What is the biochemical mechanism of self-splicing?

Self-splicing involves a mechanism in which certain RNA molecules remove their own introns without the need for enzymes. In this process, the 3' hydroxyl (OH) group of a guanine nucleotide within the intron attacks and attaches to the 5' phosphate of the first nucleotide of the intron. Then, the 3' OH group of the last nucleotide of the upstream exon attacks and binds to the 5' phosphate of the first nucleotide of the downstream exon, attaching the two exons together and removing the intron. After being removed from the RNA molecule, the excised intron undergoes a process where the 3' hydroxyl group of its last nucleotide reacts with a phosphate group within the intron itself (between the 15th and 16 nucleotides). Additional breakdown removes the intron from the process, preventing the reaction from reversing. Phosphoester transfers can revert unless the excised intron is eliminated. Overall, the number of phosphoester bonds remains unchanged during this splicing process. The self-splicing pathway consists of two transesterification reactions. The first reaction involves the hydroxyl group of the guanine nucleotides carrying out a nucleophilic attack. In the second transesterification reaction, the nucleophilic attack occurs at the 3’ splice site.