Question

Which one of the following amino acids is coded by a single codon?

• A. Valine
• B. Threonine
• C. Tryptophan
• D. Isoleucine

Hint:

Only Methionine (AUG) and Tryptophan (UGG) are coded by a single codon. All other amino acids have multiple codons.

Answer 1

The Correct Option is C

Step 1: Understand codon degeneracy.
The genetic code is degenerate, meaning most amino acids are coded by more than one codon. However, there are exceptions — some amino acids are coded by only one codon.

Step 2: Amino acids with a single codon.
There are only two amino acids that are each coded by a single codon:

• Methionine (AUG)
• Tryptophan (UGG)

Step 3: Analyze the options.
Valine – coded by 4 codons: GUU, GUC, GUA, GUG
Threonine – coded by 4 codons: ACU, ACC, ACA, ACG
Tryptophan – coded by 1 codon: UGG
Isoleucine – coded by 3 codons: AUU, AUC, AUA

Step 4: Conclusion.
Only Tryptophan is coded by a single codon, UGG. Hence, option (3) is correct.