Stop codon
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 on polypeptides, which are unique sequences of amino acids. Most codons in messenger RNA (from DNA) 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 by binding release factors, which cause the ribosomal subunits to disassociate, releasing the amino acid chain. While start codons need nearby sequences or initiation factors to start translation, a stop codon alone is sufficient to initiate termination.
Introduction
In the standard genetic code, there are three different stop codons:
in RNA:
- UAG ("amber")
- UAA ("ochre")
- UGA ("opal")
UAG ("amber")
UAA ("ochre")
UGA ("opal")
in DNA:
- TAG ("amber")
- TAA ("ochre")
- TGA ("opal" or "umber")
TAG ("amber")
TAA ("ochre")
TGA ("opal" or "umber")
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 the proximity of the SECIS element (SElenoCysteine Incorporation Sequence).
The UAG codon can translate into pyrrolysine in a similar way selenocysteine is encoded.
