An important note on the terminology of Splicing
of Exons
& Introns
'Splicing' is defined as the joining
of two things, for example the ends of two ropes. It
should not be confused with 'splitting', which is separating
two things. However, in molecular biology, ''splicing out'
refers to the removal of some RNA segments in the process of splicing together
the remaining segments.
The terms and 'exon'
and 'intron' properly refer to
regions of the DNA
that are respectively "expressed" and "intervening."
However, these terms are sometimes used loosely to refer to
the corresponding sequences in hnRNA that
are retained or removed, respectively, from
the final mRNA product.
These should properly be called intron & exon transcripts,
as in the above diagram. This means of course that the
sequences of the DNA exons in the sense strand are
identical to the corresponding
mRNA exon transcripts,
except for substitution of U
for T. Thus the
corresponding amino acid sequences can be either 'read' directly from the DNA sense strand,
or 'translated' from
the mRNA.
Avoid
thinking of splicing as splitting of
the hnRNA transcript. Avoid
thinking of exons as the translated
portions of the mRNA., while recognizing that
the word is often used incorrectly. In the figure above, the phrase "Splicing
eliminates intron transcripts" means that "Splicing,
by joining together the exon transcripts, results in the
elimination of intron transcripts," not that
splicing is the direct elimination of intron transcripts.
Figure ©2000 by
Griffiths et al. ; text
©2024 by Steven M. Carr