FIGURE 1.

The mechanisms of splicing and alternative splicing. (a) The complexes and processing steps of splicing for the major spliceosome are depicted. In the assembly phase, U1, U2, U4, U5, and U6 are sequentially assembled in the mRNA molecule resulting in the formation of complex E, complex A, and finally, complex B. The activation phase consists of conformational and compositional rearrangements of complex B involving U2, U5, and U6. Two transesterifications occur as the catalytic complex C adapts its conformation: the first between the 5′ SS and the BP and the second between the 5′ end of the first exon and the 3′ end of the second exon. Finally, the snRNPs still attached to the mRNA disassemble to start a new cycle. (b) Illustration of the regulation of splicing by cis‐ and trans‐acting factors. Cis‐acting factors are represented as boxes in introns and exons. Trans‐acting factors are represented as circles bound to their corresponding cis‐acting elements: exonic splicing enhancer (ESE), exonic splicing silencer (ESS), intronic splicing enhancer (ISE), or intronic splicing silencer (ISS). Silencers are depicted in orange and enhancers in light blue. (c) Schematic representation of alternative splicing events. Gray boxes represent constitutive exons, red and orange boxes represent alternative exons, and a light blue box represents an exonized intron. Discontinuous ends in the exons indicate that the transcript continues in that direction. Thick blue lines represent introns in the transcript before processing, and dashed lines above and below represent the alternative processing of the exons