The codons for the various amino acids are given below. Answer the question.
What will be the number of amino acids in a polypeptide chain formed from the RNA with the given codons?
AUG GUU GCA CGU UAA AAU AAG
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 4
Read the codons from the table one by one: AUG = Met (start codon), GUU = Val, GCA = Ala, CGU = Arg, UAA = stop codon.
Translation ends at the stop codon. A stop codon codes for no amino acid, and the codons after it (AAU, AAG) are not read. The polypeptide is Met–Val–Ala–Arg, so it has 4 amino acids.
Option (1) 6 — Incorrect. 6 counts every codon except UAA, as if AAU (Asn) and AAG (Lys) after the stop codon were also translated.
Option (2) 7 — Incorrect. 7 counts all seven codons, including the stop codon, which codes for no amino acid.
Option (3) 4 — Correct. Translation starts at AUG and stops at the first stop codon. AUG (Met), GUU (Val), GCA (Ala) and CGU (Arg) are read, then UAA stops it, so the chain has 4 amino acids; AAU AAG after it are not translated.
Option (4) 5 — Incorrect. 5 counts the stop codon UAA as if it added an amino acid.