Biology Reference
In-Depth Information
A
G2
G1
G1 is activated
B
G1
G2
translocation of the regulator
C
G2
G1
G2 is activated
(adapted from Kupiec, 1996)
F
IGURE
31. Random folding of DNA.
DNA movements are themselves subject to
thermal agitation. They can therefore induce the translocation of proteins between
different genes
(B)
and, thus, their random expression
(C)
. The functioning logic
of the stochastic model of expression remains unchanged because the probabilities
of interactions between the DNA sites (translocation) depend on the relative posi-
tions of the latter.
potential distributions. This condition is readily realised owing to
the non-specificity of the interactions between molecules in the
chromatin. We have already mentioned the examples of the protein
MeCP2 and the transcription factors encoded by homeogenes
(chapter 4, ยง4.1.4).
Moreover, chromatin is organised in a three-dimensional rather
than a linear structure as in our example, but that does not alter
the model's functioning principle. In a real cell, the position of the
genes in the three-dimensional nuclear space will still determine
their chronological sequence and the probability of their being
expressed in the same way as it would do on a linear chromosome.
Although the gene expression probabilities are, in this case, no
