Final answer:
After the OH binds to the 5' end of a group I intron, a guanine nucleotide cofactor is involved in forming a phosphodiester bond with the intron, and the 3'-OH of the exon initiates splicing to excise the intron and join exons.
Step-by-step explanation:
Splicing of Group I Introns
When the hydroxyl group (OH) binds to the 5' end of the intron in group I introns, it initiates the splicing process. This is a two-step reaction involving the guanine nucleotide cofactor where its 3'-hydroxyl group forms a 3'5'-phosphodiester bond with the 5' end of the intron. This results in the release of the 3'-hydroxyl group of the exon, which acts as a nucleophile to attack the 3' end of the intron, thereby excising the intron and connecting the exons. This particular mechanism of splicing differs from that of group-II introns, which form a branched lariat structure through a self-splicing mechanism, and group-III introns, which require the intervention of small nuclear RNA.
Each intron that is spliced out of the pre-mRNA must undergo this precise mechanism to form a functional mRNA, which will then undergo further modifications like 5' capping and poly-A tail addition before being translated into a protein.