Stop codon

In molecular biology, a stop codon (or termination codon) is a codon (nucleotide triplet within messenger RNA) that signals the termination of the translation process of the current protein. Most codons in messenger RNA correspond to the addition of an amino acid to a growing polypeptide chain, which may ultimately become a protein; stop codons signal the termination of this process by binding release factors, which cause the ribosomal subunits to disassociate, releasing the amino acid chain.

While start codons need nearby sequences or initiation factors to start translation, a stop codon alone is sufficient to initiate termination.

Properties

Standard codons

In the standard genetic code, there are three different termination codons:

Alternative stop codons

There are variations on the standard genetic code, and alternative stop codons have been found in the mitochondrial genomes of vertebrates, Scenedesmus obliquus, and Thraustochytrium.

Reassigned stop codons

The nuclear genetic code is flexible as illustrated by variant genetic codes that reassign standard stop codons to amino acids.

Translation

In 1986, convincing evidence was provided that selenocysteine (Sec) was incorporated co-translationally. Moreover, the codon partially directing its incorporation in the polypeptide chain was identified as UGA also known as the opal termination codon. Different mechanisms for overriding the termination function of this codon have been identified in prokaryotes and in eukaryotes. A particular difference between these kingdoms is that cis elements seem restricted to the neighborhood of the UAG codon in prokaryotes while in eukaryotes this restriction is not present. Instead such locations seem disfavored albeit not prohibited.