Question

Which of the following amino acids are represented by triplet codon?
(A) 4-Hydroxy proline
(B) Histidine
(C) Tyrosine
(D) Cysteine
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A), (B) and (C) only.
• C. (A), (B), (C) and (D).
• D. (B), (C) and (D) only.

Hint:

Post-translationally modified amino acids (like hydroxyproline, hydroxylysine, and $\gamma$-carboxyglutamate) are essential for protein structure but do not have dedicated codons in the mRNA.
They are modified only after translation is complete.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The standard genetic code consists of 64 triplet codons.
Out of these, 61 codons encode the 20 standard amino acids used during ribosomal translation to synthesize proteins.
Detailed Explanation:
Let us analyze the amino acids in the options:
- (B) Histidine: A standard basic amino acid directly encoded by the triplet codons CAU and CAC.
- (C) Tyrosine: A standard aromatic amino acid directly encoded by the triplet codons UAU and UAC.
- (D) Cysteine: A standard sulfur-containing amino acid directly encoded by the triplet codons UGU and UGC.
- (A) 4-Hydroxyproline: This is a non-standard amino acid found in high abundance in collagen.
It is not directly represented by any triplet codon in the genetic code.
Instead, proline residues are first incorporated into the growing polypeptide chain during ribosomal translation.
These proline residues are subsequently hydroxylated post-translationally by the enzyme prolyl hydroxylase to form 4-hydroxyproline.
Therefore, only Histidine, Tyrosine, and Cysteine are directly represented by triplet codons.

Step 2: Final Answer:
The correct option is (D), representing (B), (C), and (D) only.