Ribosomal elongation — direction of peptide transfer During translation elongation on the ribosome, in what direction is the growing polypeptide chain transferred between tRNA binding sites?

Introduction:
Understanding the choreography of tRNAs on the ribosome is fundamental to molecular biology. Elongation involves precise positioning of the peptidyl-tRNA and the incoming aminoacyl-tRNA in the P and A sites, respectively, followed by peptide bond formation and translocation.

Given Data / Assumptions:

• The ribosome contains A (aminoacyl), P (peptidyl), and E (exit) sites.
• Peptidyl transferase activity resides in rRNA of the large subunit.
• Aminoacyl-tRNA initially enters the A site, guided by elongation factors.

Concept / Approach:

Peptide bond formation transfers the growing polypeptide from the tRNA in the P site to the amino acid on the tRNA in the A site. After this chemical transfer, ribosomal translocation moves the new peptidyl-tRNA from A to P, and the deacylated tRNA shifts from P to E.

Step-by-Step Solution:

Verification / Alternative check:

Kinetic and structural studies show that the nucleophilic attack comes from the amino group of the A-site aminoacyl-tRNA on the carbonyl carbon of the P-site peptidyl-tRNA, confirming transfer of the chain from P to A during bond formation.

Why Other Options Are Wrong:

Option A reverses the chemical transfer step; A → P describes the positional change after translocation, not the bond-forming transfer. Options C and D involve movement to the E site, which is for deacylated tRNA, not peptide growth. Random transfer is incompatible with ribosome mechanics.

Common Pitfalls:

Confusing chemical transfer (P → A during peptide bond formation) with physical translocation (A → P after the bond forms). Keeping these steps distinct avoids errors.

Final Answer:

From the P site to the A site on the ribosome