Q.45 The number of triplet codon(s) for methionine is ___.

Methionine is encoded by only one triplet codon in the standard genetic code. This makes it unique among amino acids, as most have multiple codons due to the code’s degeneracy.

Correct Answer

The number of triplet codon(s) for methionine is 1.
In the standard genetic code, methionine (Met) is exclusively coded by the AUG codon, which also serves as the start codon for translation initiation.

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Genetic Code Basics

The genetic code consists of 64 triplets (codons) formed from four nucleotides (A, U, G, C in mRNA), specifying 20 amino acids plus stop signals. Degeneracy means most amino acids have 2-6 codons, but methionine and tryptophan each have only one.

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AUG codes for methionine in all positions, though internal AUGs may rarely code for it depending on context; no other codon substitutes.

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Why Only One Codon?

Methionine’s single codon (AUG) lacks wobble degeneracy at the third position, unlike amino acids such as leucine (6 codons) or serine (6 codons). This rarity aids precise translation start and relates to evolutionary code minimization.

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In exams like the query’s Q.45 (likely CSIR NET or similar), options might include 1, 2, 3, or 4; only 1 is correct, as confirmed across sources—no exceptions in standard code.

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Common Exam Options Explained

• Option 1 (Correct): AUG alone codes for Met; serves as initiator.

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• Option 2 or 3 (Incorrect): No second/third codon exists; confusion may arise with isoleucine (AUU, AUC, AUA).

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• Option 4 or more (Incorrect): Applies to multi-codon amino acids like alanine (GCU, GCC, GCA, GCG); Met has none.

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