408
Cell Nucleus Biogenesis, Structure and Function
3.6
Chromosome Territories
It has been known for many years that
chromosomes occupy discrete regions
within interphase nuclei. Over the past
20 years, the idea that interphase chro-
mosomes occupy discrete nuclear ‘‘territo-
ries’’ has been conFrmed using a number
of techniques. A major advance in the anal-
ysis in nuclear architecture came from
the development of fluorescence in situ
hybridization (±ISH) techniques for ‘‘chro-
mosome painting.’’ This process uses the
unique sequences from puriFed chromo-
some
sa
sp
robe
stoiden
t
i
fytheloca
t
ion
of particular chromosomes in Fxed nu-
clei. The probe DNA is tagged with a
modiFed DNA precursor analog – such as
biotin-dUTP – prior to hybridizing to the
Fxed cell (with denatured target DNA)
and is subsequently visualized using anti-
bodies to biotin and secondary antibodies
coupled to a fluorochrome that can be
visualized by light microscopy. This tech-
nique conFrmed that chromosome terri-
tories from mammalian cells appear to
have distinct borders with little mixing
of chromatin from adjacent territories.
The territories themselves appear to be
subcompartmentalized so that the space
occupied by individual chromosome arms
and even R and G bands within these
arms might be deFned within individ-
ual territories. The general principle of
chromosome territory organization can be
conFrmed using a variety of DNA label-
ing techniques, using precursors that can
be visualized in both Fxed and living cells
(±ig. 7).
The existence of chromosome territories
begs the question: do interphase chromo-
somes occupy speciFc nuclear locations
that could influence chromatin function?
Theanswertothisappearstobepartlyyes
and partly no. Territories do not generally
occupy speciFc positions. However, gene
activity does appear to influence chromo-
some location, such that chromosomes
with a high density of active genes tend
to be located toward the nuclear center,
whereas those with a low density tend
to lie closer to the nuclear periphery.
The smaller chromosomes also show a
tendency to occupy the nuclear center;
note that for those chromosomes with ri-
bosomal RNA gene clusters, location is
determined by their association with nu-
cleoli, which generally tend to be situated
centrally in mammalian nuclei. Studies on
cell populations show that individual chro-
mosomes do not assume precise locations.
However, when an individual mitotic event
is followed, the arrangement of chromo-
somes in the two daughter cells is very
similar to that of the mother nucleus im-
mediately prior to mitosis. This implies
that the mechanics of mitosis incorporate
a mechanism for maintaining chromo-
some position.
So interphase chromosomes occupy dis-
crete territories with preferred locations
that can be maintained during cell divi-
sion. However, this should not be taken
to conFrm that chromosomes are static
structures. In fact, this is far from the
truth; locally, the chromatin within each
chromosome territory can be very dy-
namic. Moreover, while the vast majority of
DNA from an individual territory remains
within the deFned zone, there are in-
stances where chromatin escapes from its
territory to form loops that might spread 1
to 2
µ
m into the surrounding nucleoplasm.
These loops will generally contain a Mbp
or more of DNA and commonly have an ex-
truded or elongated appearance suggesting
that they might occupy the interchromatin
space that lies between adjacent territo-
ries. Chromatin loops of this sort are
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