intronok

Intronok, or introns, are non-coding segments within genes that are transcribed into precursor messenger RNA but removed from the final mRNA during RNA processing. They separate coding regions, called exons, and are found in most eukaryotic genes. Most introns are removed by the spliceosome, a large RNA-protein complex, through a two-step transesterification process that joins exons together and forms a lariat intermediate.

There are several classes of introns. The majority are spliceosomal introns, which are further divided into major (U2-type) and minor (U12-type) classes based on the small nuclear RNA components that mediate splicing. Some self-splicing introns, group I and group II, can remove themselves without proteins, relying on RNA catalysis, and are often found in organellar genomes or some bacteria.

Intron number and length vary widely among organisms and genes. Some genes are intronless, while others contain many introns. Introns can influence gene expression, mRNA export, and regulation, and they harbor sequences that regulate transcription and alternative splicing. Alternative splicing of introns and exons produces multiple transcript variants from a single gene, contributing to proteome diversity.

Evolutionarily, introns are thought to arise and be lost over time, with frequent intron gain and loss events across lineages. Mutations affecting splice sites or regulatory motifs within introns can lead to mis-splicing and disease, highlighting their functional importance.