Stoppkodon

Stoppkodon, or stop codon in English, are nucleotide triplets in messenger RNA that signal the termination of translation. They mark the end of an open reading frame and do not encode amino acids.

In the standard genetic code, there are three stop codons: UAA, UAG, and UGA. Historically these are known as ochre, amber, and opal codons. In DNA the corresponding triplets are TAA, TAG, and TGA. Some organisms and organelles use variations of the code, but the three stop signals are the most common basis for termination.

Mechanism and factors: when a ribosome translating mRNA encounters a stop codon in the A site, no tRNA recognizes these codons. Instead release factors bind and catalyze the hydrolysis that releases the newly made polypeptide and promotes ribosome disassembly. In bacteria, RF1 and RF2 recognize different stop codons (with RF3 aiding recycling), while in eukaryotes a single eRF1 recognizes all three stop codons (with eRF3 assisting). This termination step is essential to producing full-length, functional proteins.

Biological context: termination efficiency can be influenced by the surrounding sequence and RNA structure. Translational read-through can occur in some contexts, producing extended proteins. In mitochondria and some unicellular lineages, the genetic code can differ, with certain codons reassigned as stops or as amino acids. Stoppkodon are a fundamental concept in molecular biology, essential for understanding gene expression, protein synthesis, and the consequences of mutations that alter termination.