Intron
From Palaeos
An intron, generically, is a piece of DNA which is transcribed into RNA but does not form part of the final gene product. In one fashion or another, it must be spliced out of the RNA before the RNA can be used for translation, ribosome formation, etc. Group I introns are large introns whose RNA product has an extremely complex secondary structure. The RNA itself (with or without cofactors such as nucleotides or metal ions) catalyzes its own removal from the initial RNA transcript. That is, it functions as a ribozyme, an enzyme made up of RNA. Most Group I introns then catalyze their own circularization. Mobile Group I elements may also encode a homing endonuclease, or use an endonuclease native to the host, to insert a DNA copy of the transcribed intron into the homologous location on a sister chromosome, or into another copy of the "home" gene in the case of genes present in multiple copies. The point of insertion is never random, and tends to be quite specific to a particular gene. A Spliceosomal Intron, at the opposite extreme, is a short (c. 40 bp) intron which serves as a substrate for splicing enzymes which remove the intron and probably also perform various regulatory roles, including adding the 3'-poly(A) "tail" which acts as a shipping label, specifying that the nuclear RNA is a messenger RNA which can be exported to the cytoplasm for translation. Various regulatory roles and one possible template for the evolution of the system are reviewed by Lynch & Kewalramani (2003). Group I and spliceosomal introns are likely to be related, with Group I introns gradually becoming simplified and more dependent on the host cell until they are eventually reduced to spliceosomal introns or disappear altogether.
References
Haugen, P, HJ Runge & D Bhattacharya (2004), Long-term evolution of the S788 fungal nuclear small subunit rRNA group I introns. RNA 10: 1084–1096.
Lynch, M & A Kewalramani (2003), Messenger RNA surveillance and the evolutionary proliferation of introns. Mol. Biol. Evol. 20:563-571.
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