Biology Reference
In-Depth Information
euchromatin
heterochromatin
x
+
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x
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F
IGURE
29. 'Variegated' gene expression.
Owing to the intrinsically random nature
of the interactions between proteins and DNA in the chromatin, the limit between
the euchromatin and the heterochromatin varies. Gene x, located in this region,
may be expressed or not depending whether it is in the euchromatin or the hete-
rochromatin. The two cases are produced randomly in each individual cell of a
tissue, leading to the 'mosaic' expression of this gene. Some cells in a tissue express
it, others do not.
types of chromatin and transform the one into the other. These
processes depend on competition between proteins promoting the
formation of heterochromatin and transcription factors promoting
euchromatin (Dillon and Festenstein, 2002). In each cell the result
of this competition is random because the competition itself
depends on the Brownian behaviour of molecules (Fig. 29).
However, while variegated expression illustrates a perfectly real
phenomenon of stochastic gene expression, the model of self-organ-
isation of chromatin suffers from the same defects as the general
theory (chapter 5). In the course of embryonic development, in the
huge majority of cases, cells do not form mosaics of differentiated
cells but homogeneous tissues. If cell differentiation depends on self-
organisation of chromatin, why this process leads to identical states
in whole tissues, while being different depending on the tissues,
needs to be explained. Stochastic expression explains why cells dif-
ferent one from the other are produced, but not the stabilisation of
cell types which is necessary to form homogeneous tissues.
Constraints must be exerted on stochastic expression in order to
