Q.42 Which one or more of the following statements correctly describe(s) the changes
upon the addition of puromycin during eukaryotic translation?
(A) Puromycin resembles aminoacyl end of the charged tRNA.
(B) Puromycin occupies the A site of the translating ribosomes.
(C) Puromycin occupies the P site of the translating ribosomes.
(D) Puromycin occupies the E site of the translating ribosomes.

Correct options: (A) and (B)

Puromycin inhibits eukaryotic translation by mimicking charged tRNA and causing premature chain termination. This CSIR NET-style question tests understanding of its molecular mechanism on ribosomal sites.​

Option Analysis

Option (A): Puromycin structurally resembles the aminoacyl end of charged tRNA, featuring a 3′-aminoacyl-like moiety with an amide bond instead of an ester linkage. This mimicry allows it to enter the ribosome and participate in peptidyl transfer, leading to truncated proteins.​

Option (B): Puromycin occupies the A site, where it binds like incoming aminoacyl-tRNA and accepts the nascent chain from P-site peptidyl-tRNA via peptidyl transferase. This triggers release of the puromycylated peptide, halting elongation.​

Option (C): Puromycin does not occupy the P site, which holds peptidyl-tRNA during elongation; it targets the A site exclusively for incorporation.​​

Option (D): Puromycin does not bind the E site, reserved for deacylated tRNA exit; its action is A-site specific.​​

Introduction
Puromycin eukaryotic translation A site mechanism is crucial for CSIR NET aspirants studying translation inhibitors. This antibiotic mimics aminoacyl end of charged tRNA, binds the A site of ribosomes, and triggers premature polypeptide release—key to mastering molecular biology questions on ribosome function and protein synthesis inhibition.​

Mechanism Overview

Puromycin enters the A site during elongation, forms a peptide bond with the P-site chain, and dissociates due to its stable amide linkage. This halts translation in eukaryotes, producing incomplete proteins detectable in assays.​

• Resembles 3′ end of aminoacyl-tRNA for peptidyl transferase recognition​

• Binds A site competitively with incoming tRNA​

• Releases puromycylated chains rapidly​

Exam Relevance

For CSIR NET, focus on distinguishing puromycin (A-site mimic) from other inhibitors like cycloheximide (E-site/P-site hybrid blocker). Practice confirms A and B as correct.​