It is a codon (that is, a sequence of three nucleotides in messenger RNA). It indicates the end of a gene. Similar to how a full stop indicates the end of a sentence, a stop codon signals the end of genetic translation.
They are also called termination codons or chain termination codons. There are three such codons: UAA, UAG and UGA.
Proteins are based on polypeptides, which are unique sequences of amino acids. Most codons in messenger RNA correspond to the addition of an amino acid to a growing polypeptide chain, which may ultimately become a protein. Stop codons indicate the end of this process, and therefore, the end of polypeptide synthesis.
- Griffiths A.J.F.; et al. (2000). "Chapter 10 (molecular biology of gene function): Genetic code: stop codons". An introduction to genetic analysis. W.H. Freeman (go to end of excerpt).