Which of the following will have the strongest effect on a gene product, i. e. most likely to completely block function of the protein?

a) a +1 base pair insertion just after the stop codon.
b) a -1 base pair deletion shortly after the start codon.
c) a missense mutation in the middle of the coding region.
d) a nonsense mutation just before the stop codon.
e) these are all equally likely to block function of the protein.

B) A -1 base pair deletion shortly after the start codon.

Explanation:

A base pair deletion right after the start codon will block the protein function because it will cause the entire reading frame to shift. Therefore the protein which will be synthesized out of such mRNA will be non-functional because it will be entirely different from that of the original protein.

In other options, the protein synthesized will not be this much different from the original protein and hence it will be functional although the efficiency of the protein will be less as compared to original protein.

The protein which will be synthesized as described in option A will be same as of original protein because a base pair insertion after stop codon does not impact the protein composition at all.

The protein which will be synthesized as per option c will have only one amino acid difference as compared to the original protein so it will be functional. Similarly the protein synthesized as per option D will also be functional because the only amino acid which will be different from the original protein will be the last amino acid of the polypeptide and hence will not affect the function of protein adversely.

D) A non sense mutation just before a stop codon

Explanation:

A non sense mutation is a mutation in which a sense codon that corresponds to one of the twenty amino acids specified by the genetic code is changed to a chain-terminating codon. As the chain is terminated, the function of the the protien is completely lost because the amino acids cannot b produced and the process is terminated. Instead of substituting one amino acid for another, however, the altered DNA sequence prematurely signals the cell to stop building a protein. This type of mutation results in a shortened protein that may function improperly or not at all.

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