In the first step, the 2'-OH of an adenosine in the intron (the branch site A) attacks the phosphodiester bond at the 5' splice site to release the upstream exon and form a branched or lariat ...

"One of the questions that was sort of missing from this story in my mind was, is it possible that the modern spliceosome is still able to take a lariat intron and insert it somewhere in the genome?" ...

The adjoining exons are covalently bound, and the resulting lariat is released with U2 ... Proponents of the "intron-early" theory suggest that all organisms (including prokaryotes) at one ...

This study demonstrates that after splicing is finished, the spliceosome is still active and can convert the lariat intron into a circle using a third reaction (green arrow 3) marked by an asterix.