Fig. 1

Overview of splicing mechanisms and alternative splicing types. (A) Simplified schematic illustration of pre-mRNA splicing. Evolutionarily conserved 5′ splice sites (5′ss; GU) and 3′ splice sites (3′ss; AG) are located at the 5′ and 3′ ends of introns, respectively. Within the upstream intron of the 3′ss, a branchpoint sequence (BPS) and a polypyrimidine tract (PPT) are present. The 5′ss is recognized by the U1 snRNP, the PPT by the U2AF2, and the 3′ss by the U2AF1. The U2 snRNP recognizes the BPS. It then undergoes two successive transesterification steps to generate splicing products (spliced exon and lariat intron). (B) Major types of alternative splicing events. Cells generate different mRNA transcripts by exon skipping of alternative exons, the selection of alternative 5 or 3′ss, retention of introns, selection of the mutually exclusive exons, and selection of alternative promoters or polyadenylation. Exons are represented by colored boxes, introns by horizontal lines, and distinct alternative splicing events by dotted lines