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 in which group I introns are spliced:
# 3OH 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.
# 3OH 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 2OH of a specific adenosine in the intron attacks the 5 splice site, thereby forming the lariat
# The 3OH of the 5 exon triggers the second transesterification at the 3' splice site, thereby joining the exons together.