Question

A matured mRNA has 300 bases with a single stop codon. What would be the length of the polypeptide synthesized from this mRNA?
A. always 100 amino acids.
B. always 99 amino acids.
C. maximum of 99 amino acids.
D. maximum of 100 amino acids.

Answer 1

Codons are triplet in nature; thus, three bases in mRNA code for one aminoacid in a polypeptide chain. This means that your chain of 300 bases should code for 100 aminoacids in a chain. However, the last codon is one of the following mutations — UAA, UAG, or UGA. These are called stop codons or nonsense codons, because they don’t code for an aminoacid.

Thus, your polypeptide chain will be 99 aminoacids long.


B is the correct answer.