Re: epigenetics

r norman wrote:

On 10 Aug 2006 10:03:01 -0700, "lucaspa" <lucaspa@xxxxxxxxxxxxx>
wrote:


r norman wrote:

On Thu, 10 Aug 2006 11:32:30 -0500, Dick <remdickhm@xxxxxxxxxxxxx>
wrote:

Some time ago, I generated a purely theoretical example of a genome
that, when put into say a pine tree egg cell would result in the
production of a pine tree but when put into a human egg cell would
result in the production of a human. My example was derided at the
time as being totally impossible to produce biologically, even if the
scheme could possibly work. However the intent was specifically to
answer the question you raise. The new organism is produced by
"reading" and "interpreting" the sequence in the DNA but the new
organism is not at all "completely determined" by the sequence in the
DNA. The new organism is the result of the DNA being read by an
appropriate DNA-interpreting machine, a cell. The kind of cell that
does the interpretation makes a big difference.

Can make a big difference. But not always. For instance, putting the
PAX gene into leg cells of a fly causes the formation eyes in the leg.

For the opposite case, the Dpp gene causes wing development in a fruit
fly. But it is a member of the BMP family that causes bone development
in mammals. So when Dpp was transfected into mouse fibroblasts and the
fibroblasts implanted into the muscle of a mouse leg, the result was
extraskeletal bone, not a fly wing.


Putting a mouse analog of PAX into a fly causes formation of a fly
eye, not a mouse eye.

But not because of epigenetic factors. It's because of the other genes
downstream in the regulatory cascade.

But all of this is confused by the fact that BMP is not really
"responsible" for producing bone nor is PAX or its equivalents
"responsible" for producing eyes of whatever type. These genes are
necessary components for the initiation of bone or of eye but a large
number of other genes are actually involved in producing the
structures that make up bones and eyes.

BMP and PAX don't actually produce any structures at all. They just make
transcription factors that bind to the promoters of other genes. What
they do -- meaning what genes they bind to, and whether the effect is to
increase or decrease transcription -- is not their problem. They're just
switches, and what the switch controls can be rewired easily. Many
transcription factors are reused for different purposes during
development of the same organism, turning different genes on and off in
different contexts. And those contexts are determined by the expression
of other genes.

Sometimes we get so tied up
in producing cute names for genes that we forget that the genes often
have only a very indirect role in the structures or processes for
which they are named.

I think the take-home message is that this was a digression from
epigenetics into developmental genetics. Nothing to see here, folks.

.