Biology Reference
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guide it, and that is indeed the case. Chromatin molecules are con-
tinually affected by signals (selectors) arising from the cell environ-
ment which cause epigenetic modifications (phosphorylation,
acetylation, methylation etc.) that change the interaction properties
of the molecules making it up (the diffusion coefficient, association
constants etc.). Chromatin organisation depends directly on these
parameters and is therefore controlled by these constraints arising
from the cell environment. Here, again, we are dealing with a hetero-
organisation process, not spontaneous self-organisation.
6.5.3
The stochastic expression of genes subject to
natural selection
Two types of interdependent constraints are exerted on gene
expression during embryo development: those that arise from its
immediate development (its ontogenesis) and those that arise from
its history (its phylogenesis). Firstly, selective constraints of the cell
microenvironment are created in the embryo owing to its own devel-
opment. They induce stabilisation (or destabilisation) of gene
expression (Figs. 16 and 22). Secondly, embryonic development is
itself constrained by its initial conditions, i.e. by the structure of
the egg which is the product of its evolutionary history (Fig. 15).
DNA plays a dominant role in the context of this phylogenetic con-
straint, because it is passed on in a (relatively) unaltered way
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to
each generation. By being unaltered it promotes the reproducibility
of ontogenesis. Our theory does not therefore deny its importance
in biological processes but attributes a different role to it. As
for the synthetic theory of evolution, for our theory DNA is the
result of external selective constraints to which the organism has
been subjected and which have been interiorised in its structure
via
genetic mutations and recombination. In ontophylogenesis, how-
ever, it acts as a random protein generator, not as a genetic pro-
gramme. The important point to emphasise in this conception is
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At each cell division, there is always a certain rate of genetic recombinations
which modify the organisation of the genome.
