Biology Reference
In-Depth Information
chromatin molecules and their DNA binding sites, and on the other,
the relative position of the genes in the DNA molecule. These
parameters determine the chronological gene expression sequences
which have the highest probability of occurring in a cell during
embryogenesis and which correspond to the different cell differenti-
ation pathways.
To illustrate these two principles, let us take the simplest exam-
ple possible. The egg cell of an embryo has a single extremely rudi-
mentary chromosome. It is made just of a single transcription
regulator protein
42
(
R
) and a linear DNA carrying three genes
G1, G2 and G3, positioned in this order with a distance d1 separat-
ing G1 and G2 which is greater than the distance d2 separating
G2 and G3 (Fig. 30).
When
R
binds to the regulator region of one of these three
genes, it activates it. In our egg cell,
R
is at G1 when embryogenesis
begins. It is the result of the differentiation of the germinal line in
the previous generation (Fig. 30A). G1 is activated as long as
R
is
bonded to its regulator region. When it dissociates from it, it moves
around randomly in the surrounding space (diffusion). Several
events may then possibly occur. It may re-associate with G1, or
escape from this space and, after translocation, bind with G2, or
with G3, activating them in turn. These three events are random
because the diffusion of
R
is itself a random phenomenon, but the
probability of each occurring is not equal. Once
R
has dissociated
from G1 (Fig. 30B), the probability of it re-associating with one of
the three genes depends on their respective positions, since it
explores the space randomly. This is a direct consequence of the laws
of diffusion. It is most probable that it will re-associate with G1,
therefore, and least probable that it will associate with G3, with the
probability of it associating with G2 being intermediate. If in a cell
the
R
molecule goes from G1 to G2 or G3, the same phenomenon is
42
Here we are giving a wide meaning to the concept of transcription regulator
that we are using. It means any molecule which influences gene expression includ-
ing those participating in giving the chromatin its structure.
