Which of the following is not a stop codon?

The correct answer is Option (3) → AUG

Option (1) UAA — A stop codon.

Option (2) UAG — A stop codon.

Option (3) AUG — Not a stop codon. AUG has dual functions: it codes for methionine and also acts as the initiator (start) codon. The three stop codons are UAA, UAG and UGA.

Option (4) UGA — A stop codon.

Note: The same question was also asked in CUET (UG) 2023 on 28-05-2023 (Slot 1).

NCERT Extract:
Genetic Code – “… The salient features of genetic code are as follows: (i) The codon is triplet. 61 codons code for amino acids and 3 codons do not code for any amino acids, hence they function as stop codons. (ii) Some amino acids are coded by more than one codon, hence the code is degenerate.”
Translation – “… A translational unit in mRNA is the sequence of RNA that is flanked by the start codon (AUG) and the stop codon and codes for a polypeptide.”
Genetic Code – “… (vi) UAA, UAG, UGA are stop terminator codons.”