Recall the experiments done by Frederick Griffith, Avery, MacLeod and McCarty, where DNA was speculated to be the genetic material. If RNA, instead of DNA was the genetic material, would the heat killed strain of Pneumococcus have transformed the R-strain into virulent strain? Explain.
RNA is more liable and prone to degradation (owing to the presence of $2^{\prime} \mathrm{OH}$ group in its ribose). Hence, heat-killed S-stain may not have retained its ability to transform the R-strain into virulent form if RNA was its genetic material.
You are repeating the Hershey-Chase experiment and are provided with two isotopes ${ }^{32} \mathrm{P}$ and ${ }^{15} \mathrm{~N}$ (in place of ${ }^{35} \mathrm{~S}$ in the original experiment). How does you expect your results to be different?
Use of ${ }^{15} \mathrm{~N}$ will be inappropriate because method of detection of ${ }^{32} \mathrm{P}$ and ${ }^{15} \mathrm{~N}$ different ( ${ }^{32} \mathrm{P}$ being a radioactive isotope while ${ }^{15} \mathrm{~N}$ is non-radioactive but is the heavier isotope of nitrogen).
Even if ${ }^{15} \mathrm{~N}$ was radioactive then its presence would have been detected, both inside the cell ( ${ }^{15} \mathrm{~N}$ incorporated as introgenous base in DNA) as well as in the supernatant, because ${ }^{15} \mathrm{~N}$ would also get incorporated in amino group of amino acids in proteins. Hence, the use of ${ }^{15} \mathrm{~N}$ would not give any conclusive results.
There is only one possible sequence of amino acids when deduced from a given nucleotides. But multiple nucleotides sequence can be deduced from a single amino acid sequence. Explain this phenomena.
Some amino acids are coded by more then one codon (known as degeneracy of codons), hence, on deducing a nucleotide sequence from an amino acid sequence, multiple nucleotide sequence will be obtained, e.g., Ile (Isoleucine) has three codons AUU, AUC, AUA. Hence, a dipeptide Met-Ile can have the following nucleotide sequence.
(i) AUG-AUU
(ii) AUG-AUC
(iii) AUG-AUA
And if, we deduce amion acid sequence from the above nucleotide sequences, all the three will code for Met-Ile.
A single base mutation in a gene may not 'always' result in loss or gain of function. Do you think the statement is correct? Defined your answer.
The statement is correct. Because of degeneracy of codons, mutations at third base of codon, usually does not result into any change is phenotype. This is called silent mutations.
On other hand, if codon is changed in away that now it specifies another amino acid, it may other the protein function as it happens in cse of $\beta$-globulin of haemoglobin protein. Where a substitution of valine instead of glutamic acid causes change in its structure and function, and resulting into sickle-cell trait.
A low level of expression of lac operon occurs at all the time. Can you explain the logic behind this phenomena.
In the complete absence of expression of lac operon, permease will not be synthesised which is essential for transport of lactose from medium into the cells. And if lactose cannot be transported into the cell, then it cannot act as inducers. Hence, cannot relieve the lac operon from its repressed state.