iGen3 05-14

Exon splicing & intron excision in eukaryotic hnRNA

    The 5' & 3' ends of introns have characteristic GU and AG motifs that signal the region to be "spliced out." The diagram shows the removal of an intron between Exons 1 and 2. An A base near the AG signal at the 3' end of the intron-equivalent covalently bonds to the GU signal at the junction of 5' end of the intron and the 3' end of Exon 1 [left hand diagram]. The 3'-OH end of Exon 1 then bonds to the 5'-PO4 of Exon 2 [right-hand diagram, below], which releases the intron as a covalently-bonded circular lariat with a short tail. Note that the RNA molecule remains unbroken  throughout the process, and that it is the splicing of the two exons that excises ('splices out') the intervening intron.

    The sequences of the GU and AG intron motifs, and the flanking 3' & 5' bases of the exons, are critical to the process. Mutation of the corresponding bases in the DNA can result in mis-splicing, either failure to remove an intron or removal of an exon along with the adjacent intron.

    [Note on terminology: The diagram refers to sequences in an hnRNA molecules as introns and exons. Properly speaking, introns and exons occur only in the DNA, and the corresponding RNA sequences are intron- and exon-equivalents].

Figure © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2010 by Steven M. Carr