Biology Reference
In-Depth Information
Tim Mitchison. Their conception is based on the distinction
between self-assembly and self-organisation (Kirschner
et al
., 2000).
We have seen that self-assembly is a process based on stereospecificity
(chapter 3, ยง3.2). It leads to the formation of unique structures in
stable equilibrium which do not require the exchange with the envi-
ronment of any matter or energy. In contrast, self-organisation
would imply consumption of energy and constant exchange of mat-
ter with the environment. In the first instance, it would produce
dynamic equilibria between several states, then, owing to con-
straints or amplified random fluctuations, instead of oscillating
between these states, the system would swing towards a specific
final state.
To illustrate their theory, Kirschner
et al
. provide several exam-
ples. Certain bacteria produce a polarised tail-like bundle of fila-
ments of actin proteins which helps them be propelled along. At the
start the bundle is symmetrical but is subject to random fluctua-
tions between which it oscillates. If one of them is too strong, it will
be amplified and cause polarisation of the bundle. Another exam-
ple concerns the differentiation of embryonic cells. The two essen-
tial elements for this process are signalling between cells and cell
competence. Cell competence means that embryonic cells have sev-
eral possible differentiation pathways at a given stage of their devel-
opment. For Kirschner
et al
. this is a question of equilibrium
between several interconvertible states, each state corresponding to
the potential initiation of one of the differentiation pathways.
Under the influence of signals from other cells, the equilibrium
would be biased in favour of one of the pathways, which would then
be selected.
Kirschner
et al
. insist on the dynamic aspects of ontogenesis and
have integrated the role of random fluctuations. In this respect they
go beyond conventional genetic determinism. However, like other
adherents of self-organisation, they do not challenge it head on.
They continue to accept the stereospecificity of interactions
between molecules and genetic information and integrate this in
their theory. They put forward a synthesis to explain how information
is transmitted in biological systems. It is more complicated than the
