1. Splicing requires numerous antiparallel RNA base-pairing rearrangements.The rearrangements accompanying the first transesterification reaction are summarized here.Black lines denote RNA sequences in snRNPs;the yellow line denotes the pre-mRNA.RNA segments involved in intra-and intermolecular base-pairing interactions are shown as colored boxes.(a)The exchange of U1 for U6 involves base-pairing rearrangements at the 5’-end of the intron(orange).(b)BBP is displaced from the branch-point sequence by base-pairing rearrangement within U2(blue-green).(d)disruption of a U4:U6 interaction to form an intramolecular stem-loop structure in U6(yellow)(e)Disruption of U4:U6 interactions in favor of a U2:U6 interaction(magenta).(f)An additional U2:U6 base-pairing interaction(green).