Stopcodons

Stop codons, also known as nonsense codons, are specific sequences in messenger RNA that signal the termination of protein synthesis. In the standard genetic code, the three stop codons are UAA, UAG, and UGA. They do not encode amino acids and are recognized during translation as signals to end the growing polypeptide chain.

Recognition of stop codons differs between organisms. In bacteria, two release factors, RF1 and RF2, recognize different stop codons and promote termination, with RF3 assisting in recycling the ribosome. In eukaryotes, a single release factor, eRF1, recognizes all three stop codons, and the GTPase eRF3 aids termination. Because stop codons do not pair with tRNAs, release factors bind to the ribosome at the A site to catalyze peptide release and release the finished protein.

Termination efficiency can be influenced by the sequence surrounding a stop codon, as well as by cellular conditions. Readthrough, where near-cognate tRNAs mispair with a stop codon or where release factor activity is reduced, can occur and is sometimes used by certain viruses to extend proteins. Some organisms and organelles use variant genetic codes, and stop codon usage can differ in mitochondria and other organelles, where codon reassignment may occur.

Premature stop codons are a major source of genetic disease, producing truncated, typically nonfunctional proteins. Cells employ quality-control mechanisms such as nonsense-mediated decay to degrade transcripts with early stop codons. Therapeutic strategies under investigation aim to promote readthrough of premature stop codons to restore full-length protein function, with varying degrees of success.