stop codon

The stop codon is the full stop at the end of a genetic sentence. When the ribosome reaches it, translation halts and the finished protein chain is released. There are three of them in the standard code: UAA, UAG, and UGA.

Unlike other codons, stop codons are not read by a transfer RNA carrying an amino acid. Instead, proteins called release factors recognize them, prompt the ribosome to cut the polypeptide free, and let the machinery disassemble. So a stop codon adds no amino acid — it ends the chain.

A mutation that creates a stop codon too early (a nonsense mutation) cuts the protein short and usually cripples it; one that destroys a normal stop codon lets the ribosome read past the proper end into sequence that should not be translated. Either way, the placement of the stop codon is essential to making a correct, complete protein.