Cell Nucleus Biogenesis, Structure and Function
421
Fig. 10
Chromatin folding. The DNA
within a mammalian nucleus is highly
folded. The required folding is achieved
Frst by wrapping DNA around the
surface of histone complexes to form
nucleosomes. Long stretches of
nucleosomes then fold to form the
higher-order chromatin structures. In
the model shown here, DNA foci – see
±igure 7 – are depicted as the critical
architectural units of higher-order
packaging. It is not certain how these
structures are formed, but one
possibility is that DNA loops are arrayed
as stable entities in a way that reflects
their functional status at sites of RNA
and DNA synthesis. In this example, a
cluster of chromatin loops is arrayed
around a transcription factory. Within
this type of higher-order structure,
chromatin might be either open, as
euchromatin or more condensed, as
heterochromatin. These chromatin
states correlate with various histone
modiFcations that either allow
chromatin to be transcribed or stimulate
local aggregation of the chromatin
clouds. (Reproduced from Cook, P.R.
(2001)
The Principles of Nuclear Structure
and Function
, John Wiley and Sons, New
York with permission of John Wiley
and Sons.)
Chromosome
domain in
nucleus
Chromatin cloud
around factory
Loop
attached
to factory
Zig-zagging
ribbon
Nucleosome
DNA duplex
about 0.5 copies per nucleosome core
in DNase sensitive euchromatin. The
solenoid represents the second level of
chromatin folding. This structure is not
thought to fold according to speciFc
design parameters, however, and is more
probably best imagined as a short-range
chromatin aggregate that is stabilized by
association with chromatin proteins such
as histone H1. ±urther condensation of
the chromatin leads to the formation of
condensed Fbers with diameters in the
range of 50 to 80 nm. These are typical
of the structures seen in heterochromatin
during the interphase and are stabilized by
association with heterochromatin proteins
such as HP1 and Pc-G proteins. The
most highly condensed chromatin Fbers
are found in mitotic chromosomes where
histone modiFcations that result from the
activity of the mitotic cdc/cyclin complexes
yield chromatin Fbers of 150 to 200 nm.
These are folded around the axis of
the chromatids, which have an overall
diameter of about 600 nm.
4.2.1
Chromatin Domains and Loops
Interactions that allow the stabilization
and modulation of chromatin status will
clearly have a profound impact on chro-
matin organization within the nuclei of
higher eukaryotes. However, these do
not seem to be the only aspects of
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