Termination of translation: When is the growing polypeptide chain released from the ribosome during protein synthesis?

Introduction:
Translation terminates when the ribosome encounters a stop codon (UAA, UAG, or UGA). Unlike sense codons, stop codons are not decoded by tRNAs but by protein release factors that trigger hydrolysis of the ester linkage between the polypeptide and the P-site tRNA. Recognizing this mechanism clarifies why no “termination tRNA” exists.

Given Data / Assumptions:

• Stop codons signal termination in both bacteria and eukaryotes.
• Release factors (RF1/RF2 in bacteria; eRF1 in eukaryotes) recognize stop codons.
• Peptidyl–tRNA hydrolysis frees the completed polypeptide.

Concept / Approach:
Upon stop-codon entry into the A site, release factors bind and position a conserved GGQ motif near the peptidyl transferase center, promoting hydrolysis of the peptidyl–tRNA ester bond. Subsequent factor-mediated steps disassemble the ribosomal complex, allowing ribosome recycling. There is no tRNA with an anticodon complementary to a stop codon under normal conditions.

Step-by-Step Solution:

Verification / Alternative check:
Mutations that convert a stop codon to a sense codon prevent termination at that position; conversely, suppressor tRNAs that read stop codons can bypass termination, demonstrating that normal termination relies on release factors at stop codons.

Why Other Options Are Wrong:

• Termination tRNA: does not exist in standard translation.
• 7-methylguanosine cap/poly(A) tail: eukaryotic mRNA features involved in initiation/stability, not termination signals.
• GTP depletion: not the trigger; stop codon recognition is.

Common Pitfalls:
Assuming a special tRNA ends translation or that ribosomes read beyond coding regions to the cap or tail; termination occurs precisely at stop codons.

Final Answer:
When a chain-terminating (stop) codon is reached on the mRNA