n. (context biology genetics biochemistry English) a sequence of three RNA nucleotides (A, C, G or U) that instruct the synthesis, or translation, of a protein to stop. The three-letter stop codon sequences have been given names: "UAG" is ''amber'', "UGA" is ''opal'', and "UAA" is ''ochre''.
In the genetic code, a stop codon (or termination codon) is a nucleotide triplet within messenger RNA that signals a termination of translation into proteins. Proteins are based on polypeptides, which are unique sequences of amino acids. Most codons in messenger RNA (from DNA) 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.