Biology Reference
In-Depth Information
to the environment. The phenomenon is deterministic with noise.
For genetics, ontogenesis is this type of phenomenon. The generation
of a phenotype from a genotype is indeed considered a deterministic
phenomenon affected to some extent by noise. This noise, which
arises from variations in the internal molecular mechanisms and
interaction with the environment, disturbs the deterministic action
of the genes. It causes small differences between individuals which
increase the diversity of phenotypes. There are countless phenomena
of this kind in nature. When noise has negative effects it must be
limited or compensated in order not to destroy the phenomenon in
question, but it can also have positive effects by permitting a sys-
tem to change state. The standard example of this is the reaction-
diffusion mechanism analysed by Alan Turing (1912-1954).
In general, if a system in equilibrium is subjected to fluctuation,
it will return after a certain lapse of time to its initial state of
equilibrium or it will oscillate around this equilibrium with minor
fluctuations. A very simple case of this is that of local fluctuation
in the concentration of a solute in a solvent. On average, the con-
centration always remains the same. Turing demonstrated that in
certain cases it may be otherwise. If the system is more complex
and the relationships between its constituents permit, instead of
returning to the initial state after fluctuation, it will evolve towards
a different equilibrium. Turing suggested that such a reaction-
diffusion mechanism seemed to be the basis of morphogenesis in
living beings
11
(Turing, 1952).
The principle of this mechanism is easy to understand. Take two
biochemical compounds, an activator (
Ac
) and an inhibitor (
I
).
Ac
activates its own synthesis and also that of
I
.
I
inhibits the synthe-
sis of
Ac
.
Ac
is diffused a great deal more slowly than
I
. Initially,
they are present along the axis of an embryo at a constant concen-
tration. This system is in equilibrium because in all respects the
activator effect of the molecules of
Ac
is compensated by the
inhibitor effect of the molecules of
I
(Fig. 3A). If, at a point
k
,
11
Numerous analogous systems have been described today. They are known as
metastable (or multi-stationary) systems.
