Schematic representation of the splicing cycle. Splicing is the removal of introns from pre-mRNA and ligation of exons in order to form the mature mRNA. The splicing cycle is a sequential set of reorganizations of the spliceosome, a complex composed of five snRNAs: U1, U2, U5, and U4/U6. U5 and U2/U6 constitute the catalytic core and catalyze the two nucleophilic attacks (catalytic steps). Four DEAH/RHA helicases (in black) and two known G-patch protein partners (in red) are involved in splicing. Prp2 is activated by Spp2 and acts prior to the first catalytic step. Prp16 acts between the two catalytic steps remodeling the spliceosome in order to permit the binding of essential factors for the second catalytic step. Prp2 is involved in the release of spliced mRNA from the lariat-spliceosome complex. Prp43 is activated by Ntr1 in the disassembly of the lariat-spliceosome complex. 5′-SS: 5′-splicing site, 3′-SS: 3′-splicing site, and BP: branch point.