codon

codon, in genetics, any of 64 different sequences of three adjacent nucleotides in DNA that either encodes information for the production of a specific amino acid or serves as a stop signal to terminate translation (protein synthesis). Codons are made up of any triplet combination of the four nitrogenous bases adenine (A), guanine (G), cytosine (C), or uracil (U). Of the 64 possible codon sequences, 61 specify the 20 amino acids that make up proteins and three are stop signals.

An example of a codon is the sequence AUG, which specifies the amino acid methionine. The AUG codon, in addition to coding for methionine, is found at the beginning of every messenger RNA (mRNA) and indicates the start of a protein. Methionine and tryptophan are the only two amino acids that are coded for by just a single codon (AUG and UGG, respectively). The other 18 amino acids are coded for by two to six codons. Because most of the 20 amino acids are coded for by more than one codon, the genetic code is called degenerate. The same codons specify the same amino acids in almost all species.

Examples of termination codons are UAG, UAA, or UGA. Translation stops when one of these codons is encountered by the ribosome (ribosomes are small particles in cells that serve as the sites of protein synthesis). Special release factors associate with the ribosome in response to these codons, and the newly synthesized protein, transfer RNAs (tRNAs), and mRNA dissociate.