In article <u58563-1308950129000001 at genetics.comd.uic.edu>, u58563 at uic.edu
(Tommy the Terrorist) wrote:
> I've been involved with a project in which the human form of a gene is
50% GC, and the mouse form is 60%. The coding sequence is only about 80%
conserved on protein and DNA levels; the changes are primarily GC ---> AT
transitions. Interestingly, about 75% of the CpG doublets in the mouse
have been lost from the human (but almost all of the human CG's are
present in the mouse). Nevertheless, the other transitions are
predominantly in the same direction.
> Thus, we have a pair of homologues, one of which seems to differ from
the other primarily due to (I presume) methylcytosine formation and
conversion to thymine. It would appear to be a case in which evolution is
driven neither by natural selection nor in a strictly random fashion.
> We have mulled a few notions of imprinting, proximity to
heterochromatin, and so forth, but haven't really come up with any
compelling ideas for experiments that could explain this odd observation.
> Any recommendations or comments would be most welcome.
there are cases of rapid (relatively) change in %GC when genes move
between isochores in mammalian genomes. check out the work of ross
hardison's group on transposons in rabbits [bunny hoppers :-)] where
elements moving into a novel isochore rapidly ameliorate to the %GC of the
surrounding DNA. Intrinsic to this hypothesis is the assumption that the
two homologues lie in different isochores. this can also be tested if
cosmid or YAC clones of each are available.
lawrence at bioscience.utah.edu