Ribosome functional sites: From which site does the tRNA donate its amino acid to the growing polypeptide chain during elongation?

Introduction / Context:
During translation, the ribosome has three key tRNA-binding sites: A (aminoacyl), P (peptidyl), and E (exit). Understanding which site contributes the next amino acid to the growing chain is essential for mapping antibiotic actions and mutation effects on translation.

Given Data / Assumptions:

• Elongation cycle on a 70S (prokaryotic) or 80S (eukaryotic) ribosome is conserved mechanistically.
• We focus on the source site of the donor amino acid added to the chain each cycle.

Concept / Approach:

The incoming aminoacyl-tRNA binds at the A site. Peptidyl transferase (on the large subunit rRNA) transfers the growing peptide from the tRNA in the P site to the aminoacyl-tRNA in the A site. Thus, the new residue is “donated” by the A-site aminoacyl-tRNA, after which translocation moves the newly elongated peptidyl-tRNA to the P site.

Step-by-Step Solution:

Verification / Alternative check:

Inhibitors like tetracycline block A-site entry, which halts addition of the next amino acid—evidence that the aminoacyl donor arrives at the A site. Puromycin mimics an aminoacyl-tRNA at the A site and causes premature chain termination, further confirming the donor role of the A site.

Why Other Options Are Wrong:

• P site: holds the peptidyl-tRNA that donates the growing chain, but the next amino acid donor is the A-site aminoacyl-tRNA.
• O site / T site: not standard ribosomal sites.

Common Pitfalls:

• Reversing roles of A and P sites.
• Assuming the same naming in bacteria and eukaryotes changes site function; it does not.

Final Answer:

A site