Question 1. A strain of E. col

Question 1.

A strain of E. coli carries a substitution mutationcausing the terminator codon 5’-UAG-3’ to occur at an interiorposition in a particular type of mRNA. This strain also carries asuppressor mutation that alters the anti-codon of a type of tRNAmolecule so that it reads 3’-AUC-5’. This type of tRNA reads thisterminator codon and inserts the amino acid tyrosine.

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

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


A). The translation will not stop at the stop codon UAG becauseit has complementary tRNA (AUC) for tyrosine. While ribosomereading the stop codon can be read as tyrosine, so the processcontinues.

B).Substitution Mutation is caused by the insertion of one base forthe other (exchange). The presence of both will alter the structureof the protein because the substitution of the 5UAG3 will alter theentry of the correct AMINOACID at that place. The presence of asuppressor tRNA with 3AUC5 will read the stop codon and keeptyrosine. This may or may not alter the sequence.

Case1: the mutation is at 3rd nucleotide of UAG will not causealteration because of UAU, UAC is for tyrosine, and the suppressortRNA also code for the TYROSINE and also the stop codon for thistranslation 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 allthis is the temninatin site for this protein than this will notstop by the mutaion at correct terminantion site leads to alter thesequence)

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

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