Termination signal
Codons: TAG TGA TAA
Stop codons (TAG, TGA, TAA) within messenger RNAs signal the termination of translation into proteins. When the translation machinery reaches a stop codon the addition of amino acids is completed and the nascent polypeptide is released. Hence it marks the end of an open reading frame and therefore of the coding region of an mRNA sequence. The counterpart to the stop codon, the start codon (ATG), simultaniously codes for the amino acid methionine, whereas no tRNAs and therefore no encoded amino acids exist for the three stop codons.
A premature stop codon, which can arise due to a DNA mutation, can cause the production of truncated polypeptides that are usually non-functional. Such mutations are therefore referred to as nonsense mutations. On the other hand, a base substitution in an existing stop codon that results in an amino acid-encoding triplet, leads to continued translation and to extremely long non-functional polypeptides. Hence this type of base change is called a nonstop mutation.
Historically, the three stop codons, or rather the corresponding nonsense mutations, were named "amber" (TAG), "ochre" (TAA) and "opal"/"umber" (TGA). The first set of nonsense mutations to be discovered, namely "amber", was named after Harris Bernstein, whose last name means amber in German. The other two were named in reference to the first one.