Final answer:
The lariat in Group II self-splicing is formed when the 2'-hydroxyl group of an adenylate residue within the intron attacks the 3′ end of the exon, without any external cofactors, thereby creating a 2',5'-phosphodiester bond and a branched lariat structure.
Step-by-step explanation:
The lariat formation during Group II self-splicing is a fascinating biological process integral to RNA processing. Unlike spliceosomal splicing, where snRNPs are required, Group II introns are capable of self-splicing. This process involves the formation of a lariat structure without the need for any external cofactors. The 2'-hydroxyl group of an adenylate residue within the intron initiates the splicing by attacking the 3′ end of the exon. This forms a distinctive branched lariat structure characterized by a 2',5'-phosphodiester bond. The clever self-catalytic activity of Group II introns suggests they might be evolutionary precursors to the more complex spliceosomal machinery.
It is the intricacy of the RNA secondary structure, similar to hairpin structures, that permits this self-splicing activity. The intron folds itself into a stem-loop structure with the essential reactive sites closely aligned to enable the catalytic action needed to carry out splicing. After the initial attack creates the lariat, the 3'-hydroxyl group of the released exon attacks the other splice site, completing the reaction. The result is the removal of the intron and the ligation of the two exons into a mature message, ready for translation.