Biology Reference
In-Depth Information
moment is not the product of its molecular state. Both depend on
their previous state and on the randomness of the interactions
between the molecules which make up the cell. Taking the history
of the structure and the random character of molecular interactions
into account in the causal explanation, to the detriment of a purely
deterministic explanation based on levels of organisation and
founded on stereospecificity, frees us from the contradiction of
genetic determinism.
This outline is obviously an extreme simplification but it illus-
trates the general principle of ontophylogenesis: evolution over time
of the structure is the continuation of its ontogenesis which is never
completed. Ontogenesis and phylogenesis are but one process
(Kupiec, 1986, 1997).
From this general principle we can detail three factors which
have an influence on the ontophylogenesis of a living organism:
DNA, the environment and the past.
1) DNA influences the probabilities of protein interaction
Proteins are subjected to thermal agitation and are moved by ran-
dom Brownian motion. The probability of their encountering each
other depends on their concentrations and their diffusion coefficients
in the medium they are in inside cells. The more numerous they are
and the more rapidly they diffuse, the more probable it is that they
will encounter each other. Once achieved, an association between
proteins is more or less stable depending on the strength of their
bonds. These parameters are directly influenced in a cell by the
DNA, its nucleotide sequence determining the amino acid sequence
of the proteins, which itself influences their own binding and diffu-
sion properties. In the same way, the concentration of proteins
depends on the level of gene expression, which in turn depends on
the structure of the genome.
2) The environment stabilises or selects certain cells rather than
others
The strength of the bonds between molecules does not indeed
depend solely on their intrinsic properties but also on the chemical
