FIGURE 10-8 Two transesterification reactions result in the splicing of exons in pre-mRNA. In the first reaction, the ester bond between the 5′ phosphorus of the intron and the 3′ oxygen (dark red) of exon 1 is exchanged for an ester bond with the 2′ oxygen (blue) of the branch-point A residue. In the second reaction, the ester bond between the 5′ phosphorus of exon 2 and the 3′ oxygen (orange) of the intron is exchanged for an ester bond with the 3′ oxygen of exon 1, releasing the intron as a lariat structure and joining the two exons. Arrows show where activated hydroxyl oxygens react with phosphorus atoms.