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Same gene, different GC%...

Tommy the Terrorist u58563 at uic.edu
Sun Aug 13 01:29:00 EST 1995

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.

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