Sequential transesterification reactions during splicing (see also Figure 28.32 on page 1098). In the first reaction, the ester bond between the 5' phosphorous of the intron and the 3' oxygen (red) of exon 1 is exchanged for an ester bond with the 2' oxygen (dark blue) of the branch-site A residue. In the second reaction, the ester bond between the 5' phosphorous of exon 2 and the 3' oxygen (light blue) 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 the activated hydroxyl oxygens react with phosphorous atoms.