Self-splicing

=Self-splicing=

**Self-splicing** is one of the methods that may be used to remove introns from a pre-mRNA molecule to form the mature mRNA transcript. It is a post-transcriptional modification. Self-splicing occurs on the rare occasion when an intron forms a ribozyme (RNA enzyme) that is capable of catalysing its own excision from the molecule, in the same manner that its excision is usually catalysed by a spliceosome complex.

There are three groups of self-splicing introns, though groups I and II require no assistance from proteins.

In group I introns, two transesterifications lead to the excision of introns and splicing of exons:
 * 1) The 3' hydroxyl (OH) group of a free guanine nucleoside (guanosine), found either within the intron or elsewhere, attacks the phosphate at the 5' end of the splice site
 * 2) The 3' hydroxyl (OH) group of the 5' exon joins (splices) with the 3' exon

In group II introns, there are also two transesterifications:
 * 1) The 2' OH of a specific adenosine nucleoside in the intron attacks the 5' splice site, forming a lariat
 * 2) The 3' OH of the 5' exon triggers the next transesterification, at the 3' splice site, splicing the two exons together