This step occurs by transesterification; in this case, an OH group at the 3′ end of the exon attacks the phosphodiester bond at the 3′ splice site. The adjoining exons are covalently bound ...

1b). Within this complex, intron excision occurs in two steps, each a single transesterification reaction. In the first step, the 2'-OH of an adenosine in the intron (the branch site A ...