Editing Intron (section)

== Distribution ==

The frequency of introns within different genomes is observed to vary widely across the spectrum of biological organisms. For example, introns are extremely common within the nuclear genome of [[jawed vertebrate]]s (e.g. humans, mice, and pufferfish (fugu)), where protein-coding genes almost always contain multiple introns, while introns are rare within the nuclear genes of some eukaryotic microorganisms,<ref>{{cite journal | vauthors = Stajich JE, Dietrich FS, Roy SW | title = Comparative genomic analysis of fungal genomes reveals intron-rich ancestors | journal = Genome Biology | volume = 8 | issue = 10 | pages = R223 | year = 2007 | pmid = 17949488 | pmc = 2246297 | doi = 10.1186/gb-2007-8-10-r223 | doi-access = free }}</ref> for example [[baker's yeast|baker's/brewer's yeast]] (''Saccharomyces cerevisiae''). In contrast, the [[mitochondrial DNA|mitochondrial genomes]] of vertebrates are entirely devoid of introns, while those of eukaryotic microorganisms may contain many introns.<ref>{{cite journal | vauthors = Taanman JW | title = The mitochondrial genome: structure, transcription, translation and replication | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1410 | issue = 2 | pages = 103–123 | date = February 1999 | pmid = 10076021 | doi = 10.1016/s0005-2728(98)00161-3 | doi-access =  | s2cid = 19229072 }}</ref>

[[Image:Pre-mRNA_to_mRNA_MH.svg|right|thumbnail|420px|Simple illustration of an unspliced mRNA precursor, with two introns and three exons (top). After the introns have been removed via splicing, the mature mRNA sequence is ready for translation (bottom).]]

A particularly extreme case is the ''[[Drosophila]] dhc7'' gene containing a ≥3.6 [[megabase]] (Mb) intron, which takes roughly three days to transcribe.<ref name=tollervey2000>{{cite journal | vauthors = Tollervey D, Caceres JF | title = RNA processing marches on | journal = Cell | volume = 103 | issue = 5 | pages = 703–709 | date = November 2000 | pmid = 11114327 | doi = 10.1016/S0092-8674(00)00174-4 | doi-access = free }}</ref><ref name=reugels2000>{{cite journal | vauthors = Reugels AM, Kurek R, Lammermann U, Bünemann H | title = Mega-introns in the dynein gene DhDhc7(Y) on the heterochromatic Y chromosome give rise to the giant threads loops in primary spermatocytes of Drosophila hydei | journal = Genetics | volume = 154 | issue = 2 | pages = 759–769 | date = February 2000 | pmid = 10655227 | pmc = 1460963 | doi = 10.1093/genetics/154.2.759 }}</ref> On the other extreme, a 2015 study suggests that the shortest known [[metazoan]] intron length is 30 base pairs (bp) belonging to the human ''MST1L'' gene.<ref name=piovesan2015>{{cite journal | vauthors = Piovesan A, Caracausi M, Ricci M, Strippoli P, Vitale L, Pelleri MC | title = Identification of minimal eukaryotic introns through GeneBase, a user-friendly tool for parsing the NCBI Gene databank | journal = DNA Research | volume = 22 | issue = 6 | pages = 495–503 | date = December 2015 | pmid = 26581719 | pmc = 4675715 | doi = 10.1093/dnares/dsv028 }}</ref> The shortest known introns belong to the [[heterotrich]] ciliates, such as ''[[Stentor coeruleus]]'', in which most (> 95%) introns are 15 or 16 bp long.<ref name=slabodnick2017>{{cite journal | vauthors = Slabodnick MM, Ruby JG, Reiff SB, Swart EC, Gosai S, Prabakaran S, Witkowska E, Larue GE, Fisher S, Freeman RM, Gunawardena J, Chu W, Stover NA, Gregory BD, Nowacki M, Derisi J, Roy SW, Marshall WF, Sood P | display-authors = 6 | title = The Macronuclear Genome of Stentor coeruleus Reveals Tiny Introns in a Giant Cell | journal = Current Biology | volume = 27 | issue = 4 | pages = 569–575 | date = February 2017 | pmid = 28190732 | doi = 10.1016/j.cub.2016.12.057 | pmc = 5659724 | bibcode = 2017CBio...27..569S }}</ref>