RNA splicing

• In molecular biology, splicing is the editing of the nascent precursor messenger RNA (pre-mRNA) transcript into a mature messenger RNA (mRNA). After splicing, introns are removed and exons are joined together (ligated).

• For nuclear-encoded genes, splicing takes place within the nucleus either during or immediately after transcription. For those eukaryotic genes that contain introns, splicing is usually required in order to create a mRNA molecule that can be translated into protein.

• For many eukaryotic introns, splicing is carried out in a series of reactions which are catalyzed by the spliceosome, a complex of snRNPs. Self-splicing introns, or ribozymes capable of catalyzing their own excision from their parent RNA molecule, also exist.

• • Self-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.

• Two transesterifications characterize the mechanism through which group of introns is spliced:

1. 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.
2. 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:

1. The 2’OH of a specific adenosine in the intron attacks the 5′ splice site, thereby forming the lariat
2. The 3’OH of the 5′ exon triggers the second transesterification at the 3′ splice site, thereby joining the exons together.