Biology Reference
In-Depth Information
different point in the cell, so that specific responses are thus gener-
ated (Kitano, 2002; Levchenko, 2003; Prill
et al
., 2005). Before
describing the mechanisms alleged to control the activity of the
networks, the following must be stated. Under the principle pro-
pounded by genetic determinism of order from order, the regulation
and macroscopic organisation of biological systems is supposed to be
explained by the structure of the protein networks, itself resulting
from molecular stereospecificity and genetic information. This is
however not the case. Regulation of the networks themselves must
now be put forward to explain their specificity.
4.3.2
Negating the principle of order from order
The mechanisms put forward to explain how a molecular network
generates specific behaviour despite the non-specificity of the pro-
teins which compose it have been the subject of in-depth descrip-
tions (Dumont
et al
., 2002; Schwartz and Madhani, 2004; Komarova
et al
., 2005). We shall look at the essential points here. We shall see
that these mechanisms, unanimously accepted by molecular biolo-
gists, reintroduce holism, yet this absolutely contradicts the princi-
ples of genetic determinism.
The sequestration of proteins consists of limiting contact between
proteins, in order to prevent certain interactions from occurring and
only let those that occur exert a supposed specific effect. It is itself
the result of several mutually non-exclusive mechanisms.
— Spatial compartmentalisation: proteins are not uniformly dis-
tributed in a cell. They are preferentially located in certain com-
partments such as the nucleus, the cytoplasm, the membranes or
other organelles. Compartmentalisation therefore prevents interac-
tions between physically separated molecules.
— Temporal separation: some proteins are not present at the same
time at the same place in the cell because they are not expressed
with the same kinetics. Their interacting is thus avoided.
— Micro-compartmentalisation: there exist proteins called 'scaffold'
proteins which bind to the various proteins of a single signalling
