Nuclear pre-mRNA splicing is catalyzed by the spliceosome which assembles in a step-wise manner. Complex B undergoes a number of structural rearrangements and U4 and U1 snRNPs dissociate to generate complex C, the catalytic spliceosome. (Adapted from PMID:24452469 and PMID:23118483).
Notes on Group
Complex B goes through transition intermediates Bact and B* prior to forming complex C (PMID:24452469 and PMID:23118483). These have not been curated as gene groups.