intron

An intron is a noncoding segment of a gene that is transcribed into RNA but removed from the precursor mRNA during RNA processing. Introns interrupt exons, which encode portions of a protein or a functional RNA. After transcription, the remaining exons are joined by splicing to produce a mature mRNA for translation. Introns are common in eukaryotic genes and in some organellar genomes and viruses, but are absent or rare in most prokaryotic genes.

Most introns in eukaryotes are spliceosomal introns, removed by the spliceosome, a complex of small nuclear RNAs and proteins. They typically begin with a GU donor site and end with an AG acceptor site, and contain a branch point that forms a lariat during splicing. The process also involves a polypyrimidine tract and various splicing factors that recognize sequence motifs to determine alternative splice sites. In addition to spliceosomal introns, two self-splicing types exist: group I and group II introns, which catalyze their own removal without the spliceosome. Group II introns resemble the spliceosomal reaction and are considered likely evolutionary precursors of spliceosomal introns.

Splicing can occur cotranscriptionally in the nucleus and is frequently regulated by splicing factors. Alternative splicing allows a single gene to produce multiple mRNA and protein isoforms, contributing to proteome complexity. Intron retention and other splicing patterns can also regulate gene expression and may trigger quality-control pathways such as nonsense-mediated decay.

Introns can harbor regulatory elements, noncoding RNAs, and signals for RNA export. The presence, number, and size of introns vary among organisms and lineages, informing theories about their evolution (introns-early vs introns-late) and their role in genome architecture. In practice, intron–exon structure aids gene annotation and has clinical relevance when splicing is disrupted by mutation.