Do you think that the alternate splicing of exons may enable a structural gene to code for several isoproteins from one and the same gene? If yes, how? If not, why so?
Functional mRNA of structural genes need not always include all of its exons. This alternate splicing of exons is sex-specific, tissue-specific and even developmental stage-specific. By such alternate splicing of exons, a single gene may encode for several isoproteins and/ or proteins of similar class.
In absence of such a kind of splicing, there should have been new genes for every protein/isoprotein. Such an extravagancy has been avoided in natural phenomena by way of alternate splicing.
Comment on the utility of variability in number of tandem repeats during DNA fingerprinting.
Tandemness in repeats provides many copies of the sequence for finger-printing and variability in nitrogen base sequences present in them. Being individual-specific, this proves to be useful in the process of DNA fingerprinting.
Give an account of Hershey and Chase experiment. What did it conclusively prove? If both DNA and proteins contained phosphorus and sulphur do you think the result would have been the same?
Hershey and Chase conducted experiments on bacteriophage to prove that DNA is the genetic material.
Hershey and Chase experiment
(i) Some bacteriophage virus were grown on a medium that contained radioactive phosphorus ( ${ }^{32} \mathrm{P}$ ) and some in another medium with radioactive sulphur ( ${ }^{35} \mathrm{~S}$ ).
(ii) Viruses grown in the presence of radioactive phosphorus ( ${ }^{32} \mathrm{P}$ ) contained radioactive DNA.
(iii) Similar viruses grown in presence of radioactive sulphur ( ${ }^{35} \mathrm{~S}$ ) contained radioactive protein.
(iv) Both the radioactive virus types were allowed to infect E. coli separately.
(v) Soon after infection, the bacterial cells were gently agitated in blender to remove viral coats from the bacteria.
(vi) The culture was also centrifuged to separate the viral particle from the bacterial cell.
Observations and Conclusions
(i) Only radioactive ${ }^{32} \mathrm{P}$ was found to be associated with the bacterial cell, whereas radioactive ${ }^{35} \mathrm{~S}$ was only found in surrounding medium and not in the bacterial cell.
(ii) This indicates that only DNA and not protein coat entered the bacterial cell.
(iii) This proves that DNA is the genetic material which is passed from virus to bacteria and not protein.
If both DNA and proteins contained phosphorus and sulphur, the result might change.
In case (i)
Radioactive ${ }^{35} \mathrm{~S}$ and + Bacteriophage ${ }^{32} \mathrm{P}$ labelled protein capsule $\longrightarrow$ No radioactive
${ }^{35} \mathrm{~S}$ and ${ }^{32} \mathrm{P}$ Detected in cells + Radioactivity ( ${ }^{35} \mathrm{~S}$ and ${ }^{32} \mathrm{P}$ ) detected in supernatant
In case (ii)
Radioactive ${ }^{35} \mathrm{~S}$ and ${ }^{32} \mathrm{P}$ lebelled DNA + Bacteriophage $\longrightarrow$ Radioactive ${ }^{32} \mathrm{P}$ and ${ }^{35} \mathrm{~S}$
Detected in cells + No radioactivity detected in supernatant
During the course of evolution why DNA was choosen over RNA as genetic material. Give reasons by first discussing the desired criteria in a molecule that can act as genetic material and in the light of biochemical differences between DNA and RNA.
A molecule that can act as a genetic material must fulfil the following
(i) It should be able to generate its replica (replication).
(ii) It should chemically and structurally be stable.
(iii) It should provide the scope for slow changes (mutation) that are required for evolution.
(iv) It should be able to express itself in the form of Mendelian.
Biochemical differences between DNA and RNA
(i) Both nucleic acid (DNA and RNA) are able to direct their duplication proteins fails for the first criteria.
(ii) RNA is reactive, it also acts are catalyst, hence DNA is less reactive and structurally more stable than RNA.
(iii) Presence of thymine at the place of uracil also confers additional stability to DNA.
Give an account of post transcriptional modifications of a eukaryotic mRNA.
Post-transcriptional Modifications
The primary transcripts are non-functional, containing both the coding region, exon and non-coding region, intron in RNA and are called heterogenous RNA or hnRNA. In eukaryotes, three types of RNA polymerases are found in the nucleus
(i) RNA polymerase I transcribes rRNAs ( 28 S and 5.8 S ).
(ii) RNA polymerase II transcribes the precursor of mRNA (called heterogeneous nuclear RNA or hnRNA).
(iii) RNA polymerase III transcribes $t$ RNA, 5 S rRNA and snRNAs (small nuclear RNAs).
The hnRNA undergoes two additional processes called capping and tailing.
In capping, an unusual nucleotide, methyl guanosine triphosphate is added to the 5 '-end of hnRNA.
In tailing, adenylate residues (about 200-300) are added at 3'-end in a template independent manner.
Now the hnRNA undergoes a process where the introns are removed and exons are joined to form $m$ RNA by the process called splicing.
$$ \text { Diagram representation of a post transcriptional modification in eukaryotes } $$