Re: On influencing factors

Fri, 30 Oct 1998 12:51:00 -0500 (EST)

Subject: Re: On influencing factors
Date: Fri, 30 Oct 1998 12:51:00 -0500 (EST)

On Tue, 27 Oct 98 15:14:27 -0600 Mark Mills <>

> To quote your above comment, 'differential gene expression is the
> controlling factor,' sounds like a claim that 100% of control goes to the
> nucleus.
> I am just guessing. Perhaps you can clarify your position.
> Do you mean 100% control resides in the DNA?

Not quite 100%, since even identical twins, who are near enough 100%
genetically identical (somatic mutations and immune lineage
rearrangements aside), are not 100% phenotypically identical. But they
are much more identical than people who are, for instance, only

> Do you suggest outside influences are immaterial to cellular
> differentiation?

I wouldn't say immaterial, but I would say much less important. The
geno-pheno correspondence is, as Waddington used to say, canalized
(Gatherer 1996), meaning that it's rather like running a ball bearing
down a grooved landscape. The ball bearing will tend to get into one
groove and stay in it. Extreme environmental conditions could cause it
to pop out of one groove and into an adjacent one, but most of the
time it will stay within the groove it starts out in.

> If both cellular rhythms and protein production can be influenced by
> forces outside the cell, how clearly can we draw the boundary between
> what is controlled by DNA vs 'the outside.'

They are influenced by outside forces, but never in such a way as to
cause a differentiated cell to de-differentiate. Once a cell loses its
toti/pluri-potency there is no going back. Neurons respond extensively
to things like nerve growth factor, neurotransmitters etc, but they
always stay neurons.

> The other ingredient to add to the puzzle is self-organizing features of
> auto-catalytic protein systems (see Kauffman). I bring this up to replace
> the homunculus. If we admit the role of self-organization, the
> homunculus is serves no purpose.

Kauffman has often touched on developmental biology (eg. Kauffman
1991). But he works solidly in the paradigm of genes controlling
development. One interesting contribution he has made to dev. biol. is
the observation that "the number of cell types in organisms seems to be
related mathematically to the number of genes in the organism."
(Kauffman 1991, p.70). Circumstantial evidence, I admit, but difficult
to explain away.

Gatherer D (1996) A stroll along the epigenetic landscape - bringing
Waddington's ideas into molecular biology. Early Pregnancy: Biology
and Medicine 2, 241-243.

Kauffman SA (1991) Antichaos and adaptation. Scientific American
August 1991, pp. 64-70. (British edition - numbering is slightly
different in US ed. I believe...)


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