Stopkodon

A stopkodon, also called a stop codon, is a nucleotide triplet in messenger RNA that signals the end of translation. In the standard genetic code, the three stop codons are UAA, UAG, and UGA. In DNA, these correspond to the triplets TAA, TAG, and TGA.

During translation, codons are read by the ribosome with the help of tRNAs. When a stop codon is encountered, there is no corresponding tRNA with an anticodon that matches it. Instead, release factors recognize the stop codon and catalyze hydrolysis of the bond between the polypeptide and the tRNA in the P site, releasing the completed polypeptide and causing the ribosomal subunits to dissociate.

Stop codons are found at the end of open reading frames and are universal features of almost all organisms. In bacteria, termination involves RF1 and RF2 release factors that recognize different stop codons, while in eukaryotes a single release factor, eRF1 (often with eRF3 as an accompanying GTPase), recognizes all three stop codons.

Occasionally, stop codons can be read through, allowing incorporation of an amino acid and production of an extended protein; this can be natural in some genes or induced in research. Nonsense mutations introduce premature stop codons and can cause diseases; cells may employ nonsense-mediated decay to reduce such transcripts. Therapeutic approaches aim to promote readthrough or modulate decay pathways, though challenges remain.