Biology Reference
In-Depth Information
Now, it has to be said that this raises a critical question. Gene
expression is the elementary stage of the genetic programme: how
is it possible for it to be a stochastic phenomenon while the genetic
programme is deterministic by definition? Two answers to this can
be envisaged. Either stochasticity is noise affecting the controlled
functioning of the genetic programme, in which case the cell must
have developed mechanisms to eliminate its potentially negative
effects, or normal gene expression is really an intrinsic stochastic
process and the cell exploits this stochasticity to bring about
its functions. In this case the idea of a deterministic genetic pro-
gramme would have to be abandoned. The latter option seems to
have been reinforced by a series of experiments, which have indeed
shown that it is the stochastic expression of genes that is at
the origin of cell differentiation in unicellular and multicellular
organisms (Becksei
et al
., 2001; Blake
et al
., 2003; Isaacs
et al
.,
2003; Wernet
et al
., 2006; Maamar
et al
., 2007). However, these
results require to be further generalised, above all to embryonic
development.
6.3
The Darwinian theory of cell differentiation
6.3.1
From differentiation to cell identification
In the instructive model, cells differentiate due to the influence of
signals they receive. The Darwinian model is based on quite differ-
ent logic. It takes as its starting point the non-specificity of proteins
which induces a number of stochastic molecular interactions, the
occurrence of any of which is probable.
In an individual cell, certain of all the interactions that are pos-
sible take place, but owing to their probabilistic character, the same
interactions do not occur in each cell of a cell population. Cell
diversity originates from this probabilistic process.
In the diagram shown in Fig. 20, the two cells A and A
are
identical. Two random molecular events may be produced there,
b
or
c
(each corresponding to non-specific interactions between at
least two molecules. For an example, see Fig. 21). These events
permit the stochastic expression of two different sets of genes and
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