text: Spliceosomal splicing and self-splicing involve a two-step biochemical process.  Both steps involve transesterification reactions that occur between RNA nucleotides.  tRNA splicing, however, is an exception and does not occur by transesterification.
  Spliceosomal and self-splicing transesterification reactions occur via two sequential transesterification reactions.  First, the 2OH of a specific branchpoint nucleotide within the intron, defined during spliceosome assembly, performs a nucleophilic attack on the first nucleotide of the intron at the 5 splice site, forming the lariat intermediate.  Second, the 3OH of the released 5 exon then performs a nucleophilic attack at the first nucleotide following the last nucleotide of the intron at the 3' splice site, thus joining the exons and releasing the intron lariat.