Question

Question 1.

A strain of E. coli carries a substitution mutation causing the terminator codon 5’-UAG-3’ to occur at an interior position in a particular type of mRNA. This strain also carries a suppressor mutation that alters the anti-codon of a type of tRNA molecule so that it reads 3’-AUC-5’. This type of tRNA reads this terminator codon and inserts the amino acid tyrosine.

a.What effect would the presence of both of these mutations together have on the polypeptide translated from the mRNA carrying the mutant terminator codon?

b.Do you expect that there might be other genes whose translation would be altered by the presence of the tRNA produced by this suppressor gene? If so, describe the circumstances under which translation would be altered and explain how it would be altered.

Answer 1

A). The translation will not stop at the stop codon UAG because it has complementary tRNA (AUC) for tyrosine. While ribosome reading the stop codon can be read as tyrosine, so the process continues.

B).
Substitution Mutation is caused by the insertion of one base for the other (exchange). The presence of both will alter the structure of the protein because the substitution of the 5UAG3 will alter the entry of the correct AMINOACID at that place. The presence of a suppressor tRNA with 3AUC5 will read the stop codon and keep tyrosine. This may or may not alter the sequence.

Case1: the mutation is at 3rd nucleotide of UAG will not cause alteration because of UAU, UAC is for tyrosine, and the suppressor tRNA also code for the TYROSINE and also the stop codon for this translation not be UAG at the end of translation.

(this happen when the third nucleotide is not be the alanine (A) because UAA is a stop codon and no tRNA for this codon. if at all this is the temninatin site for this protein than this will not stop by the mutaion at correct terminantion site leads to alter the sequence)

Case2: if the alteration in the 1st or 2nd nucleotide will alter the sequence.