Q.37 Which set of the following events occurs during the elongation step of translation?
P. Attachment of mRNA with the smaller subunit of ribosome
Q. Loading of correct aminoacyl-tRNA into the A site
R. Formation of a peptide bond between the amino acyl-tRNA in the A site and the peptide
chain that is attached to the peptidyl-tRNA in the P site
S. Dissociation of the ribosomal subunits
T. Translocation of peptidyl-tRNA from the A site to the P site of the ribosome
(A) P, Q and R (B) P, Q and T (C) Q, R and T (D) R, S and T

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Understanding the Elongation Step of Translation

Protein synthesis, or translation, unfolds in three main phases: initiation, elongation, and termination. The elongation step of translation is the core cycle where the polypeptide chain grows. It repeats multiple times, adding amino acids one by one based on mRNA codons. This process happens on the ribosome, which has three key sites: the A (aminoacyl), P (peptidyl), and E (exit) sites.

During elongation, specific events ensure accurate peptide chain extension. A popular MCQ tests this knowledge—let’s break it down with the correct answer and analyze every option.

Correct Answer: (C) Q, R and T

Why Q, R, and T Define the Elongation Step

The elongation step of translation cycle includes three precise events:

• Q. Loading of correct aminoacyl-tRNA into the A site: Aminoacyl-tRNA (charged with the right amino acid) enters the empty A site, matching the mRNA codon via EF-Tu (in prokaryotes) or eEF1 (in eukaryotes). This decoding step sets up chain growth.

• R. Formation of a peptide bond: Peptidyl transferase catalyzes the bond between the amino acid in the A site’s tRNA and the growing chain on the P site’s peptidyl-tRNA. The chain transfers to the A site tRNA, powered by ribosome RNA catalysis.

• T. Translocation of peptidyl-tRNA from the A site to the P site: EF-G (prokaryotes) or eEF2 (eukaryotes) drives ribosome movement along mRNA by one codon. The peptidyl-tRNA shifts to the P site, A site empties, and deacylated tRNA exits via E site.

These steps repeat until a stop codon appears, making Q, R, and T the hallmark translation elongation events.

Explanation of All Options: Why Not P or S?

Let’s evaluate each option to clarify misconceptions:

• P. Attachment of mRNA with the smaller subunit of ribosome: Incorrect for elongation. This occurs in initiation, where the small ribosomal subunit binds mRNA and the initiator tRNA at the start codon (AUG). By elongation, the full ribosome (large + small subunits) is already assembled.

• Q. Loading of correct aminoacyl-tRNA into the A site: Correct. Essential first sub-step of every elongation cycle.

• R. Formation of a peptide bond between the amino acyl-tRNA in the A site and the peptide chain that is attached to the peptidyl-tRNA in the P site: Correct. This defines peptide elongation, shifting the chain to the A site.

• S. Dissociation of the ribosomal subunits: Incorrect. Subunit dissociation happens in termination, triggered by release factors at stop codons, freeing the completed polypeptide.

• T. Translocation of peptidyl-tRNA from the A site to the P site of the ribosome: Correct (with a minor note). After peptide bond formation, the new peptidyl-tRNA moves from A to P site during translocation, advancing mRNA.

Options A and B include P (initiation), while D includes S (termination)—ruling them out.

Key Takeaways for Biotech and Molecular Biology Students

Mastering the elongation step of translation is crucial for understanding microbial growth kinetics, enzyme production in fermentation, and genetic engineering. Disruptions here (e.g., antibiotics like erythromycin blocking translocation) impact biotech applications.

Visualize it like an assembly line: tRNA delivers parts (Q), bonds them (R), and shifts forward (T)—repeat for protein length.