Spliceosome assembly on pre-mRNAs involving base pairing to snRNAs. The splicing process is described in the text. In this figure, exons are green; introns, pink; snRNAs, blue. Notice that in step 2, U6 snRNA base-pairs near the 5′ exon binding site where U1 snRNA was formerly bound. As U4 dissociates, the U2-U6-U5 complex remains assembled on the pre-mRNA. U5 base-pairs to both sides of the splice junction to align the RNA for the splicing reaction, and U2 and U6 base-pair to each other.