introns

Introns are non-coding segments within genes that are transcribed into RNA but removed from the transcript before the mRNA is translated. In most eukaryotic genes, transcripts contain both exons, which encode amino acids, and introns, which do not.

Introns are common in eukaryotes and vary widely in length from a few dozen to tens of thousands of base pairs. Prokaryotes generally have few or no introns; some archaeal and bacterial genes harbor self-splicing introns, but such cases are exceptions in bacteria. Introns are especially abundant in many plant and animal genes, and organellar genomes (such as chloroplasts and mitochondria) can also contain introns.

Splicing is carried out by the spliceosome in nuclear genes; recognition signals include the 5' splice site (typically GU), the branch point with an adenine, the polypyrimidine tract, and the 3' splice site (typically AG). The intron is removed as a lariat by two sequential transesterifications, joining the flanking exons to form mature mRNA.

Alternative splicing uses different splice sites or intron retention to generate multiple mRNA variants from a single gene, increasing proteome diversity. Splicing can influence transcript stability and localization, and intronic sequences can harbor regulatory elements such as enhancers or silencers that modulate gene expression.

Some introns are self-splicing ribozymes, classified as group I and group II introns, found in organelles and some fungi. Nuclear introns are usually spliceosomal and thought to have co-evolved with the spliceosome; hypotheses about intron evolution include intron late and intron early.

Mutations that disrupt splicing can cause disease; antisense therapies and splice-switching oligonucleotides are used to modulate splicing in research and medicine.