and foramen ovale ceases; the adult pattern of blood flow-through the heart, aorta, and pulmonary arteries-begins.
    • In some infants, the switch-over is incomplete and blood flow through the pulmonary arteries is inadequate. Failure to synthesize enough nitric oxide is one cause.
  • After birth: Within 10-15 min after delivery, the placenta and the remains of the umbilical cord, called “after birth,” are expelled out.

Lactation

• Although estrogen and progesterone are essential for the physical development of breasts during pregnancy, a specific effect of both these hormones is to inhibit the actual secretion of milk. Conversely, the hormone prolactin has exactly the opposite effect on secretion- promotion of milk secretion.
• This hormone is secreted by the mother’s anterior pi- tuitary gland and its concentration in her blood rises steadily from the fifth week of pregnancy until the birth of the baby at that time it has risen 10-20 times the normal non-pregnant level.
• In addition, placenta secretes large quantities of human chorionic somatomammotropin, which probably also has lactogenic properties, thus, supporting the prolactin from the mother’s pituitary during pregnancy.
• The fluid that is secreted in the last few days before and in the first few days after parturition is called colostrums. It contains essentially the same concentrations of proteins and lactose as milk but almost no fat.

Ejection (or “Let-Down”) Process in Milk Secretion

• Milk is secreted continuously into the alveoli of the breasts, but it does not flow easily from the alveoli into the duct system and, therefore, does not continually leak from the breast nipples.
• Instead, milk must be ejected from the alveoli into the ducts before the baby can obtain it. This is caused by a combined neurogenic and hormonal reflex that in- volves the posterior pituitary hormone oxytocin.
• When the baby suckles, sensory impulses are trans- mitted through somatic nerves from the nipples to the mother’s spinal cord and then to her hypothalamus, there causing nerve signals that promote oxytocin secretion; at the same time they cause prolactin secretion.
• Oxytocin is carried in the blood to the breasts, where it causes myoepithelial cells (that surround the outer walls of the alveoli) to contract, thereby, expelling milk from the alveoli into the ducts.

NEET Biology Notes For Human Reproduction Assertion-Reasoning Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

(1) If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

(2) If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

(3) If Assertion is true but Reason is false, then mark (3).

(4) If both Assertion and Reason are false, then mark (4).

Question 1. Assertion: Scrotum provides optimum temperature conditions for spermatogenesis.

Reason: Dartos and cremaster muscles in scrotum contract and relax involuntarily in response to temperature.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 2. Assertion: The process of reproduction does not suffer if one ovary is removed.

Reason: The other ovary enlarges to take over the function of the missing ovary too.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 3. Assertion: “Nothing lives forever, but life continues.”

Reason: Death keeps the population growth under check.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 4. Assertion: Placenta is connected to the fetus by an umbilical cord.

Reason: Fetal components of placenta are derived from endometrium.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 5. Assertion: Placenta is contra-deciduate and even the fetal placenta is absorbed in mole.

Reason: Mole’s egg contains abundant yolk in ooplasm.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 6. Assertion: Polar bodies have small amount of cytoplasm.

Reason: It is formed by unequal mitotic division.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 7. Assertion: Ovulation takes place when the blood level of luteinizing hormone is high.

Reason: Leutinizing hormone is responsible for ovulation.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 8. Assertion: Umbilical cord contains 100% fetal blood.

Reason: It has single umbilical artery and single umbilical vein.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 9. Assertion: The activation of sperm is called capacitation.

Reason: Capacitation takes about 5-6 h.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 10. Assertion: Before fusion, spermatozoa have to penetrate egg membrane,

Reason: The activated spermatozoa undergo acrosomal reactions and release sperm lysin.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 11. Assertion: In post natal life, oocyte development occurs in mature follicle,

Reason: After ovulation, the Graafian follicle transforms in corpus luteum.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

Question 12. Assertion: Placenta is combined structure of fetal tissue and maternal tissue.

Reason: Placenta formation is completed before 6 weeks.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 13. Assertion: Seminal vesicle is known as the accessory sex organ of males.

Assertion: Seminal vesicle conserves sperm energy and provides fuel to sperm.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 14. Assertion: Testes are retroperitoneal organ in man.

Reason: Peritoneal layer covers the testes on the dorsal side.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 15. Assertion: Cervix contains the most weak sphincter muscle in the body.

Reason: Cervix opens into fallopian by os external.

Answer. 4. If both Assertion and Reason are false, then mark (4).

Question 16. Assertion: In ovarian cycle, corpus luteum is exocrine gland.

Reason: It secretes pheromones.

Answer. 4. If both Assertion and Reason are false, then mark (4).

NEET Biology Notes For Sexual Reproduction In Flowering Plants Introduction

R. Camerarius was the first to describe sexual reproduction in plants. The sexual reproductive organ in angiosperms is flower. The male sex organs are known as stamen or microsporophyll and the female sex organs are known as pistil or carpel.

NEET Biology Notes For Sexual Reproduction In Flowering Plants Stamen Or Microsporophyll: Male Sex Organ

The fertile portion of stamen is called anther. Each anther is usually made up of two lobes connected by a connective. A typical anther consists of two lobes or four microsporangia; such an anther is called bithecous. In the members of Malvaceae, arther consists of one lobe or two micro-sporangia; such an an er is called monothecous and is bisporangiate. In Arceutho- bi, the smallest dicot parasite, anther consists of only one microsporangium (monosporangiate).

reproduction flower

Development of Anther

A young anther consists of homogeneous mass of meristematic cells called primary sporogenous cells surrounded by anther wall. Primary sporogenous cells form microspore mother cells (2n) inside the microsporangium.

The anther wall consists of the following layers:

• Epidermis: It is single layered and protective in function.
• Endothecium: The cells of this layer have a-cellulosic fibrous bands arising from inner tangential wall which help in the dehiscence of anther. Fibrous bands are ab- sent in hydrophytes.
• Middle layer: The cells of this layer are ephemeral and are three-layered.
• Tapetum: This is the innermost layer of anther wall which surrounds the sporogenous tissue. Tapetal cells are nutritive. They are multinucleate and polyploid. The tapetum has two types of cells:
  • Secretory or glandular: These cells secrete sporopollenin, pollen kit, and compatibility proteins. The Ubisch bodies are present in these cells, which help in the ornamentation of exine, as they have a chemical called sporopollenin deposited on them.
  • Amoeboid or plasmodial or invasive: Cells undergo breakdown and entire protoplast moves in the center and nourishes microspores.

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Microsporogenesis

The formation and differentiation of microspores (pollen grains) is called microsporogenesis. In the cavity of microsporangium, the microspore mother cells divide meiotically to produce microspore tetrads or pollen tetrads.

Cytokinesis may occur either after each meiotic division (successive type) leading to the formation of isobilateral tetrad of microspores (e.g., monocots) or after both meiotic (1 and 2) divisions (simultaneous type) leading to the formation of tetrahedral tetrad of microspores (e.g., dicots). Successive type of cytokinesis is advanced type.

Tetrads are of five types: (a) tetrahedral, (b) isobilateral, (c) T-shaped, (d) linear, and (e) decussate.

In Aristolochia elegans, all five types of tetrads are present.

Some Exceptions to Remember

• After formation, the microspores are separated from the tetrad. But in Elodea, Drosera, and Typha, the microspores do not separate from each other. Thus, com- pound pollen grains are formed.
• In Asclepiadaceae (Calotropis) and Orchidaceae, all microspores in a sporangium adhere together in a single mass known as pollinium.
• In Calotropis, the pollinia of adjacent anthers of different stamens are attached by thread-like caudicles to a sticky disc called corpusculum. The whole structure is called translator.
• In Cyperaceae, out of four microspores formed, three are degenerate. So, ultimately one microspore mother cell (2M) produces only one microspore or pollen grain.
• Sometimes, more than four pollen grains are produced from one microspore mother cell. It is called polyspory. For example, Cuscuta reflexa.

” reproduction in angiosperms”

Structure of Microspore or Pollen Grain

The cell wall of microspore (sporodern) consists of two layers. The outer layer is exine and the inner layer is intine. Exine is made up of sporopollenin and intine is pectocellulosic in nature. Sporopollenin, a fatty substance, is resistant to physical and biological decomposition. So pollen wall is preserved for long periods in fossil deposits. Pollen kit is a sticky layer found on the outside of mature pollen grains of many insect pollinated species. The material of pollen kit is contributed by the tapetal cells. It acts as an insect attractant.

Exine has proteins for enzymatic and compatibility reactions. It is two-layered:

• Ektexine: Highly sculptured and is differentiated into tectum, baculum, and foot layer
• Endexine: Not sculptured

Some weak points are present on the exine. These are called germ pores.

Pollen grains can be monocolpate (having one germ pore called germinal furrow, e.g., monocots), bicolpate (two germ pores), and tricolpate (three germ pores, e.g., dicots).

The study of pollen grains is called palynology (term given by Hyde and Williams).

Male Gametophyte

Microspore is the first cell of male gametophyte. Its germination starts in situ (in the mother place). It can be best defined as partially developed male gametophyte.

Microspore divides mitotically into large tube cell and small generative cell. At this two-celled stage (in some cases at three-celled stage), pollination takes place. Further development of male gametophyte takes place on the stigma.

Pollen grain expands by absorbing liquid from the moist surface of stigma. Stigma provides boron, sugar, amino acids, etc. The exine bursts and the intine comes out in the form of pollen tube.

Pollen grains are either monosiphonous (with one pollen tube) or polysiphonous (with more than one pollen tubes). For example, members of Cucurbitaceae and Malvaceae. Pollen tube was first observed by G. B. Amici (1824) in Portulaca.

The generative nucleus divides mitotically to form two male gametes called sperms. The male gametes are non-motile and amoeboid. They are slightly unequal in size.

The function of the pollen tube is to carry sperm. In the pollen tube, the tube nucleus enters first (it is a vestigial structure) and soon disintegrates. The growth of the pollen tube is apical and the entire cytoplasm of pollen grain is confined to the tip of the pollen tube.

NEET Biology Notes For Sexual Reproduction In Flowering Plants Carpel Or Megasporophyll: Female Sex Organ

A carpel consists of ovary, style, and stigma. Ovary contains ovules.

Structure of Ovule or Integumented Megasporangium

Ovule is an outgrowth of placenta. Each ovule is attached to its placenta by a stalk known as funicle. The point of attachment of the funicle with the main body of the ovule is called bilum. Sometimes funicle gets fused with the body of the ovule along one side and forms a ridge known as raphe. The basal region of the ovule is known as chalaza.

The main body of an ovule is called nucellus (called mega- sporangium) which consists of mass of parenchymatous tissue. On the basis of the development of nucellus, ovules are of two types:

• Crassinucellate: The nucellus is well developed, e.g., polypetalae.
• Tenuinucellate: The nucellus is poorly developed, e.g., gamopetalae.

The nucellus is invested all around by one or two ring-like coverings called integuments except at the apex where a small passage is formed known as microplyle. On the basis of the number of integuments, ovules are of the following types:

• Unitegmic: These are ovules with one integument, e.g., members of gamopetalae and gymnosperms.
• Bitegmic: These are ovules with two integuments, e.g., members of polypetalae and monocots.
• Ategmic: These are ovules without integument, e.g., Santalum, Loranthus (parasites), and Viscum.

The third integument in the form of aril develops from the base of ovule or funicle in many plants, e.g., litchi and Inga dulce. In litchi and Inga dulce, aril is fleshy and edible.

Types of Ovules

• Orthotropous: The micropyle, chalaza, and funicle are in straight line. This is the most primitive type of ovule. For example, piper, polygonum, and cycas.
• Anatropous: The ovule turns at 180° angle. Thus, it is inverted ovule. Micropyle lies close to hilum or at the side of hilum. For example, it is found in 82% angiosperm families.
• Hemianatropous: The ovule turns at 90° angle upon the funicle or the body of ovule and is at right angle to the funicle. For example, Ranunculus.
• Campylotropous: The ovule is curved more or less at right angle to the funicle. The micropylar end is bent down slightly, for example, in the members of Leguminosae and Cruciferae.
• Amphitropous: The ovule as well as embryo sac is curved like horseshoe. For example, Lemna, poppy, and Alisma.
• Circinotropous: The ovule turns at more than 360° angle and, so, the funicle becomes coiled around the ovule. For example, Opuntia (Cactacea).

reproduction of flowering plants

Megasporogenesis

Any cell of nucellus towards the micropylar end is differentiated from the other cells. This cell is called the megaspore mother cell (MMC). It divides meiotically to produce linear megaspore tetrad. In majority of angiosperms, the chalazal megaspore is functional and the other three megaspores degenerate.

Female Gametophyte of Embryo Sac

P. Maheshwari classified embryo sac, on the basis of number of megaspore nuclei participating in its formation, into the following:

• Monosporic embryo sac: Only one megaspore nucleus forms embryo sac. For example, Polygonum and Oenothera.
• Bisporic embryo sac: Two megaspore nuclei take part in the development of embryo sac. For example, Allium and Endymion.
• Tetrasporic embryo sac: All four megaspore nuclei take part in the development of embryo sac. For example, Adoxa, Plumbago, Drusa, Fritillaria, Penaea, Plumbagella, and Peperomia.

Development of Monosporic Embryo Sac (Polygonum Type)

The normal type of embryo sac development has been studied in Polygonum by E. A. Strasburger. Since this embryo sac develops from one megaspore, it is monosporic embryo sac. It develops from chalazal functional megaspore (fourth from micropyle). The nucleus of functional megaspore divides by three mitotic divisions to form eight nuclei.

• Three cells at the micropylar end form egg apparatus. One is egg cell (n) and two are synergids (n) or cooperative cells.
• Three cells at the chalazal end form antipodals (n) or vegetative cells of gametophyte.
• Two nuclei (one from each pole) in the center are called polar nuclei (n).

flowering plants reproduction

As the embryo sac matures, these polar nuclei get fused to form a secondary nucleus (2n) or definitive nucleus. An embryo sac is seven-celled and eight-nucleated structure.

NEET Biology Notes For Sexual Reproduction In Flowering Plants Pollination

The transfer of pollen grains from the anther of a flower to the stigma of the same or different flower of the same species is called pollination. It is of two types:

• Self-pollination or autogamy: When pollen grains are transferred from the anther to the stigma of the same flower, the process is called self-pollination or autogamy.
  • Bisexuality: The flower should be bisexual or hermaphrodite. For example, Cartharanthus.
  • Homogamy: Male and female reproductive parts in a bisexual flower mature at the same time. For example, Mirabilis.
  • Cleistogamy: Sometimes bisexual flowers remain closed and never open. Such flowers are known as cleistogamous. For example, Commelina benghalensis, Viola, Oxalis, and Arachis.
  • Bud pollination: Self-pollination occurs in the bud stage before the opening of flowers. For ex- ample, Pisum, wheat, and rice.
• Cross-pollination or allogamy: When pollen grains are transferred from the anther to the stigma of the flower of another plant of the same or different species, the process is called cross-pollination or allogamy. It is of two types:
  • Geitonogamy: Pollination taking place between two flowers of the same plant (genetically self- pollination but ecologically cross-pollination)
  • Xenogamy: Pollination taking place between two flowers of different plants (genetically and ecologically cross-pollination)

Contrivances for Cross-Pollination

• Unisexuality or dicliny: It is the formation of unisexual flowers. In unisexual flowers, allogamy becomes obligatory. Unisexuality can be seen in monoecious plants, e.g., maize and Vallisneria.
• Dichogamy: In bisexual flowers, the two sexes mature at different timing. When anthers mature first, it is called protandry. For example, sunflower and cotton. When gynoecium matures first, it is called protogyny. For example, Vallisneria.
• Heterostyly: Flowers are dimorphic with regard to the length of style and, thus, facilitate cross-pollination. For example, Primula (primrose) jasminum.
• Herkogamy: It is the presence of natural and physical barrier between androecium and gynoecium. For example, Gloriosa and Salvia.
• Self-sterility or incompatibility: Due to physiological or genetic reasons, the pollen fails to germinate on its own stigma. For example, tobacco.

Agencies for Cross-Pollination

• Entomophily (pollination by insects): 80% of in- sects’ pollination occurs by bees (chief pollination). All flowers pollinated by bees are brightly colored, have a sweet smell, and produce nectar. Entomophil- ous flowers produce a small amount of pollen which has a spinous and sticky exine due to the presence of pollen kit. The stigmas of such flowers are long, rough, and sticky. Salvia is an excellent example of insect pollination, which occurs by lever or turn-pipe mechanism. Calotropis exhibits translator mechanism. Aristolochia shows pitfall or flytrap mechanism. Moth-pollinated plants are white flowered and fragrant. Pollination in orchid (Ophrys speculum) occurs by wasp (pseudocopulation mechanism). In Ficus carica, pollination occurs by an insect Blastophaga (trapdoor mechanism). Yucca is pollinated by Pronuba yuccasella (obligate relation between two).
• Anemophily (pollination by wind): It is a non-directional and wasteful process. Windpollinated flowers produce a large number of pollen grains to compensate for the wastage. Female flowers have large feathery brush like stigmas to catch the pollen grains. Anemophilous flowers are small and inconspicuous with long and versatile stamens. Pollen grains are dry and powdery and are produced in large numbers. For example, sugarcane, maize, wheat, bamboo, Pinus (winged pollen), and papaya.
• Hydrophily (pollination by water): All aquatic plants are not hydrophilous.
  Some hydrophytes are anemophilous, such as Potamogeton and Myriophyllum, or entomophilous, such as Alisma and lotus. Hydrophilous plants may be pollinated inside the water (hypohydrophily) as in Zostera and Ceratophyllum or outside the water (epihydrophily) as in Vallisneria (tape grass and ribbon weed).

” reproduction in flowering plant”

• Ornithophily (pollination by birds): Flowers are brightly colored but are odorless and produce plenty of nectar and large quantities of pollen. For example, Bombax, Callistemon, Strelitzia, and Erythrina.
• Chiropterophily (pollination by bats): Bats pollinate the flowers of tropical regions. For example, Anthocephalus, Kigelia, and Adansonia.
• Malacophily (pollination by snails): For example, arum lillies, Arisaema, and Lemna.
• Ophiophily (pollination by snakes): For example, Santalum and Michelia.

Entry of Pollen Tube into Ovule

A pollen tube mostly enters into an ovule through the micropyle; it is called porogamy, as seen in most of the flowering plants. In some plants, such as Casuarina, the pollen tube enters the ovule through chalaza; it is called chalazogamy. Sometimes it enters through integuments; it is called mesogamy. For example, Cucurbita.

NEET Biology Notes For Sexual Reproduction In Flowering Plants Double Fertilization

Pollen tube enters one of the synergids and bursts releasing two male gametes. One male gamete fuses with the egg to form dip- loid zygote. The fusion is called syngamy or generative fertili- zation. It was discovered by Strasburger.

Male gamete (n)+ Egg (n) → Zygote (2n)

The other male gamete fuses with the secondary nucleus (2n after the fusion of polar nuclei) to form primary endosperm nucleus (3n). This fusion is also called triple fusion because three nuclei take part in this. It is also known as vegetative fertilization or pseudofertilization or trophomixis.

Male gamete (n)+ Secondary nucleus (2n) → Primary endosperm nucleus (3)

Both syngamy and triple fusion are called double fertilization. As the fusion in the embryo sac occurs twice, this is called double fertilization. Triple fusion and double fertilization were discovered by S. G. Nawaschin and Guignard in Lilium and Fritillaria, respectively. Double fertilization occurs in angiosperms only. In all, five nuclei are involved in double fertilization.

Endosperm

An endosperm is a product of triple fusion and develops from the central cell of embryo sac. It is generally a triploid tis- sue. This endosperm is the nutritive tissue for the developing embryo. It is absent in families such as Orchidaceae and Podostemaceae, and trapaceae.

Nature of Endosperm

• Cells are isodiametic and polyploid.
• Starch endosperm in cereals.
• Proteinaceous endoperm (aleurone layer) in cereals.
• Oily endosperm in castor and coconut.
• Cellulosic endosperm in ivory palm (hard endosperm).
• Hemicellulosic endosperm in date palm.

Types of Endosperms

• Nuclear endosperm: Primary endosperm nucleus of the central cell divides without wall formation (free nuclear division). It is the most common type of endosperm. For example, cotton, maize, Capsella, and coconut (milk).
• Cellular endosperm: Primary endosperm nucleus divides many times and each division is followed by wall formation. For example, Petunia, Utricularia, and coconut (copra).
• Helobial endosperm: It is intermediate between nuclear and cellular types. For example, members of order Helobiales (monocot).

NEET Biology Notes For Sexual Reproduction In Flowering Plants Embryo

The study of the development of embryo is called embryogeny.

Development of Embryo in Dicots

The normal type of dicot embryo development has been studied in shepherd’s purse (Capsella bursa-pastoris) which belongs to family Cruciferae. This is called as crucifer or onagrad type of embryo development.

The development of embryo is endoscopic. The zygot (oospore) divides into two unequal cells a larger sus- pensor cell towards micropyle and a smaller embryonal cell (or terminal cell) towards the antipodal region. The suspensor cell undergoes transverse divisions forming 6-10 celled long sus- pensor. The first cell of the suspensor (towards embryo cell) is known as hypophysis. It forms the radicle tip.

The embryonal cell divides twice vertically and once trans- versely to produce a two-tiered eight-celled embryo. The epibasal tier forms two cotyledons and a plumule while the hypobasal tier produces only hypocotyl and most of the radicle. For this, the octant embryo undergoes periclinal divisions producing protoderm, procambium, and ground meristem. It is initially globular, but with the growth of cotyledons, it becomes heart-shaped and then assumes the typical shape. For example, Capsella bursapastoris. In orchids, Orobanche, and Utricularia, the embryo does not show the distinction of plumble, cotyle- dons, and radicle.

Development of Embryo in Monocot

The normal type of monocot embryo development has been studied in Luzula forsteri and is called Sagittariatype.

The early developments of dicot and monocot embryos are similar up to the octant stage. Later on differentiation starts. Suspensor is single celled.

The zygote of oospore divides transversely producing a vesicular suspensor cell towards the micropylar end and an embryo cell towards the chalazal end. The embryo cell divides transversely again into a terminal and a middle cell. The terminal cell divides vertically and transversely into globular embryo. It forms a massive cotyledon and a plumule.

The growth of cotyledon pushes the plumule to one side. The remains of the second cotyledon occur in some grasses. It is called epiblast. The single cotyledon of monocots is called scutellum. It is shield-shaped and appears terminal. The middle cell gives rise to hypocotyl and radicle. It may add a few cells to the suspensor. Both radicle and plumule develop covering sheaths called coleorrhiza and coleptile, respectively.

Incompatibility

Incompatibility is the inability of certain gametes, even from genetically similar plant species, to fuse with each other. This is also called intraspecific incompatibility, self-sterility, and self- incompatibility. It involves many complex mechanisms associated with the interaction of pollen and stigmatic tissues. It is of two types:

• Sporophytic incompatibility: It is the incompatibility due to the genotype of sporophytic/stigmatic tissues.
• Gametophytic incompatibility: It is the incompatibility due to the genotype of the pollen. This may be due to the prevention of pollen germination, deorientation of pollen tube, or even failure of nuclear fusion. It is controlled by genes with multiple alleles (S-allele). A plant carries two such alleles, e.g., S₁, S₂, S₂, S₃, S₁, S₃, S₂, S₄, S₃, S₅, etc. A pollen carries only one allele. If it happens to be one of the two alleles of pistil, the pollen fails to form pollen tube.

Apomixis and Polyembryony

Apomixis is the formation of new individuals directly through asexual reproduction without involving the formation and fusion of gametes. It is of two types: (a) agamospermy and (b) vegetative propagation.

NEET Biology Notes For Sexual Reproduction In Flowering Plants Agamospermy

In this type of asexual reproduction, the embryo is formed by a process in which normal meiosis and syngamy have been eliminated. This type of apomixis occurs within the seed.

Types of Agamospermy

• Adventitive embryony (sporophytic budding): The embryo arises from diploid sporophytic cells such as nucellus or integuments (other than egg). Examples are Citrus and Opuntia.
• Recurrent agamospermy: In this method, a diploid embryo sac is formed from MMC which has a diploid egg or oosphere. The diploid egg grows parthenogenetically into diploid embryo. Examples are apple, pear, and Allium.
  Diploid embryo sac can develop directly either from the diploid MMC (diplospory) or the diploid nucellai cell (apospory).
• Non-recurrent agamospermy: The embryo develops parthenogenetically from the haploid egg. Example is banana.

Polyembryony is the phenomenon of having more than one embryo. There may be more than one egg cell in an embryo sac or more than one embryo sac in an ovule. All the eggs may get fertilized. Synergids and antipodal cells may also form embryos. The occurrence of polyembryony due to the fertilization of more than one egg is called simple polyembryony.

The formation of extra embryos through sporophytic budding is called adventive polyembryony. Polyembryony is quite common in onion, groundnut, mango, lemon, and orange. In some of these cases, a stimulus of pollination may be required. Citrus seed has 2-40 embryos-one normal and the rest adventitive mostly nucellar.

NEET Biology Notes For Sexual Reproduction In Flowering Plants Assertion-Reasoning Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).
2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
3. If Assertion is true but Reason is false, then mark (3).
4. If both Assertion and Reason are false, then mark (4).

Question 1. Assertion: All the pollen grains of a microsporangium are held together and form rollinium.

Reason: Pollinium is very suitable for anemophily.

Answer. 3. If Assertion is true but Reason is false, them mark (3).

Question 2. Assertion: An endosperm represents the triploid condition.

Reason: It is formed due to the fusion of triploid nuclei.

Answer. 3. If Assertion is true but Reason is false, them mark (3).

Question 3. Assertion: Allele of pollens happens to be one of the two alleles of pistil; the pollen fails to form pollen tube.

Reason: The incompatibility is due to the genotype of the pollen.

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 4. Assertion: The pollen grains of dicot are tricolpate.

Reason: It has three distinct lens shaped apertures,

Answer. 1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 5. Assertion: The main body of an ovule is called nucellus.

Reason: The nucellus is well-developed in polypetalac.

Answer. 2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).

NEET Biology Notes For Plant Kingdom Systems Of Biological Classification

The following systems of classification have been proposed from time to time. Some early attempts for the classification of plants are as follows:

1. Aristotle: Grouped plants into trees, shrubs, and herbs based on their habit. He divided animals into “Enaima” and “Anaima” on the basis of the presence and absence of RBC, respectively.
2. Charaka (Father of Ayurveda): Listed 200 kinds of animals and 340 kinds of plants in his book “Charak Samhita.”

Artificial System Of Classification: The artificial system of classification is based on a few morphological characters for grouping the organisms. Early systems of classifications given by Aristotle, Theophrastus, Pliny, Bauhin, John Ray, and Linnaeus are artificial systems.

• Theophrastus proposed the first system of artificial classification of plants on the basis of habit of plants into herbs, shrubs, and trees;
• Carolus Linnaeus (1707-1778) proposed the artificial system of classification based exclusively on the nature and number of stamens and carpels. It was called as a sexual system of classification.
• The classification consisted of 24 classes, in which 23 classes were of flowering plants (Phanerogamia) and the 24th class had flowerless plants (Cryptogamia).

The details of his classification were published in Genera Plan/arum (1737). Total 24 classes given by Linnaeus were: Monandria, Diandria, Triandria, Tetrandria, Pentandria, Hexandria, Heptandria, Octandria, Enneandria, Decandria, Dodecandria, Icosandria, Polyandria, Didynamia, Tetradynamia, Monadelphia, Diadelphia, Polyadelohia, Syngenesia, Gynandria, Mcnoecia, Dioecia, Polygamia, and Cryptogamia.

Artificial System Of Classification Drawbacks

1. Because this system was based on few characters, hence diverse animals and plants were placed into a limited number of groups.
2. No natural or phylogenetic relationships were considered.

Natural System Of Classification: The natural system of classification is known as the horizontal system of classification or 2D system. Organisms in this system are classified on the basis of natural affinities.

• This system uses as many taxonomic characters as possible to group organisms. This classification is mainly based on form relationships realizing all information available at the time of collection of plants.

” plant kingdom”

• The classification systems proposed by John Ray, de Jus- siaeu, de Candolle, Bentham, and Hooker were natural.
• George Bentham and J.D. Hooker (1833-1862) gave the most important system of classification of angiosperms and published it in three volumes of “Genera Plantarum.’’
• They described 202 families. In this system, the description of plants was based on their detailed studies and dissections. It is widely acceptable and all British commonwealth countries including India widely follow this system for practical purposes, hence, it is called practical classification.

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Bentham And Hooker’s Classification: Bentham and Hooker divided plant kingdoms into two subking-doms—Cryptogamia (non-seeded plants) and Phanerogamia (seeded plants). Phanerogamia was further divided into three classes—dicotyledons, gymnosperms, and monocotyledons.

An outline of the classification of Phanerogamia is given below.

Class 1: Dicotyledonae: Pentamerous flower, reticulate venation in leaves, two cotyledons in seed, vascular bundles open (with cambium), secondary growth present, wood formation occurs. It is divided into three sub-classes

1. Sub-class 1 Polypetalae: Petals free and separate. It has three series.
   • Series 1 Thalamiflorae: Flower hypogynous, stamens and pistils many (indefinite), petals free, distinct sepals are free from ovary. It has six orders.
   • Series 2 Disciflorae: Flower hypogynous, calyx consists of free or united sepals, petals free, a prominent cushion-shaped disc is present below the ovary. It has four orders.
   • Series 3 Calyciflorae: Flower perigynous or epigy- nous. Calyx contains united sepals (rarely free); ovary inferior. It has five orders.
2. Sub-class 2 Gamopetalae: Petals united or fused. It is divided into three series.
   • Series 1 Inferae: Flower epigynous, ovary inferior; stamens as many as corolla lobes or fewer. It has three orders.
   • Series 2 Heteromerae: Ovary usually superior; carpels more than two. it has three orders.
   • Series 3 Bicarpellatae: Ovary usually superior, two carpels (rarely one or three). It has four orders.
3. Sub-class 3 Monochlamydeae: Flower incomplete; no distinction between calyx and corolla present which is usually sepaloid and may be absent. It is divided into eight series.
   • Series 1 Curvembryeae: Embryo curved; generally one ovule in the ovary which is one in each locule.
   • Series 2 Multiovulatae: Aquaticae-Plants are aquatic, submerged herbs, syncarpous ovary.
   • Series 3 Multiovulatae Terrestres: Plants are terrestrial, syncarpous ovary.
   • Series 4 Microembryae: Very minute or small embryo.
   • Series 5 Daphnales: Ovary with one carpel and one ovule.
   • Series 6 Achlamydosporeae: Ovary unilocular with one to three ovules. Ovary inferior.
   • Series 7 Uniscxulaes: Flower unisexual.
   • Series 8 Ordines Anomali: Families having plants with anomalous (abnormal) characters.

Class 2 Gymnospermae: Sex organs on cones, perianth absent, ovule naked (not found inside ovary), seeds are also naked, haploid endosperm. It has three families: Gnctaceae, Coni ferae, and Cycadaccae.

Class 3 Monocotyledonae: Parallel venation in leaves, embryo with one cotyledon, flower usually trimerous, wood absent, and no secondary growth. It is divided into seven series.

• Series 1 Microspcrmae: Ovary inferior, seeds minute.
• Series 2 Epigynae: Ovary inferior, seeds larger.
• Series 3 Coronarieae: Ovary superior, perianth colored.
• Series 4 Calycineae: Ovary superior, perianth green.
• Series 5 Nudiflorae: Perianth absent, ovary superior.
• Series 6  Apocarpae: Carpels free (apocarpous).
• Series 7 Glumacae: Flowers arranged in spikelets with bracts, Perianth reduced, bracts large and scaly.

In this classification, class is like division, series is like class, cohort is like order, and order is like family.

Merits Of Bentham And Hooker System Of Classification

1. It is useful for practical value. In India, it is the most used system.
2. Ranales have been given a primitive position and monocots are considered as advanced.

Demerits Of Bentham And Hooker System Of Classification

1. Gymnosperms were placed between dicots and mono-cots.
2. Many important floral characters have been neglected.
3. It is not a phylogenetic system. The idea was to believe on the fixity of species.
4. Some closely related families were separated and placed under different cohorts, and unrelated families have been put nearer.
5. Advanced families such as Orchidaceae have been considered as primitive.

Phylogenetic System Of Classification: The phylogenetic system of classification is also known as the vertical system or 3D system of classification. The term phylogeny was given by Lamarck. Phylogeny is the evolutionary history of an organism.

• According to this system of classification, plants are classified according to their evolutionary and genetic affinities. In this system organisms belonging to the same taxa are believed to have a common ancestor and may be represented in the form of a family tree called a cladogram.
• A.W. Eichler modified Bentham and Hooker’s system by placing gymnosperms in the beginning. He is also known as the pioneer in the phylogenetic system of classification.

Adolph Engler and Karl A.E. Prantl, two German botanists, adopted their system in “Die Naturalichen Pilanzen Familien” (1887-1915). It was a German work that was later translated in English. The work had 23 volumes.

An Outline Of Engler And Prantl System Of Classification

Phylogenetic Merits Of This System: In this system, families are arranged according to the increasing complexity of flowers.

Phylogenetic Demerits Of This System

1. Monocots were considered primitive to dicots.
2. According to this system, primitive forms having naked flowers were kept in the beginning. The more advanced families have distinct perianth, while the highly evolved families have fused perianth.

John Hutchinson presented the phylogenetic classification in his famous book Families of Flowering Plants in 1959. In Hutchinson’s classification,

1. Trees and shrubs are considered to be more primitive than herbs.
2. Dicots have been considered more primitive than monocots.
3. Polypetalous, actinomorphic, and solitary flowers are considered more primitive than gamopetalous, zygomorphic, and inflorescence, respectively.

An outline of Hutchinson’s classification is given. The divisions include orders which again include families. Division does not include classes. Takhtajan stated that “taxonomy without phylogeny is like bone without flesh.”

An Outline Of Hutchinson’s System Of Classification

NEET Biology Notes For Plant Kingdom Branches Of Taxonomy

Classical Taxonomy: It deals with employing all available information to classify organisms on the basis of their origin, evolution, affinities, and variations.

Numerical Taxonomy/Phenetics/Taximetrics/Adansonian Taxonomy

1. It uses statistical methods for evaluating similarities and differences between species. All selected characters are given equal importance. The comparison has been made easier with the availability of sophisticated calculating machines and computers.
2. Dendrogram is the family tree of organisms on the basis of numerical taxonomy.

Biosystematics: It deals with the variations within a species and its general evolution.

Cytotaxonomy/karyotaxonomy: Based on the cytological information on cell, chromosome number, and their structure and behavior during meiosis.

Chemotaxonomy: Based on chemical constituents of plants, for example, betacyanin pigment in beetroots, raphides, and cystolith, sequencing DNA, and the chemical nature of proteins.

plant kingdom chart

Turril Divided Taxonomy Into Three Types:

1. α-taxonomy: It is based on gross morphological features including compilation of monographs and flora.
2. β-taxonomy: It is based on morphology and evidence from genetics, cytology, anatomy, physiology, etc.
3. Ω-taxonomy: It includes all microscopic and biochemical evidence.

NEET Biology Notes For Plant Kingdom Classification Of Plant Kingdom

There are five major groups of Plant Kingdoms or Kingdom Plantae. These are:

1. Thallophyta
2. Bryophyta
3. Pteridophyta
4. Gymnosperms
5. Angiosperms

NEET Biology Notes For Plant Kingdom Thallophyta (Algae)

General Characters Of Algae Are As Follows:

1. Eukaryotic, autotrophic, chlorophyllous, cryptogamic, thallophytes.
2. Aquatic, either freshwater or marine.
3. They are found in many forms: unicellular (for example, Chlamydomonas), colonial (for example, Volvox), palmelloid (for example, Tetraspora), dendroid (for example, Prasinocladus), filamentous unbranched (for example, Spirogyra), or branched (for example, Cladophora), heterotrichous (for example, Fritschiella, Coleochciete, Stigeoclonium), siphonaceous (for example, Vaucheria), parenchymatous (for example, Ulva).
4. Reserve food is mainly starch.
5. Vegetative Reproduction: It takes place by fragmentation.
6. Asexual Reproduction: Zoospores, aplanospores, hypnospores, akinetes, palmella stage, autospores, etc.
7. Sexual Reproduction
   • Sex organs are unicellular or multicellular with all fertile and non-jacketed.
   • The embryonic stage is absent.
   • Sexual Reproduction Three Types:
     1. Isogamous,
     2. Anisogamous, and
     3. Oogamous.
   • Sporic or gametic or zygotic meiosis occurs in life cycle.

R.E. Fritsch classified algae into 11 classes in his book Structure and Reproduction of the Algae mainly on the basis of pigmentation, reserve food, flagellation, thallus structure, modes of reproduction, and life cycles.

Whittaker included only Chlorophyceae (green algae), Phaeophyceae (brown algae), and Rhodophyceae (red algae) in algae under Kingdom Plantae.

Green Algae (Chlorophyceae) General Characters

1. Mostly freshwater (Spirogyra. Ulothrix, Chara, Chla- mydomonas, Volvox, etc.) and some arc marine (Ac- etabularia, Ulva, etc.).
2. Photosynthetic pigments are chlorophyll-a, chloro-phyll-b, and carotenoids.
3. Isokont flagellation.
4. Thylakoids are in the group of 2-20 per lamella.
5. Pyrenoids present.
6. Three types of life cycles occur in green algae:
   1. Haplontic life-cycle, for example, Ulothrix, Spirogyra. Chlamydomonas (zygotic meiosis occurs).
   2. Diplontic life-cycle, for example, Caulerpa (gametic meiosis).
   3. Diplohaplontic life cycle, for example, Ulva, Cladophora. In this type of lifecycle, haploid and diploid phases are well-developed and multicellular. The isomorphic alternation of generation is found.

Chlamydomonas: It is unicellular, motile, pyriform, or pear-shaped. Cell wall is made up of hydroxyproline (glycoprotein). Flagella are isokont, whiplash type. Presence of neuromotor apparatus (basal granules + paradesmos + rhinoplasty + centrosome).

Presence of single cup-shaped chloroplast with a single pyrenoid. Two contractile vacuoles, single eye spot or stigma as photoreceptor organ is present.

Chlamydomonas Reproduction

1. Asexual Reproduction: It is of the following types:
   • By Zoospores: Under favorable conditions.
   • Palmella Stage: It is formed in response to desiccation conditions and toxic salts. Palmella spores are green, non-motile, and are capable of growth and division.
   • By Aplanospore (Thin-Walled Spore) Or Hypno- Spores (Thick-Walled Spore): These are formed under drought conditions. Aplanospore formation occurs in C. caudata.
2. Sexual Reproduction: It is of the following types
   • Isogamy: Fusion of similar gametes, for example, C. debaryana (homothallic) or C. reinhardii (heterothallic).
   • Anisogamy: Fusion between a motile, smaller male gamete and a motile larger morphologically dissimilar female gamete, for example, C. braunii.
   • Hologamy: Fusion of young cells, for example, C. media.
   • Oogamy: Fusion between a motile male gamete and a non-motile female gamete, for example, C. coccifera.

Spirogyra: It is commonly known as summer alga or “hair of princess” “pond silk” “water silk” or “pond scum” (due to mucilage covering found in stagnant water). The cell wall is two-layered; the outer wall of pectose (water soluble which forms mucilage) and the inner wall of cellulose.

• The primordial utricle is present as a thin peripheral cytoplasm due to a large central vacuole. Presence of 1-16 ribbon-shaped, spiral chloroplast. Coiling is sinistral or left-handed. Many pyrenoids are present.
• Spirogyra adnata and S.jogensis remain attached to the substratum by basal cells called holdfast or hapteron.

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Spirogyra Spirogyra Reproduction

1. Vegetative Reproduction: By fragmentation
2. Asexual Reproduction: It is generally absent except in the following:
   1. By Parthenospores Or Azygospores: Example, Spirogyra various. If placed in a sugar solution, it develops parthenospores.
   2. By Akinetes: Example, S. Farlow
   3. By Aplanospores.
3. Sexual Reproduction: It occurs by conjugation. It is physiologically anisogamous type. Types of conjugation are:
   • Scalariform Conjugation: The most common type; occurs in heterothallic and homothallic species. A minimum of two filaments are involved, for example, S. tmvensis.
   • Lateral Conjugation: Two adjacent cells of the same filament function as gametangia. It is a primitive type and uncommon.
   • Direct Lateral Conjugation: Example, S.jogensis
   • Indirect Lateral Conjugation: Example, S. affinis.

Economic Importance Of Green Algae

1. Codium and Ulva (Sea lettuce) is used as salad or vegetables in European countries after drying and salting.
2. Chlorella pyrenoidosa (called space alga) is used by exobiologists for food, oxygen, and disposal of CCE and organic waste in prolonged space flight.
3. Cephaleuros virescence is a parasitic green alga that causes red rust of tea disease.

Brown Algae (Phaeophyceae) General characters

1. The members of this group are called brown algae due to their characteristic brown color derived by the presence of an extra brown pigment, fucoxanthin.
2. The brown algae are almost exclusively marine plants found primarily in the coastal zones. These occur mainly in the cooler seas. However, the Sargassum is a well-known warm-water form that occurs in the North Atlantic Ocean region and forms the Sargassum Sea.
3. No unicellular brown algae is known. Except for a few members, most brown algae are macroscopic. Some of these are of massive sizes such as Laminaria (2 – 9 m); Nereocystis (45 m); Macrocystis (60-90 m). Because of their bulky appearance, these are called giant kelps. The body of a typical brown alga (for example, Laminaria) is differentiated into
   • A holdfast,
   • A stipe, and
   • Lamina.
   • In some large brown algae, certain cells are modified into long filaments called trumpet hyphae. These carry food from the lamina to the holdfast.
4. The cell walls of brown algae are generally composed of two layers: an inner cellulosic layer and an outer mucilaginous layer. The cellulosic walls are covered by a colloidal covering called phycocoUoids.
5. Alginic acid is an important component of hydrocolloids and is commercially obtained from kelps. It has hemostatic properties and has been used in emergency transfusions in the treatment of shock. Sodium alginate is used as a stabilizer in food industries; and calcium alginate in plastics. Forms of alginic acid are used in textile, rubber, and paint industries. It is also used for making smooth ice creams.
6. The cells of brown algae contain in addition to other normal structures small colorless vesicles called fu- cosan vesicles. The function of these vesicles is not known.
7. The cells contain pigment fucoxanthin in addition to chlorophyll-a and c. Fucoxanthin dominates over the green color of the chlorophyll, imparting a brown color to the algae.
8. The reserve food material is in the form of laminarin starch and mannitol.

Brown Algae (Phaeophyceae) Asexual Reproduction: Asexual reproduction by fragmentation is quite common. Detached vegetative parts of brown algae have great potential to develop into new individuals. Asexual reproduction also occurs by motile or non-motile spores formed inside large-sized cells called sporangia which may be unicellular (unilocular) or multicellular (multicellular.)

Brown Algae (Phaeophyceae) Sexual Reproduction: Sexual reproduction takes place by the formation of flagellate gametes that are generally formed inside gametangia. In some cases, the female gamete is non-motile. The gametes may be isogametes (both male and female gametes exactly similar), isogametes (both similar, but female, gamete larger than male gamete), and oogamous (female larger non-motile; male motile smaller).

Economic Importance Of Brown Algae

1. Fucus and Laminaria are rich sources of iodine.
2. Laminaria yields a food product rich in carbohydrates called kombu and similarly, A/aria yields a product called sarumen in Japan.
3. Durvillea has antiworm properties.

Red Algae (Rhodophyceae) General Characters

1. They are called so because of their reddish color due to the presence of special pigments: r-phycocyanin and r-phycoerythrin. They are mostly marine.
2. In deeper oceans, these acquire deeper color. Actually, only the blue-green light reaches the deep water. The pigment r-phycoerythrin in red algae does the job of capturing light of available wavelengths for red algae.
3. Red algae show a variety of forms. Their thallus ranges from microscopic, unicellular to neatly hull a meter in length. These may be unicellular, filamentous, ribbons-shaped, or leaf-like. Some of these secrete calcium carbonate over their walls and form coral-line structures. In certain coral reefs, red algae such as Corallina are the main producers.
4. Some red algae are parasitic. For example, llarveyella is parasitic on other red algae. The parasitic forms are colorless.
5. The cell wall contains cellulose and pectic materials together with certain polysaccharides called hydrocolloids, some of which contain sulfur, for example, agar is used widely in preparing solid food media for the growth of bacteria or fungi in the laboratory. Agar has no food value for bacteria or fungi. It is also used by human beings as a thickening and binding agent in various food products. Agar agar is obtained commercially from the red algae Geliclum and Gracilaria.
6. The cell walls of red algae also contain another bio-chemical carrageenin which is also widely used as a thickening and binding agent in food products, especially in puddings.
7. The cells may be uninucleated or multinucleated with one or more plastids that may be with or without pyrenoids. The food reserves lie in the form of floridean starch and a soluble sugar fluoridoside (a galactoside of glycerol).
8. Asexual reproduction takes place with the help of non-motile spores. Reproduction by fragmentation is not common, though some red algae are able to regenerate the full plant from a severed holdfast.
9. Sexual reproduction is highly advanced and oogamous. Flagellated gametes are absent in red algae. The male gametes called spermatia develop in specialized structures called spermatangia while the female cells are called carpogonia. The carpogonium is a flask-shaped body having a neck-like trichogyne.
10. Alternation of haploid and diploid multicellular generations occurs in many algae.

NEET Biology Notes For Plant Kingdom Thallophyta (Algae) Points To Remember

Red snow is caused by hypnospores of Chlamydomonas nivalis. They contain a carotenoid pigment called hematochrome

Bald spot: Receptive, colorless spot of oogonium through which male gamete enters. Chlamydomonas prefers habitats rich in ammonium salts.

NEET Biology Notes For Plant Kingdom Bryophyta

Bryophyta General Characters

1. They are found in moist and shady places, i.e., sporophytes. They are often found to grow during the rainy season forming green carpets or mats on damp soil, rocks, walls, tree trunks, etc.
   • Bryophyta Exceptions
     1. Aquatic Forms
        • Ricciafluitans
        • Ricciocarpus nutans
        • Riella; Funtinalis
     2. Saprophytic Forms:
        • Cryptothallus mirabilis
        • Buxbaumia aphylla
     3. Epiphytic Form:
        • Frullania
2. The dominant phase or plant body is a free-living gametophyte.
3. Vascular tissues are absent.
4. Roots are absent, instead, rhizoids occur. The latter may be unicellular or multicellular.
5. Vegetative reproduction is quite common through fragmentation, tubers, gemmae, buds, adventitious branches, etc. Mitospores are not formed.
6. Sex organs are multicellular and jacketed. They are of two types, male antheridium and female archegonium.
7. An external layer of water is essential for the swimming of male gametes and to reach the archegonia.
8. Fertilization produces an embryo inside the archegonium. The embryo grows into a sporophyte.
9. Sporophytes is parasitic over gametophyte.
10. The sporophyte of bryophytes is also called sporogonium. It produces haploid spores inside its capsule part while attached to the gametophyte.
11. On germination, each spore produces a gametophyte either directly or through a juvenile filamentous stage called protonema.
12. Bryophytes show heteromorphic or heterologous alter-nation of generations in the life cycle.

Classification Of Bryophyta: Shows the classification of Bryophyta.

Life Cycle Of Funaria hygrometrica

• Funaria is a radially symmetric plant that is differentiated into stem or central axis, leaves or phyllodes, and rhizoids. The rhizoids are multicellular, branched with oblique septa. It is monoecious and autogenous (i.e., both sex organs on the same plant but on different branches).

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• Antheredia are borne in groups at the tip of the main axis and surrounded by perigonial leaves. Archegonial clusters on lateral branch tip and surrounded by perichaetial leaves. Thus, both male and female gametangia are acrocarpous. Male gametes or sperms are elongated biflagellates with curved bodies. Number of neck canal cells (NCC) in archegonium is 6-10.

Diploid oospore develops into sporophyte. Mature sporogo- nium is differentiated into foot, seta, and capsule. The foot is embedded in a gametophytic plant body meant for the absorption of water, mineral salts, and fixation. Seta is a narrow stalk that lifts the capsule in the air. The capsule is differentiated into three parts: apophysis, theca, and operculum.

Apophysis contains assimilatory tissue; and stomata with a single annular guard cell. Theca contains central sterile columella, a spore sac, air cavity, and assimilatory tissue. The operculum is separated from theca by a one-celled elongated diaphragm. Above the hill lies a few layers of thin-walled cells fonning annulus.

Peristomial teeth are 32, acellular, arranged in two rings of 16 each. The outer peristomial teeth are exostome, and bear transverse thickening of cellulose, showing hygroscopic movement. Endostome or inner peristome teeth are without cellulose thickening, act as sieves, and check the sudden dispersal of spores.

NEET Biology Notes For Plant Kingdom Bryophyta Points To Remember

The smallest bryophyte is Zoopsts.

• The largest bryophyte is Dawsonia,
• The largest archegonium in the plant kingdom is of Funaria.

1. Gemmae in Funaria arc formed at the tips of chloroma branches of protonema.
2. Apospory is present in Funaria. It introduces polyploidy in the race.
3. Sphagnum upon death gets compressed and fossilized over thousands of years to produce a dark spongy mass called peat which is dried, cut, and used as fuel as well as good manure.
4. Dry Sphagnum can absorb 18-26 times its weight of water. In older times, Sphagnum moss was used in the place of absorbent cotton.
5. Polytrichum commune was employed in removing kid¬ney stones.
6. The oldest bryophyte fossils are 350 million years old, for example, Hepaticites, and Ricciopsis.
7. Bryophytes are adapted to land by having a waxy coating on aerial parts.
8. Bryophytes become seldom taller than 20 cm due to the lack of vascular tissue.
9. Elaters in Marchanlia are diploid and have spiral thickenings. They help in spore dispersal.
10. The air sacs in the capsule ofmoss act as shock absorbers.
11. Bryopsida: Common Names
    1. Polytrichum: Hair cap moss
    2. Sphagnum: Peat/bog moss
    3. Andrea: Granite moss
    4. Grimmia: Black moss
    5. Tortula: Twisted moss
    6. Ptilium: Feather moss
    7. Rhodobiyum: Rose moss
    8. Merceya: Copper moss
    9. Climacium: Tree moss
    10. Dicranum: Wind-blown moss

NEET Biology Notes For Plant Kingdom Pteridophyta

Pteridophyta General characters

1. The term pteridophyta was given by E. Haeckel for plants with feather-like fronds.
2. The first group of tracheophytes. The plant body is sporophytic.
3. Vascular tissues are present. They are of two types, xylem, and phloem. In the xylem, true vessels are absent. In phloem, companion cells are absent and sieve cells are present.
4. Meiospores are formed inside sporangia. Sporangia are associated with leaves called sporophylls.
5. Sporangium development is of two types:
   • Leptosporangiate: When sporangium develops from the single superficial cell, for example, Pteris, Diyopteris, and A-chan turn.
   • Eusporangiate: When sporangium develops from a group of cells, for example, Selaginella, or Equisetum.
6. Sex organs are multicellular and jacketed; antheridia are reduced. Archegonia are partially embedded.
7. Sperms arc flagellate. They require an external supply of water for swimming to reach Archegonia.
8. The embryo stage is present.
9. Embryogeny is of two types:
   • Exoscopic Embryogeny: Axis of the embryo is directed towards the archegonial neck, for example, the Equisetum.
   • Endoscopic: Axis of the embryo is directed inward towards the center, for example, Selaginella.
10. Development Of Embryo
    • Holoblastic: When the entire zygote is involved in embryonic development, for example, ferns.
    • Meroblastic: When a part of the zygote is involved in embryonic development, for example, Selaginella.
11. Heteromorphic or heterologous alternation of generations is present in the life cycle.

Steler System: The Steler theory was proposed by Van Teigham and Douliot.

Types Of Steler

1. Protostele: Simplest type, without pith

Haplostele: Example, Rhynia.

Actinostele: Example, Lycopodium serratum.

Plectostele: Example, Lycopodium clavatum.

Mixed Protostele: Example, Lycopodium cemum.

2. Siphonostele: With pith.

Ectophloic Siphonostele: Example, Osmunda, Botrychium.

Amphiphloic Siphonostele: Marxilea.

Solenostele: Stele with one or few leaf gaps. For example, Adiantum caudatum.

Dictyostele: Stele with a large number of overlapping leaf gaps, for example, Dryopteris, Pteris, etc.

Eustele: Conjoint, collateral open vascular bundles, for example, dicot stem, gymnosperm.

Atactostele: Closed vascular bundles scattered in the ground tissue, for example, monocot stem.

Life Cycle Of Selaginella Salient Features

1. The plant body is an evergreen sporophyte. It is differentiated into stems, leaves, and roots. The roots often develop at the tips of special structures called rhinophores.
2. The plant body can multiply vegetatively by fragmentation, bulbils, and tubers. Bulbils and tubers help in perennation as well.
3. Sporangia develop in the axil of fertile leaves or sporophylls at the tips of small branches called spikes. There are two types of sporangia, microsporangia and mega¬sporangia.
4. Each microsporangium produces a large number of small haploid microspores. A megasporangium develops only four haploid megaspores. Growth of the gametophytes is precocious.
5. Microspore produces an endosporic 13-celled male gametophyte. The latter has one urothelial cell eight jacket cells and four androgenic cells. The androgenic cells form 128-256 astrocytes or sperm mother cells. A sperm mother cell gives rise to a biflagellate sperm.
6. Megaspore produces a partially exospheric female gametophyte. The female gametophyte contains an exposed generative apical cushion and a storage tissue. The two are separated by a diaphragm. The apical cushion produces archegonia and rhizoids. Each archegonium has a single female gamete or oosphere. Fertilization requires the help of external water for the swimming of sperm. It produces diploid oospores.
7. The oospore divides to form a suspensor and embryo. The development of the embryo is meroblastic. A suspensor helps in pushing the embryo into food-laden storage tissue. The embryo has a foot for absorbing nourishment, a shoot tip, and a root tip. The latter two elongate and produce an independent sporophytic plant body.
8. Selaginella shows two distinct generations, sporophyte and gametophyte, one producing the other. They are morphologically different. The phenomenon is called hetero-morphic or heterologous alternation of generations.
9. Under dry conditions, the xerophytic species of Selaginella roll on into brown balls. The phenomenon is called cespitose habit. In this state, it may be uprooted. The brown balls become green and unroll again under moist conditions. Because of this characteristic, these plants are known as resurrection plants, for example, S. lepidophylia, and S. bryopteris.
10. The plant body of Selaginella chrysocaulos and S. selaginoides is erect. Its leaves are anisophyllous and arranged in two rows. In S. kraussiana, the plant body is prostrate. Its leaves are anisophyllous or dimorphic and arranged in four rows.
11. The stem in Selaginella is distelic. Rarely, the stem is monostelic.
12. Bower and Goebel named the rhizosphere of Selaginella as an organ suigeneris, i.e., the organ having the characteristics of both, i.e., stem as well as root, but independent in origin.
13. The mucilage oozing out of the neck of archegonium in Selaginella and fern contains malic acid. The neck of archegonium in Selaginella and fern contains only one neck canal cell.
14. Selaginella rupcstris shows a near approach to seed habit.

Ferns General Characters

1. Plant body is a sporophyte. It is differentiated into true stems, leaves, and roots.
2. The stem is an underground rhizome in most of the ferns. Some are called tree ferns, for example, Cyathea, and Celotia.
3. Leaves or their leaflets show furcate venation in which veins branch dichotomously without forming interconnections.
4. The younger parts of the stem, young leaves, petiole, and rachis of mature leaves possess hairs or scales called ramenta. Ramcnta protects them from mechanical injury and desiccation.
5. Young leaves show circinate ptyxis that is they are coiled in the form of a watch spring. This coiling protects the growing point which comes to lie in the center.
6. Sporangia occur on the leaves in clusters called sori (singular-sows). The fertile leaves are known as sporophylls. A sorus is covered by a flap-like outgrowth from its surface (true indusium, for example, Dryopteris) or turned margin of the sporophyll (false indusium, for example, Adiantum).

Life Cycle Of Dryopteris

1. Dryopteris Salient Features: It is found in moist shady places in tropical, subtropical areas. The plant body is perennial, independently living evergreen sporophyte having vascular tissues. It is differentiated into stems, leaves, and roots. Roots are adventitious.
   • Stem is an underground, dark brown rhizome. The large aerial leaves or fronds are incompletely bipinnate compound leaves. The young leaves show circinate ptyxis. Persistent leaf bases of the dead leaves arc found in the older parts of the rhizome.
   • In Dryopteris, a leaflet receives a single midrib which gives rise to lateral veins showing dichotomous divisions. The young leaves, young parts of the rhizome, petiole, and rachis of mature leaves are covered over by brown to black scales called paleae or ramenta.
2. Dryopteris Reproduction: The fern multiplies vegetatively by fragmentation of rhizome and development of adventitious buds. In Dryopteris, the adventitious bud present at the leaf base separates and grows into a new plant. In Adianlum caudatum and a few other species, adventitious buds develop at leaf tips.

When such leaf tips happen to touch the soil, they form new plants the process helps in spreading the fern over a large area. Because of this reason, Adianlum caudatum is also called walking fern and maidenhair fern.

The leaflets of a mature leaf bear sporangia in clusters called sori. The leaflets and leaves having sori or sporangia are called sporophylls. In Adianlum, the sori are borne submarginally at the distal end on the undersurface of the leaflets. The margin of the leaflet is reflexed to cover the sprus. The reflexed margin is known as false indusium.

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In Diyopteris, the sori develop in two rows, one on either side of the midrib. Each row contains 4-6 sori except in smaller leaflets which may have 1-2 sori or can be sterile. Each sorus is covered by a membraneous sheath of its own. This covering is carted true indusium. The covered sori of Dryopteris are kidney-shaped in outline. This has given the name of the male shield fern to Dryopteris.

A sorus consists of a parenchymatous cushion or placenta. The placenta bears a number of stalked biconvex sporangia. In each sporangium, there is a single-layered jacket that encloses 12-16 diploid spore mother cells. A marginal row of jacket cells are differentially thickened to form an annulus.

The remaining marginal cells constitute stomium. The diploid spore mother cells divide meiotically to form haploid spores. With the maturity of spores, the indusium shrivels. The exposed sporangia dehisce in the region of the stomium is due to differential contraction of the annulus. The spores are dispersed by air currents.

After failing on suitable soil, each spore germinates and forms a thalloid gametophyte called prothallus. Prothallus is cordate, green, and flat in structure. Prothallus bears antheridia and archegonia at the ventral surface.

Antheridium has a three-celled jacket and about 32 sperm mother cells. The sperms are multiflagellate (multiciliate) and spirally twisted. Flask-shaped archegonium has a single binucleate neck canal cell, a single venter canal cell, and an oosphere. Sperms are attracted to the opened archegonia by malic acid present in their mucilage. Diploid oospore gives rise to an embryo that grows in size to form the fern plant.

NEET Biology Notes For Plant Kingdom Pteridophyta Points To Remember

1. Common names
   1. Spike moss (Selaginella)
   2. Bird’s nest moss (Selaginella rupesiris)
   3. Club moss (Lycopodium)
2. Smallest ptcridophyte – Azolla (an aquatic fern)
3. Largest ptcridophyte = Cyathea (tree term)
4. Apogamy: It is the development of haploid sporophytes from gametophytes without fertilization. It was reported by Farlow in Pier is erotica.
5. Apospory: It is the development of diploid gametophyte from sporophyte without meiosis. It was reported by Druery in Athyrium. Due to apospory, polyploidy is common in ferns.
6. Psilofales such as Rliyniu were the first trachcophytes.
7. Azolla is a water fern used as a biofertilizer.

NEET Biology Notes For Plant Kingdom Gymnosperms

Gymnosperms General Characters

1. All gymnosperms are perennial woody plants forming either bushes or trees. Some of these are very large and live for thousands of years, for example, Sequoia sempervirens.
2. It is the smallest group in plantae with 70 genera and 900 living species. In India, 14 genera and 54 species have been found.
3. Leaves are generally dimorphic, foliage, and scale leaves.
4. Foliage leaves do not have lateral veins. Transfusion tissue (hydrostereom) occurs internally or in lateral transport.
5. Wood is homozygous but Gnetalcs bear vessels in the xylem, for example, Ephedra, Gnetum, and Welwitschia.
6. Types Of Wood
   • Manoxylic: Softwood, vascular tissues with medullary rays, commercially less important, for example, Cycas.
   • Pycnoxylic: Compact wood without or with narrow medullary rays; commercially more important, for example. Pious.
   • Monoxylic: With single persistent cambium rings and bundles, for example, Pious.
   • Polyxylic: With many persistent cambium rings and bundles, for example, Cycas.
7. Flowers absent. There are two types of sporophylls, microsporophylls and megasporophylls.
8. The two types of sporophylls are usually aggregated to form distinct cones or strobilli, pollen cones (male cones), and seed cones (female cones).
9. Seeds do not occur inside a fruit. They are naked or lie exposed on the surface of megasporophylls.
10. Ovules lie exposed on the megasporophyll. Ovules are unitegmic and orthotropous.
11. Pollination is direct as stigma is absent, and the pollen grains directly reach the micropylar ends of ovules. Pollination is usually accomplished by wind (anemophily).
12. The male gametophyte produces only two male gam¬etes or sperms. Generally one of them is functional.
13. External water is not required for the transport of male gametes. Instead, a pollen tube is formed by the male gametophyte to affect fertilization (siphonogamy).
14. Seeds contain a food-laden tissue for the future growth of embryos into seedlings.

Life Cycle Of Pinus: Pinns is a coniferous gymnosperm having pyramidal or excurrent shape like a “Christmas tree.” The sporophytic plant body is divided in stems, leaves, and roots. Stem branches are of two types, long and dwarf. The dwarf branches possess 1-5 needlelike foliage leaves which are surrounded by a sheath of scale leaves.

The number of needles is 1, 2, 3, 4, and 5 in a spur of P monophylla, P. sylvestris, P. roxburghi, P. quciddfolia, and P. wallichiana, respectively. It has a tap root system with long horizontal roots. Mycorrhiza occurs near the soil surface. Mycorrhizal roots lack root hairs and root caps.

• Vegetative reproduction is absent. Pinus is monoecious and develops clusters of shortly stalked male cones subterminally on the lower branches and female cones in the circle of 2-6 on the upper long branches.
• Each male cone has a short stalk, a central axis, and a number of spirally arranged microsporophylls. A microsporophyll bears two oblong, parallel microsporangia on its lower surface.
• In a microsporangium, the diploid microspore mother cells undergo meiosis and form haploid microspores or pollen grains.

At maturity, the pollen grains are released from the male cones and dispersed by air currents. Pollen grain has at this time two prothallial cells, a generative cell, and a tube cell. They are from yellow clouds in the pine forests. A pollen grain has two air sacs or wings to make it light. Male cones are homologous to dwarf branches.

• The female or seed cones develop in groups. 2-6 on the upper long branches of the tree. Each female cone has a long stalk and a central axis on which are a number of spirally arranged paired scales. The lower of the pair is called the bract scale while the upper scale is the ovuliferous scale or megasporophyll. The ovuliferous scale bears two ovules towards the basal region on the upper side.
• Each ovule has a Three-layered integument with a terminal wide, oblique pore or micropyle and a nucellus (megasporangium). In the middle of the nucellus, a megaspore mother cell differentiates. It forms four haploid megaspores by meiosis but only one survives.

The functional megaspore gives rise to the female gametophytes. The female gametophyte bears 1-8 archegonia. An archegonium has a short neck and a broad venter. The neck canal cell is absent. The Venter canal cell is ephemeral. The venter contains a large egg or oospore. Female cones are ho¬mologous to long branches.

• The female cones open in the year of theft formation for pollination. Pollination is anemophilous (by air) and direct. The pollen grains pass into the opened micropyle and rest on the top of the nucellus.
• The micropyle of each ovule contains a mucilage or pollination drop for catching the pollen. Here it germinates and forms a pollen tube but further growth is stopped due to the arrival of winter in the first year, in the spring of the next year, fertilization occurs.
• Fertilization occurs after 13 months from the time of pollination. The pollen tube grows and pierces an archegonium. One male gamete or male nucleus fuses with the oosphere to form a diploid zygote or oospore. The oospore forms an embryo while the ovule matures into a seed.

Part of the upper surface of the ovuliferous scale is peeled off along with the seed to form its wing. A female cone takes 26 months to reach maturity. It then opens to release winged seeds which are dispersed by air. After falling on suitable soil, each seed gives rise to a new plant.

NEET Biology Notes For Plant Kingdom Gymnosperms Points To Remember

The number of cotyledons in Finns ranges from 3 to 18.

1. There are four tiers proembryo basal embryo tier, suspenser tier, rosette tier, and upper tier. The development of sporangium in Pinus is eusporangiate and fertilization is siphonogamous type.
2. Resin is collected by injuring the Pinus.
3. Resin is a semifluid secreted by special tubes. It solidifies on exposure to air. Therefore, it plugs the places of injury. It is an antiseptic and commercially distilled to obtain turpentine and resin. Resin is used in waterproofing, sealing joints, and preparation of writing paper. Turpentine is used as a solvent in paints, polishes, and wax.
4. Seeds of P. gerardiana (chilgoza) are edible after roasting.
5. Smallest gymnosperm = Zamia Largest gymnosperm = Sequoia dendron
6. The three generations in a seed are:
7. Testa, tegmen, and perispenn represent parental sporophyte;
8. Endosperm represents female gametophyte;
9. Plumule, radicle, suspensor, and cotyledons (embryo) represent future sporophytes.
10. Polyembryony: It is the formation of more than one embryo inside a single seed. It was reported by Leu-wenhoek in oranges. Simple polyembryony is due to the fertilization of many eggs, for example, the Pinus ovule has 2-8 archegonia. Cleavage polyembryony is true polyembryony and very common. It is due to the splitting of embryo tissue. Adventive polyembryony is the formation of extra embryos directly from diploid cell (for example, rosette cells) other than embryonal cells.

Cycas

1. It is a dioecious plant.
2. Female cone absent.
3. The stem is unbranched, cylindrical, i.e., caudex.
4. Coralloid roots are present
5. Cycas show fern ancestors in the presence of ramenta, circinate ptyxis, and multicilliated sperms.
6. Vascular bundles are arranged like an inverted omega in the rachis.
7. It has the largest ovule, largest male gamete, and largest egg in the plant kingdom.
8. Fertilization occurs at three stages.
9. Fertilization is siphonogamous and zooidogamous.
10. Seed has two cotyledons.

NEET Biology Notes For Plant Kingdom Angiosperm (Flowering Plants)

These are the most advanced plants, characterized by the presence of flowers, covered ovules, and seeds. These form the largest group of the plant kingdom, containing about 300 families, 8000 genera, and about 300,000 species. These are the highest-evolved plants on this earth.

From the Cretaceous period, the angiosperms eclipsed all other vegetation, and now they are the most dominant plants. They are found almost everywhere in all possible habitats. Angiosperms include all varieties of plants such as hydrophytes, xerophytes, epiphytes, parasites, saprophytes, insectivorous, symbionts, mangroves, etc.

Characteristic Features Of Angiosperms

1. The presence of flowers is the most striking feature of angiosperms. It contains the sex organs of the plant.
2. Ovules are covered and are present inside the ovary.
3. pollen grains are received by the stigma of the carpel, so maturation of pollen grain (male gametophyte) takes place on the stigma.
4. Double fertilization (syngamy and triple fusion) is also characteristic only to angiosperms.
5. Endosperm is formed after fertilization, and it is generally triploid.
6. Vessels and companion cells are present in the xylem and phloem elements, respectively.
7. Fertilized ovules ripen into seeds. The seeds are cov¬ered by fruits. A fruit is technically a ripened ovary7. Fruits not only protect the seeds but also help in their dispersal.
8. The xylem contains vessels.
9. Phloem possesses sieve tubes and companion cells.
10. Spanish moss (Tillandsia) is an epiphytic angiosperm.
11. A gametophyte can be haploid or diploid but always produces gametes.
12. Monocots do not grow in girth though they grow in length and produce new leaves and flowers. Dicots have indefinite growth. New roots, shoots, leaves, bark, and wood are formed year after year.
13. Petals are showy and are meant to attract animal pollinators, especially insects.
14. In most of the plants, pollination is effected by animal insects, worms, birds, bats, and even human beings. Insectaccomplished pollination is termed as entomophily.
15. Angiosperms are divided into two sub-groups, i.e., dicots and monocots.

Dicots: They are angiosperms or flowering plants that are characterized by the presence of two cotyledons in the seed, and generally reticulate venation in leaves (with a few exceptions Calophyllum and Etyngium). Concentric tissues in the stem with open vascular bundles are arranged in a ring: penta or tetramerous flowers.

Monocots: They are angiosperms or flowering plants which are characterized by the presence of a single cotyledon in the seed, generally parallel venation in the leaves (except Smilax, Colocasia, and relatives), scattered closed vascular bundles in the stem, and trimerous flowers, for example, banana, cereals, palms, grasses, bamboo, lilies, orchids, etc.

Differences Between Various Plant Groups

NEET Biology Notes For Plant Kingdom Angiosperm (Flowering Plants)  Points To Remember

Bessey, Oswald Tippo, Cronquist, and Thom also proposed a phylogenetic system of classification.

1. Thallophyta term was given by Endicher.
2. M.O.P. Iyenger is known as the father of Indian psychology.
3. Also known as vascular cryptogams, vascular amphibians, snakes of the plant kingdom, are seedless tracheophytes.
4. Hironymus divided the genus Selaginella in two subgenera, viz., Homeophyllum and Heterophyllum.
5. The term gymnosperm was used by Theophrastus in his book “Enquiry into Plants.”
6. C. revolution is called as sago palm. Sago is obtained from C. revolute.
7. Drug ephedrine is obtained from Ephedra which is used in curing respiratory ailments including asthma.
8. Ginkgo biloba is known as the “maidenhair tree.”

NEET Biology Notes For Plant Kingdom Different Life Cycles And Alternation Of Generation

The life cycle of a plant is called the alternation of generations. In plant life, there are two phases, one diploid sporophyte and one haploid gametophyte, which give rise to each other. In this process syngamy and meiosis are instrumental.

Sporophyte produces spore after meiosis which develops into gametophyte (gamete-producing plant). The gametes undergo fusion/syngamy to form a diploid zygote. The fate of the zygote depends upon the kind of life cycle.

Types Of Life Cycles or Patterns Of Alternation Of Generations: There are three chief kinds of life cycle. They fundamentally differ in the duration of the haploid and diploid phases. They are as follows.

Haplontic: It is characterized by the dominance of haplophasc. The plant body is gametophyte and independent. The sporophyte is dependent on gametophyte and is not a free-living body. The major part of life is enjoyed by gametophytes, for example, the majority of green algae viz, Chlamydomonas, Ulothrix, Spirogyra, etc.

Diplo-haplontic Or Haplo-diplontic: it is characterized by the existence of sporophyte as well as gametophyte. Both of them are photosynthetic and free-living and the two phases, diplophase and haplophse, are nearly equal.

If sporophytes and gametophytes are morphologically different, then the life cycle is termed as diplo-haplontic; heteromorphic, for example, All pteridophytes; all bryophytes; some brown algae, viz., Laminaria and other kelps.

If the two phases are morphologically identical, then the life cycle is termed as diplo-haplontic isomorphic, for example, Green algae, viz., Ulva and claclophora\brown algae, viz., Eclocarpns.

Diplontic: It is characterized by the dominance of diplopia. The plant body is sporophyte which is independent and free living. The gametophytes are extremely reduced and are dependent physically as well as nutritionally on (lie sporophyte. the major purl of life is enjoyed by the sporophyte, for example, all gymnosperms and angiosperms; diatoms; some brown algae, viz., Fuats and Sargussum.

NEET Biology Notes For Plant Kingdom Assertion Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: Thallophytes are non-vascular, non-archego- niate. and non-cormophytic plants.

Reason: Thallophytes lack vascular bundles, archegonia, and differentiated plant body.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 2. Assertion: Funaria archegonium has a maximum concentration of sucrose at the tip of the neck.

Reason: Male gametes show chemotropic movement.

Answer: 3. If Assertion is true, but Reason is false.

Question 3. Assertion: Pyrenoids may or may not be surrounded by a sheath of starch plates in algae.

Reason: In higher plants, these are replaced by amyloplasts.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 4. Assertion: Seeds are formed by some species of spike moss.

Reason: All conditions for seed habit are fulfilled by these species of spike moss.

Answer: 4. If both Assertion and Reason are false.

Question 5. Assertion: The resin duct in coniferales is schizogenous in origin.

Reason: The resin duct helps to retain water as well as seals the injured areas of plants.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 6. Assertion: Chlorella, a green alga, is commonly known as space alga.

Reason: It is used by exobiologists for oxygen and disposal of nitrogen in prolonged space flight.

Answer: 3. If Assertion is true, but Reason is false.

Question 7. Assertion: Bryophytes are not the amphibians of Plant Kingdom.

Reason: An external layer of sucrose instead of water is necessary for the movement of antherozoids.

Answer: 4. If both Assertion and Reason are false.

Question 8. Assertion: Polyploidy is very common in the members of Filicophyta.

Reason: It is due to the development of gametophytes

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 9. Assertion: The micropyle of Pinus ovule contains pollination.

Reason: The ovule of Pinus is unitegmic and orthotrodirectly from sporophyte without meiospore formation.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 10. Assertion: Calophyllum leaf has parallel venation and ligation drop for catching the pollen.

Reason: It is the plant of the dicot group of angiosperms.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

NEET Biology Notes For Reproduction In Organisms Introduction

Every organism can live only for a certain period of time. The period from the birth to the natural death represents its life span.

Life spans of organisms are not necessarily correlated with their sizes. The sizes of crows and parrots are not very different, yet their life spans show a wide difference. Similarly, a mango tree has a much shorter life span as compared to a peepal tree. Whatever be the life span, the death of every individual organ- ism is certain, i.e., no individual is immortal, except single- celled organisms.

Reproduction is defined as a biological process in which an organism gives rise to young ones (offsprings) similar to itself. The offspring grows, matures, and, in turn, produces new off- spring. Reproduction enables the continuity of the species, generation after generation.

Based upon whether there is participation of one organism or two, reproduction is of two types: asexual and sexual. When an offspring is produced by a single parent without the involvement of gametes, the reproduction is called asexual reproduction. When two parents (opposite sex) participate in the reproductive process and also involve the fusion of male and female gametes, it is called sexual reproduction.

NEET Biology Notes For Reproduction In Organisms Vegetative Reproduction

In vegetative reproduction, new plants or individuals are produced from the vegetative parts of plants. Newly formed in- dividuals are genetically identical to the parent plant. It is common in flowering plants.

Vegetative reproduction is of two types: natural vegetative reproduction and artificial vegetative reproduction.

Natural Vegetative Reproduction

12th class biology notes

Natural vegetative reproduction takes place by the following means:

• By roots: A portion of root breaks and gives rise to new plant as seen in Dalbergia sissoo, Populus, Psidium, Murraya, Albizia, sweet potato, tapioca, yam, Dahlia, and Asparagus.
• By underground stem: A portion of underground stem bearing bud forms a new plant as seen in rhizome (e.g., banana, turmeric, ginger, Aspidium, and Adian- tum), corm (e.g., Gladiolus, Colocasia, Freesia, and Crocus), bulb (e.g., garlic, Narcissus, and onion), and tuber (e.g., potato and artichoke).

Read and Learn More NEET Biology Notes

• By creeping stem: Examples are runners (e.g., grass), stolons (e.g., strawberry, Vallisneria), and offset (e.g., Eichhornia).
• By aerial shoots: Examples are Opuntia and sugarcane.
• By leaves: Examples are walking fern (Adiantum), Bryophyllum, Begonia, Streptocarpus, and Saint- paulia.
• By bulbils: These are fleshy buds which produce new plants. Examples are Agave, Oxalis, Ananas, Di- oscorea, lily, and Chlorophytum.
• By turions: These are fleshy buds in aquatic plants. Examples are Potamogeton and Utricularia.

Artificial Vegetative Reproduction

Artificial vegetative reproduction takes place by the following ways:

• Cutting: Senseviera is propagated by leaf cuttings. Stem cutting is employed in case of rose, Duranta, and Bougainvillea. Root cuttings are used in blackberry and raspberry.
• Layering: A lower branch is bent down and a ring of the bark is removed. This part is covered by soft soil. Roots develop in 2-3 months. This branch is cut off and grown independently. It is common in lemon and grapes. It is of four types:
  • Tip layering: The tip of the current season’s shoot is bent in the sloping hole. For example, blackberry and raspberry.
  • Serpentine layering: Basal branch is pegged in the soil at several places. For example, Clematis.
  • Mound layering: Basal part of a lower branch is bent down and the tip is kept outside the soil. For example, currant and gooseberry.
  • Air layering: In air layering (gootee), a ring of bark is removed from an aerial shoot. It is covered by grafting clay (water, clay, cow dung, and hay) with small quantity of root-promoting hormone and is wrapped in polythene. After 1-3 months, roots appear and the shoot is removed to be used for planting. For example, litchi and pomegranate.
• Micropropagation: Micropropagation is the raising of new plants from a small plant tissue with the help of tissue culture technique. Tissue culture is the technique of maintaining and growing cells, tissues, etc., and their differentiation on artificial medium under aseptic conditions inside suitable containers.
• Grafting: In grafting, a new plant is developed by the association of stock and scion. Scion grows and retains all its qualities. It is useful in Citrus, mango, rose, apple, pear, Lathyrus, orange, etc.
  The shoot (scion or graft) of one plant is joined to the stump (root system or stock) of a related plant through different unions such as tongue grafting (whip or slice grafting), wedge grafting, crown grafting, and side. grafting. In crown grafting, several scions are joined to a single stock. In approach grafting, the shoots of two independently growing plants are brought together. It involves the removal of a slice of bark. A small tongue- like cut is given before joining together by grafting wax. The cut must be at least 2.5-5 cm away from the removal area of the bark. After union, the stock is cut above the graft while the scion plant is cut below the graft. In bud grafting, the scion is a bud with a small piece of bark, e.g., rose, apple, peach, etc.
  Grafting is used for quick multiplication and proper growth of better varieties with weak roots, e.g., mango, apple, pear, rubber, orange, etc.

NEET Biology Notes For Reproduction In Organisms Asexual Reproduction

In the asexual method of reproduction, a single individual (parent) is capable of producing offsprings.

Asexual reproduction is common among single-celled organisms and in plants and animals with relatively simple organizations. In protists and monerans, the organism or the parent cell divides into two to give rise to a new individual.

Members of the kingdom fungi and simple plants such as algae reproduce through special asexual reproductive structures. The most common of these structures are conidia and zoospores.

In Penicillium, the conidia are produced exogenously at the tips of conidiophores by constriction. The conidiophores may be unbranched (monoverticillate) or branched (biverticillate). The branches of conidiophores are known as metulae. Each metula bears 2-6 flask shaped structures called sterigmata (phialides). Each sterigma produces a chain of pigmented conidia. Each conidium is multinucleated. The conidia in the chain are arranged in basipetal manner.

chapter 1 class 12 biology notes

In Chlamydomonas, under favorable condition, asexual re- production takes place by zoospores formation. The protoplast of cell divides to form 8-16 zoospores. They are pyramid shaped and anteriorly bifiagellate resembling the parent cell. The parent cell wall breaks and the zoospores are liberated in water. They enlarge and behave as an adult individual.

NEET Biology Notes For Reproduction In Organisms Sexual Reproduction

Sexual reproduction involves the formation and fusion of the male and female gametes, produced either by the same individual or by different individuals of opposite sex. The gametes fuse to form a zygote which develops to form a new organism. Sexual reproduction results in offsprings that are not identical to the parents or amongst themselves.

All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. This period of growth is called the juvenile phase. It is known as vegetative phase in plants.

The end of juvenile/vegetative phase, which marks the be- ginning of reproductive phase, can be seen easily in higher plants when they come to flowering.

Plants that are annual and biennial show clear-cut vegeta- tive, reproductive, and senescent phases, but in the perennial species, it is very difficult to clearly define these phases. Bam- boo species flower only once in their lifetime generally after 50-100 years. Strobilanthus kunthiana (neelakuranji) flowers once in 12 years.

Events in Sexual Reproduction

After the attainment of maturity, all sexually reproducing organisms exhibit events and processes that have remarkable fundamental similarity, even though the structures associated with sexual reproduction are indeed very different. These sequential events may be grouped into three distinct stages: (a) pre-fertilization, (b) fertilization, and (c) post-fertilization events.

• Pre-fertilization events: Two main pre-fertilization events are gametogenesis and gamete transfer.
  • Gametogenesis: It refers to the process of formation of gametes-male and female. Gametes are haploid cells. In some algae, both gametes are structurally and functionally similar. It is not possible to categorize them into male and female gametes. They are, hence, called isogametes or homogametes. However, in a majority of sexually reproducing organisms, the gametes produced are of two morphologically distinct types (hetero- gametes). In such organisms, the male gamete is called the antherozoid or sperm and the female gamete is called the egg or ovum.
    The male and female reproductive structures may be found in the same plant (bisexual) or in different plants (unisexual). In several fungi and some plants, terms such as homothallic and monoecious are used to denote bisexual condition while heterothellic and dioecious are used to describe unisexual condition.
    In flowering plants, the unisexual male flower is called staminate (bearing stamens) while the female flower is called pistillate (bearing pistils). In some flowering plants, both male and female flowers may be present on the same individual (monoecious) or on separate individuals (dioe- cious). Some examples of monoecious plants are Acalypha, cucurbits, and coconuts and those of dioecious plants are mulberry, papaya, and date palm.

Several organisms belonging to monera, fungi, algae, and bryophytes have haploid plant body; but organisms belonging to pteridophytes, gymnosperms, and angiosperms, and most of the animals including human beings have diploid parental body. In diploid organisms, specialized cells called meiocytes (gamete mother cell) un- dergo meiosis. At the end of meiosis, only one set of chromosomes gets incorporated into each gamete.

Sexual reproduction in Chara and Marchantia: Chara is green algae. It is oogamous. The sex organs are highly specialized. While some workers preferred to call the male sex organ as antheridium and the female one as oogonium, others did not favor this terminology. They called the male sex organ as globule and the female one as nucule; this terminology is largely followed in Chara. The sex organs are borne on the adaxial face of the short lateral branch almost on each node.

ch 1 class 12 biology

The nucule occupies a higher position than the globule. While most of the species of Chara are monoecious, Catreus wallichii is dioecious. The globule matures prior to the nucule (protandrous). Each antheridium produces band-shaped, spirallycoiled biflagellate antherozoids. The oogonium contains a single egg. The egg is laiden with starch and oil globules.

In Marchantia, the archegonia are borne on spe- cial branches called archegoniophores or the female receptacles. The archegonia may be stalked or sessile. The archegoniophore, or carpocepha- lum, has rows of archegonia protected by invo- lucre or perichaetium. In some plants, a perianth also protects an archegonial group. The arche- gonia are flask-shaped structures distinguishable into a long neck and globular, swollen venter.

A multicelled stalk is also present in the archegonia of mosses, but in others, it is very short. The neck is one-cell thick. It is generally made up of six vertical rows of cells but in Jungermamnniales, it is composed of four or five vertical rows only. The neck is capped by four cover cells and contains varying number of neck canal cells inside.

• • Gamete transfer: After the formation of male and female gametes, they must be physically brought together to facilitate fusion (fertilization). Exceptions are a few fungi and algae in which both types of gametes are motile. There is a need of a medium through which the male gametes move. In several simple plants such as algae, bryophytes, and pteridophytes, water is the medium through which gamete transfer takes place a large number of male gametes, however, fail to reach the female gametes. To compensate this loss of male gametes, the number of male gametes produced is several thousand times the number of female gametes produced.
    In seed plants, pollen grains are the carriers of male gametes and ovule bears the egg. Pollen grains are produced in anthers and, therefore, have to be transferred to the stigma before it can lead to fertilization.

class 12th biology reproduction in organisms

• Fertilization events: The most important event of sexual reproduction is the fusion of gametes. This process is called syngamy. It results in the formation of a diploid zygote.
  In most aquatic organisms, such as in majority of algae and fishes as well as amphibians, syngamy occurs in the external medium (water). This type of gametic fusion is called external fertilization. In a majority of plants, such as bryophytes, pteridophytes, gymnosperms, and angiosperms, syngamy occurs inside the body of the organism. Hence, this process is called in- ternal fertilization.
• Post-fertilization events: In sexual reproduction, the events that take place after the formation of zygote are called post-fertilization events.
  The process of development of embryo from the zygote is called embryogenesis. In animals, the zygote starts developing soon after its formation. In flowering plants, after fertilization, ovary develops into fruit and ovules mature into seeds. Inside the mature seed is the progenitor of the next generation-the embryo.