Spliceosomal Introns: Features, Functions, and Evolution.

Spliceosomal introns, which have been found in most eukaryotic genes, are non-coding sequences excised from pre-mRNAs by a special complex called spliceosome during mRNA splicing. Introns occur in both protein- and RNA-coding genes and can be found in coding and untranslated gene regions. Because intron sequences vary greatly due to a high rate of polymorphism, the functions of intron had been for a long time associated only with alternative splicing, while intron evolution had been viewed not as an evolution of an individual genomic element, but rather considered within a framework of the evolution of the gene intron-exon structure. Here, we review the theories of intron origin, evolutionary events in the exon-intron structure, such as intron gain, loss, and sliding, intron functions known to date, and mechanisms by which changes in the intron features (length and phase) can affect the regulation of gene-mediated processes.