A hypothetical cell is exposed to a chemical agent that alters the structure of serine converting it to alanine but only when the serine is activated on an appropriate tRNA. What is the most likely outcome of this modification relative to protein synthesis?

Question

Quizplus · Accepted Answer

The translational machinery does not have a mechanism to detect the change from serine to alanine once the tRNA is charged, so alanine will be incorporated into the protein instead of serine. It is absolutely necessary that the discrimination of correct amino acid and correct tRNA be made by a given aminoacyl tRNA synthetase prior to release of the aminoacyl-tRNA from the enzyme. Once the product is released there is no further way to proof-read whether a given tRNA is coupled to its corresponding amino acid. Erroneous coupling would lead to the wrong amino acid being incorporated into the polypeptide since the discrimination of amino acid during protein synthesis comes from the recognition of the anticodon of a tRNA by the codon of the mRNA and not by recognition of the amino acid. Desulfuration of cys-tRNAcys with Raney nickel generates ala-tRNAcys. Alanine is then incorporated into an elongating polypeptide where cysteine should have been.

A Hypothetical Cell Is Exposed to a Chemical Agent That