In molecular biology and genetics, splicing is a modification of an RNA after transcription, in which introns are removed and exons are joined. This is needed for the typical eukaryotic messenger RNA before it can be used to produce a correct protein through translation. For many eukaryotic introns, splicing is done in a series of reactions which are catalyzed by the spliceosome a complex of small nuclear ribonucleoproteins (snRNAs), but there are also self-splicing introns.Self-splicingSelf-splicing occurs for rare introns that form a ribozyme, performing the functions of the spliceosome by RNA alone. There are three kinds of self-splicing introns, Group I, Group II and Group III. Group I and II introns perform splicing similar to the spliceosome without requiring any protein. This similarity suggests that Group I and II introns may be evolutionarily related to the spliceosome. Self-splicing may also be very ancient and may have existed in an RNA world present before protein. Although the two splicing mechanisms described below do not require any proteins to occur, 5 additional RNA molecules and over 50 proteins are used and hydrolyzes many ATP molecules. The splicing mechanisms use ATP in order to accurately splice mRNA's. If the cell were to not use any ATP's the process would be highly inaccurate and many mistakes would occur. Two transesterifications characterize the mechanism in which group I introns are spliced :3' OH of a free guanine, nucleoside (or one located in the intron) or a nucleotide cofactor (GMP, GDP, GTP) attacks phosphate at the 5' splice site. 3' OH of the 5 'exon becomes a nucleophile and the second transesterification results in the joining of the two exons.The mechanism in which group II introns are spliced (two transesterification reaction like group I introns) is as follows :The 2'OH of a specific adenosine in the intron attacks the 5' splice site, thereby forming the lariat. The 3'OH of the 5' exon triggers the second transesterification at the 3' splice site thereby joining the exons together.Spliceosomal and self-splicing transesterification reactions occur via two sequential transesterification reactions. First, the 2' OH of a specific branch point nucleotide within the intron that is defined during spliceosome assembly performs a nucleophilic attack on the first nucleotide of the intron at the 5' splice site forming the lariat intermediate. Second, the 3' OH of the released 5' exon then performs a nucleophilic attack at the last nucleotide of the intron at the 3' splice site thus joining the exons and releasing the intron lariat.'