Editing RNA splicing (section)

=== Self-splicing ===<!-- This section is linked from [[Intron]] -->
'''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 catalytic intron|Group I]]'', ''[[Group II intron|Group II]]'' and ''[[Group III intron|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.{{cn|date=July 2024}}

Two [[transesterification]]s characterize the mechanism in which group I introns are spliced:{{cn|date=July 2024}}
# 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.