In the genetic code, a stop codon (or termination codon) is a nucleotide triplet within messenger RNA that signals a termination of translation. Proteins are based upon polypeptides, which are unique sequences of amino acids; and 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 signal the termination of this process, releasing the amino acid chain.
In the standard genetic code, there are several stop codons:
- in RNA:
- UAG ("amber")
- UAA ("ochre")
- UGA ("opal")
- in DNA:
- TAG ("amber")
- TAA ("ochre")
- TGA ("opal" or "umber").
See also: variations.
Mnemonic UGA: "U Go Away" UAA: "U Are Away" UAG: "U Are Gone
The UGA codon has recently been identified as the codon coding for Selenocysteine (Sec). This amino acid is found in 25 selenoproteins where it is located in the active site of the protein. Transcription of this codon is enabled by proximity of the SECIS element (SElenoCysteine Incorporation Sequence). The UAG codon can translate into pyrolysin in a similar way selenocysteine is encoded.
Nonsense mutations are changes in DNA sequence that introduce a premature stop codon, causing any resulting protein to be abnormally shortened. This often causes a loss of function in the protein, as critical parts of the amino acid chain are no longer created. Because of this terminology, stop codons have also been referred to as nonsense codons.
Amber, ochre, and opal nomenclature
Stop codons were historically given many different names, as they each corresponded to a distinct class of mutants that all behaved in a similar manner. These mutants were first isolated within bacteriophages (T4 and lambda), viruses that infect the bacteria Escherichia coli. Mutations in viral genes weakened their infectious ability, sometimes creating viruses that were able to infect and grow within only certain varieties of E coli.
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