Stop codon: signal that ends translation in the genetic code

Common stop codons and basic mechanism

There are three canonical stop codons in the universal code: UAA, UAG and UGA. A codon is any sequence of three nucleotides, and when one of these triplets appears in the ribosomal A site it does not recruit a tRNA with an amino acid. Instead, proteins called release factors recognize the stop signal and catalyze the release of the nascent polypeptide from the tRNA in the P site.

Key components and steps

The ribosome reads mRNA codons and builds a chain of amino acids linked into a polypeptide, which may fold into a functional protein. When a stop codon is reached, bacterial release factors (for example RF1 and RF2) or the eukaryotic release factor eRF1 recognize the signal and promote hydrolysis of the bond holding the polypeptide to its tRNA. After release, the ribosomal subunits dissociate and translation is terminated.

Variations, exceptions and special cases

• Some organisms and organelles have altered assignments: certain mitochondria, ciliates and other lineages reassign one of the conventional stop codons to code for an amino acid.
• Certain signals allow insertion of unusual amino acids at a stop codon: selenocysteine can be incorporated at UGA when a downstream SECIS element and specific factors are present; pyrrolysine is similarly incorporated in some archaea at UAG.
• Stop codon readthrough—where the ribosome ignores a stop codon—occurs in viruses and in regulated gene expression to extend protein C‑termini.

Genetic, medical and biotechnological relevance

Premature stop codons (nonsense mutations) truncate proteins and underlie many genetic disorders; cells may reduce the impact of such mRNAs by mechanisms like nonsense‑mediated decay. Molecular biologists exploit stop codons in cloning and synthetic biology, either to terminate translation intentionally or to engineer reassignment of codons for expanded genetic codes. Suppressor tRNAs, release factor modulation and directed evolution are tools used to alter termination behavior.

Notable distinctions and closing notes

Although called "stop" or "termination" codons, these sequences are dynamic elements of the genome: their interpretation can depend on context, additional RNA signals and the translational machinery. They play a central role in defining proteins specified by a gene and, analogously, mark the end of a coding sentence much like a full stop ends a written sentence. For further background on translation and coding triplets see introductory resources linked here: genetic code overview, nucleotides, translation process, codon definition, gene structure, punctuation analogy, protein basics, polypeptide synthesis and amino acid properties.