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.

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")

Mnemonics:

• UGA: "U Go Away"
• UAA: "U Are Away"
• UAG: "U Are Gone"

• TAG: "They Are Gone"
• TAA: "They Are Away"
• TGA: "They're Going Away"

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 pyrrolysine in a similar way selenocysteine is encoded.

Distribution of stop codons within the genome of an organism are non-random and can correlate with GC-content. For example, the E. coli K-12 genome contains 2705 TAA (63%), 1257 TGA (29%), and 326 TAG (8%) stop codons (GC content 50.8%).

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.