VARIATION IN DNA CONTENT AMONGST DIFFERENT SPECIES
Mark D. Garfinkel
garfinkl at iitmax.iit.edu
Thu Jan 27 20:05:59 EST 1994
Robert Dawley <rdawley at acad.ursinus.edu> posts:
>The huge disparity in genome size among organisms of roughly similar
>structural complexity was once called "the c-value paradox."
*As I learned it, the C-value paradox had two components. First,
the apparent excess of nuclear DNA relative to "required" coding capacity.
Second, the discrepancy between genome sizes sometimes observed between
even closely related species. Allotetraploidy is only a partial
explanation of the latter phenomenon.
>The paradox has since been
>resolved, as Andrew Roger explained, by the discovery that many
>eukarytoic genomes are composed partly or largely of DNA that does not
>code for proteins. This leaves, however, the very interesting question of
>why so much non-coding DNA exists in the genome. This has proven to
>be a difficult question to resolve.
*In other words, the paradox still exists.
Part of the non-coding DNA is now known to be found inside
intervening sequences, and in some cases these result in *very* long
transcription units. A time-delay therefore is built-in to such genes,
whereby an inducing signal at time zero will not manifest itself by
induced-mRNA production until (at least) the transit time for RNA
polymerase has passed. Whether this mechanism of temporal regulation
is of real significance is being experimentally tested with some
Drosophila homeotic genes and steroid-regulated genes, but there is
little clear data as yet. Large transcription units (and large mRNAs, too)
impose limitations on the rate of production of the corresponding mRNAs
and proteins. Perhaps in a passive way some of the "junk DNA" actually
has a function after all.
>Robin from Cal Tech is right.
*"Caltech." One word, initial capital only.
>there also are
>DNA sequences called transposons that can actively spread copies of
>themselves throughout the gemone. This DNA is called selfish DNA
>because it expands at the expense of the organism that houses it.
*Such transposable DNA elements are likely to be agents of
evolutionary change for several reasons. They are "portable regions of
homology," to use a Nancy Kleckner coinage, and therefore might contribute
to ectopic pairing & chromosome rearrangments such as inversions,
duplications, deletions, etc. They are insertional mutagens, accounting for
a large number of "spontaneous" mutations in several organisms. They
introduce promoters and/or enhancers that can render neighboring genes
subject to new regulatory inputs. Such cis-acting changes in gene
expression may or may not be deleterious; quite the opposite. Barbara
McClintock championed the idea of these elements being involved
beneficially in developmental regulation and/or evolutionary change.
Her views deserve mention, even if there are problems with them.
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