How does the ribosome know where to terminate translation at the end of a coding region?

• A. It encounters a stop codon (UAA, UAG, UGA) recognized by release factors; the ribosome then dissociates
• B. The ribosome runs into a poly(A) tail to terminate
• C. Release factors cause the ribosome to skip the stop codon
• D. tRNA binds to a stop codon to terminate

Answer: (A)

When theribosome reaches a stop codon—UAA, UAG, or UGA—the signal for ending translation is the recognition by release factors, not a tRNA. These factors bind to the stop codon in the ribosome’s A site and promote hydrolysis of the bond attaching the polypeptide to the tRNA, releasing the finished protein. After that, the ribosome, mRNA, and release factors dissociate to start a new round of translation if needed. In eukaryotes, a single release factor recognizes all stop codons and works with a GTPase to promote recycling; in bacteria, different release factors recognize specific stops. The poly(A) tail isn’t involved in terminating translation; termination is governed by stop codon recognition by release factors, which is why this option is the correct explanation.