Final answer:
Pre-mRNA splicing removes introns and joins exons in eukaryotic cells, involving a lariat formation. Group-II introns self-splice via the 2'-hydroxyl group attacking the exon-intron boundary. In spliceosomal splicing, snRNPs form the spliceosome complex, facilitating intron removal and exon ligation.
Step-by-step explanation:
Pre-mRNA splicing is a critical process in eukaryotic cells where introns are removed and exons are joined to form a continuous sequence that can be translated into a protein. The splicing process involves the formation of a lariat structure. In group-II introns, this is a self-splicing mechanism that relies on the intramolecular attack by the 2'-hydroxyl group of an adenylate residue within the intron on the exon-intron border.
This forms a 2',5'-phosphodiester bond resulting in a lariat structure. In contrast, the removal of spliceosomal introns involves the spliceosome, which consists of snRNPs (small nuclear Ribonucleoproteins) recognizing splice sites and facilitating the splicing reaction. Here, a similar lariat structure is formed at an A residue branch site.