Biology Reference
In-Depth Information
6.3.3
Role of signals in the Darwinian model
The signal is always interpreted according to an instructive and
deterministic model. If this model is wrong, what is its role, since
in general it has no nutritive value
per se
?
The Darwinian model relies on selection of stochastic molecular
events which change the properties of cells and let them adapt. The
frequency of these events in this model is a key parameter. If it is
very low, the cells will have a stable phenotype: their characteris-
tics will not change. In contrast, if this frequency is high, the cells
will be very unstable. Now it happens that this frequency itself
depends on the stability of the molecular interactions.
Let us consider the simplest example possible. A single molecule
of a regulator
R
can activate two genes
a
and
b
by binding to their
promoter sites. When the molecule
R
has bonded to the promoter of
a
, it can escape by dissociating and diffusing as far as
b
in order to
activate it (Fig. 21). The probability of this event of stochastic
molecular recombination occurring depends on the stability of the
molecule's bond with
a
. The more stable it is, the less dissociation
there is and consequently the lower the probability of the molecule
diffusing towards
b
. Conversely, the less stable the bond, the higher
is this probability. It will be the same for all remodelling of the
cell's protein complexes, whatever function they are involved in.
R
a
b
F
IGURE
21. Influence of the stability of random molecular interactions in gene
expression.
The DNA wound around the spherical protein structures in the chro-
matin, called nucleosomes, is not expressed (thick line). This fibre has to be
unwound to be accessible to gene transcription proteins (thin line). In these regions
open to transcription, in order for a regulator
R
to be able to diffuse from gene
a
to gene
b
, it must first detach itself from
a
. How frequently this event occurs
depends on the stability of the bond between
R
and
a
.
