Mechanism of pre-mRNA splicing.
The snRNPs of the spliceosome were left out of this figure, but it shows the sites within the intron whose interactions are catalyzed by the spliceosome. Initially, the conserved G which starts an intron is cleaved from the 3' end of the exon upstream to it and the G is covalently attached to an internal A within the intron. Then the 3' end of the just-released exon is joined to the 5' end of the next exon, cleaving the bond that attaches the 3' end of the intron to its adjacent exon. This both joins the two exons and removes the intron in lariat form.
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