Figure 2.

Splicing occurs in two sequential chemical steps (inset) to excise the intron (red) and ligate the coding exons. The spliceosome assembles in an ordered manner from preassembled components responsible for identifying the splice sites, forming the active site, and catalysis. The spliceosome is then released and the components recycled for the next round of splicing. Splicing complex names are shown in blue, and helicases that help mediate major transitions in the splicing cycle are shown in gray.