Exon mechanism

The mechanisms whereby introns are removed from the primary transcript in the nucleus, exons are ligated to form the mRNA molecule, and the mRNA molecule is transported to the cytoplasm are being elucidated. Four different splicing reaction mechanisms have been described. The one most frequently used in eukaryotic cells is described below. Although the sequences of nu-... 

Figure 37-13. Mechanisms of alternative processing of mRNA precursors. This form of RNA processing involves the selective inclusion or exclusion of exons, the use of alternative 5 donor or 3 acceptor sites, and the use of different polyadenylation sites.

Figure 6.1 The group I intron exon-spUcing mechanism. (Adapted with permission from Figure 1 of reference 24. Copyright 2004, with permission from Elsevier.)...

The splicing of pre-mRNAs often allows several possible combinations of exons. Despite this fact, only a few of the theoretically possible combinations are realized. This accentuates that the choice of alternative sphce sites must be strictly controlled. The exact mechanism by which a sphce site is chosen is not imderstood. The foUowing control mechanisms are conceivable (review Lopez, 1998) ... 

The fourth class of introns, found in certain tRNAs, is distinguished from the group I and II introns in that the splicing reaction requires ATP and an endonuclease. The splicing endonuclease cleaves the phosphodiester bonds at both ends of the intron, and the two exons are joined by a mechanism similar to the DNA ligase reaction (see Fig. 25-16). 

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