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
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.
Figure
© 2002 by Griffiths et al.; all text
material
© 2008 by Steven M. Carr