Which of the following is a stop codon?

The correct answer is Option (2) → UAG

A stop codon codes for no amino acid; it tells the ribosome to end translation and release the polypeptide.

Option (1) UAC — Incorrect. UAC codes for the amino acid tyrosine.

Option (2) UAG — Correct. UAA, UAG and UGA are the stop (termination) codons: they code for no amino acid and end translation. UAG is one of them.

Option (3) UAU — Incorrect. UAU also codes for tyrosine; the code is degenerate, so one amino acid can have more than one codon.

Option (4) AUG — Incorrect. AUG is the start (initiator) codon. It also codes for methionine.

Note: A similar question was also asked in CUET (UG) 2026 on 07-06-2026 (Slot 2).

NCERT Extract:
Genetic Code – “… UUU would code for Phenylalanine (phe). Some exceptions to this rule have been found in mitochondrial codons, and in some protozoans. (v) AUG has dual functions. It codes for Methionine (met) , and it also act as initiator codon. (vi) UAA, UAG, UGA are stop terminator codons.”