When does translation terminate?

Translation terminates when the ribosome reaches a termination codon. This codon, which can be UAA, UAG, or UGA in messenger RNA (mRNA), signals the end of the polypeptide synthesis process. At this point, the ribosome recognizes that there are no corresponding transfer RNA (tRNA) molecules that can bring an amino acid to match these codons. Instead, release factors bind to the ribosome, prompting the release of the newly synthesized polypeptide chain and dissociation of the ribosomal subunits. This is a critical step in the expression of genes, as it ensures that proteins are synthesized correctly from the genetic code.

The other choices do not accurately describe when translation terminates. Adding an amino acid to a growing polypeptide does not signal the end; rather, it is part of the elongation phase. The small ribosomal unit being released is a result of the termination process but does not indicate its initiation. Lastly, stating that translation concludes when the mRNA has been completely translated is misleading since the discontinuation is specifically tied to reaching the termination codon.