Splicing involves how many transesterification reactions?
Pre-messenger RNA splicing proceeds via two sequential transesterification reactions, chemically similar to group two self-splicing introns, with ester bonds exchanged without net ATP hydrolysis for chemistry. First reaction: two prime hydroxyl of branch adenosine attacks phosphodiester at five prime splice site, severing five prime exon from intron and generating free three prime hydroxyl on five prime exon plus lariat intermediate where intron five prime end linked to branch adenine. Second reaction: three prime hydroxyl of five prime exon attacks three prime splice site, ligating exons and
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Splicing involves two transesterifications; Lodish 9th ed., Chemistry of splicing