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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.

Read and Learn More NEET Biology Notes

• 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.

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

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.
• 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.

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.

• 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).

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.

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.

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.”

• 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.

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.

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.

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.

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.

 

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).

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.
• 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

Genetics Terms And Symbols

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.

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.

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.

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.

• • 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.

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.

• • 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

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.

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.

 

• 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.

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.

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.

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.

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).

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.

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.

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

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

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

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.

Formula Chart

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.

 

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).

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.

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.

Read and Learn More NEET Biology Notes

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.

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.

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.

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.

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.

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.

 

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).

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.

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.

Read and Learn More NEET Biology Notes

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.

• • 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.

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.

• 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.

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.

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).

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

Events In Human Reproduction

Gametogenesis→ Insemination → Fertilization → Implantation → Gestation → Parturition

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.

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.

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.

 

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.

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.

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.

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.

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.

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.

 

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).

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.

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).

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.

Read and Learn More NEET Biology Notes

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.

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.

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).

 

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).

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.

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).
• 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.

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).

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.

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.

 

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).

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.

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

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.

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.

 

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.

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 or- gans are highly specialized. While some workers preferred to call the male sex organ as antheridium and the female one as oogonium, others did not fa- vor this terminology. They called the male sex or- gan 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.

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 fe- male 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.
• 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.