stopkodonerne

Stopkodonerne (stop codons) are the three codons that mark the end of translation in most organisms. In the standard genetic code they are UAA, UAG, and UGA in messenger RNA, corresponding to the DNA codons TAA, TAG, and TGA. These codons terminate the coding sequence and signal the ribosome to release the completed polypeptide.

Termination is mediated by release factors. In bacteria, RF1 recognizes UAG and UAA while RF2 recognizes UGA and UAA; in eukaryotes, the single factor eRF1 works with eRF3 to promote hydrolysis of the peptidyl-tRNA and disassembly of the ribosome. The result is release of the newly synthesized protein and recycling of the translation machinery. The efficiency of termination can be influenced by the surrounding nucleotide context and regulatory elements.

Variations and exceptions exist. Some organelles and organisms use alternative genetic codes, and certain codons can be repurposed in specific contexts. Notably, UGA and sometimes UAG can encode amino acids such as selenocysteine or pyrrolysine when specialized machinery and RNA structures (for example, SECIS elements) are present. Stop codons can also be read through under certain circumstances, producing extended protein products in a regulated or pathological manner, a feature studied for both basic biology and therapeutic potential.

Clinical relevance is significant. Premature stop codons, or nonsense mutations, truncate proteins and underlie many genetic diseases. Therapeutic approaches explored include promoting translational readthrough of stop codons or targeted gene therapies to restore full-length protein function.