It's all about splicing of introns ... the 3′ end of the intron is brought into proximity, cut, and joined to the 5′ end. This step occurs by transesterification; in this case, an OH group ...

However, this discovery does not explain all aspects of 3 ... Within this complex, intron excision occurs in two steps, each a single transesterification reaction. In the first step, the 2 ...