Final answer:
Group II self-splicing involves the removal of group II introns from certain RNA molecules through a self-catalyzed mechanism that forms a branched lariat structure, indicating an evolutionary connection to eukaryotic spliceosomal intron splicing.
Step-by-step explanation:
Group II self-splicing is a mechanism that involves the excision of group II introns from pre-messenger RNA (pre-mRNA) within chloroplast, mitochondrial rRNA, mRNA, tRNA and some bacterial mRNAs without the need for any external cofactor or proteins. These introns are able to self-splice thanks to the complex stem-loop tertiary structures they form, which facilitate the splicing process. Remarkably, the removal of group-II introns is initiated by the 2'-hydroxyl group of an adenylate residue within the intron, which attacks the exon's 3' end, thus forming a branched lariat structure with a 2',5'-phosphodiester bond. This mechanism of action suggests that group II introns may have been the evolutionary precursor to the spliceosomal intron splicing observed in eukaryotes.