Fig. 1

Mechanism and mode of pre-mRNA splicing. a The U1 snRNP particle recognizes the 5′ splice site while U2 recognizes the adenosine (A) of the branch point. Subsequently to the entry of the heterotrimeric (pre-assembled) U4/U6-U5 particle, several rearrangements lead to the catalytic activation of the spliceosome. This results in the displacement of U4 and U1 snRNPs with a concomitant attack of the 2′ OH of the adenosine of the branch site to the 5′ splice site generating a lariat intron-3′ exon intermediate. Finally, the 3′ OH of the free 5′ exon attacks the 3′ splice site, thus releasing the lariat intron and ligating the two exons. b Modes of alternative splicing reactions. Alternatively, spliced mRNAs are generated through inclusion (or skipping) of cassette exons, mutually exclusive exons, usage of 5′ and 3′ splice sites, intron retention, alternative promoters, or polyadenylation sites