Which one of the following pairs of amino acids is not coded by more than one codon?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Methionine and Tryptophan
The genetic code has 64 codons. Of these, 61 code for the 20 amino acids and 3 are stop codons. Since there are more codons than amino acids, most amino acids are coded by more than one codon. This is why the code is called degenerate. Only two amino acids have a single codon each: methionine (AUG) and tryptophan (UGG).
(1) Glutamate and Glutamine — Coded by more than one codon. Each of these has two codons (glutamate: GAA, GAG; glutamine: CAA, CAG).
(2) Methionine and Tryptophan — Not coded by more than one codon. Methionine is coded only by AUG, which also works as the initiator (start) codon. Tryptophan is coded only by UGG.
(3) Leucine and Isoleucine — Coded by more than one codon. Leucine has six codons and isoleucine has three.
(4) Phenylalanine and Glycine — Coded by more than one codon. Phenylalanine has two codons (UUU, UUC) and glycine has four.
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.”
Genetic Code – “… (v) AUG has dual functions. It codes for Methionine (met) , and it also act as initiator codon.”