A codon is a three-base unit in a genetic sequence that specifies an amino acid or a stop signal. In standard gene expression, codons are read in messenger RNA during translation.
The genetic code uses 64 possible three-base codons. Sixty-one specify amino acids, while three—UAA, UAG, and UGA in messenger RNA—signal termination. Several codons can specify the same amino acid, making the code degenerate.
The start codon AUG usually begins translation and also specifies methionine. During translation, a transfer RNA molecule carries a matching anticodon and delivers the appropriate amino acid to the ribosome.
A codon is often confused with an anticodon. The codon belongs to messenger RNA, while the complementary anticodon belongs to transfer RNA. DNA sequences are transcribed into RNA, so the corresponding DNA triplets use T where messenger RNA uses U.