It's all about splicing ... 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 at the 3 ...

The spliceosome and Group II introns are believed to share ... released the mRNA that it had finished splicing introns from, it hung onto intron lariats and reshaped them into true circles.