Question

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)

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.