That background information did Watson and Crick have made available for developing a model of DNA? What was their contribution?
Watson and Crick had the following informations which helped them to develop a model of DNA.
(i) Chargaff's Law suggesting $\mathrm{A}=\mathrm{T}$ and $\mathrm{C}=\mathrm{G}$
(ii) Wilkins and Rosalind Franklin's work on DNA crystal's X-ray diffraction studies about DNAs physical structure.
Watson and Crick proposed
(a) Pattern of complementary bases pair
(b) Semi-conservative replication
(c) Mutation through tautomerism
What are the functions of
(i) methylated guanine cap?
(ii) poly-A 'tail' in a mature on RNA?
(i) Methylated guanine cap helps in binding of $m$ RNA to smaller ribosomal sub-unit during initiation of translation.
(ii) Poly-A tail provides longevity to $m$ RNA's life. Tail length and longevity of $m$ RNA are positively correlated.
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