A 900 bp open-reading frame (ORF) that begins with an AUG start codon and ends with a stop codon would encode a polypeptide containing __________ amino acids.

Question

A 900 bp open-reading frame (ORF) that begins with an AUG start codon and ends with a stop codon would encode a polypeptide containing __________ amino acids.

Why is the answer 299?

Answer 1

In general, three base pairs code for one amino acid, so given 3x base pairs you will have x amino acids. However, the stop codon does not code for any amino acids; it simply tells the ribosome where to begin translation. The AUG start codon does however code for the special amino acid f-Met (formyl-methionine). As a result, the number of base pairs that will correspond to amino acids is 897 after you subtract the 3-base pair stop codon. 897 divided by 3 is 299, giving you the number of amino acids in the polypeptide. Hope this helps!