Question

Newly synthesized tRNA undergoes post transcriptional modifications which include
(A). Nucleotide alkylation
(B). Attachment of \(\text{C}_\text{p}\text{C}_\text{p}\text{A}_\text{OH}\) terminal at 3' end
(C). Attachment of \(\text{C}_\text{p}\text{C}_\text{p}\text{A}_\text{OH}\) terminal at 5' end
(D). Methylation of some bases
Choose the correct answer from the options given below:

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

Hint:

Remember: All mature tRNAs end in 3'-CCA-5'. This sequence acts as the physical attachment site for the incoming amino acid, which is esterified to the 3'-terminal adenosine.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Primary tRNA transcripts (pre-tRNAs) are inactive and must undergo extensive post-transcriptional processing and modification to become functional adapters for translation.

Step 2: Detailed Explanation:
- Statements A and D are correct: Mature tRNAs contain many modified bases (up to \(10-15\%\) of their residues). These modifications include methylation (D) and alkylation (A) of standard bases, producing unusual nucleotides like pseudouridine, dihydrouridine, and ribothymidine that stabilize tRNA tertiary structure.
- Statement B is correct: Eukaryotic pre-tRNAs do not encode the terminal \(\text{CCA}\) sequence. A template-independent enzyme, tRNA nucleotidyltransferase, adds the sequence \(\text{C}_\text{p}\text{C}_\text{p}\text{A}_\text{OH}\) to the 3'-terminus of the processed tRNA, which is required for amino acid attachment.
- Statement C is incorrect: The \(\text{CCA}\) sequence is attached exclusively to the 3'-hydroxyl end, not the 5'-end.
- Thus, (A), (B), and (D) are correct modifications.

Step 3: Final Answer:
The correct post-transcriptional modifications are represented by (A), (B), and (D) only.