nonsense triplet

A name given originally to triplets of bases that do not specify any amino acid and were thus once considered as having no function. There are three such triplets, UAG, UGA, and UGG, all of which have subsequently been found to act as 'stop-signals', i.e. they define the end of a polypeptide chain. Polypeptide initiation also requires a 'start signal', the signals being AUG or GUG, which also code for valine or methionine (or a methionine derivative) respectively.