Question

Choose the correct answer from the alternatives given :
Amino acids which are specified by single codons are

• A. Phenylalanine and arginine
• B. Tryptophan and methionine
• C. Valine and proline
• D. Methionine and arginine

Answer 1

Answer: (B)

Tryptophan and methionine

Since there are 64 triplet codons and only 20 amino acids, the incorporation of some amino acids must be influenced by more than one codon. Only tryptophan (UGG) and methionine (AUG) are specified by single codons. All other amino acids are specified by two (e.g., phenylalanine UUU, UUC) to six (e.g., arginine CGU, CGC, CGA, CGG, AGA, AGG) codons. The latter are called degenerate or redundant codons. In degenerate codons, generally, the first two nitrogen bases are similar while the third one is different. As the third nitrogen base has no effect on coding, the same is called wobble position.

So, the correct answer is 'Tryptophan and methionine'.