splicing & intron excision in
The 5' & 3' ends of introns
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
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
unbroken throughout the process, and that it is the
splicing of the two exons that excises ('splices out') the intervening
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
[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.