Chromosome Organization within the Nucleus
3
interaction
may
involve both
nuclear
lamins
and
nuclear
pore
complexes.
Specifc nuclear envelope attachments,
together with a persisting remnant oF
the anaphase chromosome confguration, leads to chromosomes being organized
into nonoverlapping territories with specifc orientations. Superimposed on this
organization is a high degree oF chromosome mobility driven by diFFusion. Because
chromosomes are constrained by nuclear envelope attachments, their diFFusion is
constrained, with each locus able to explore only a limited subregion oF the nucleus.
This constrained mobility, together with the nonrandom positional organization
oF chromosomes, predicts a high degree oF nonrandomness in the pattern oF
interchromosomal interactions.
1
Introduction
The genome is packaged within the nu-
cleus on many levels. Not only is the
DNA wound around nucleosomes and
compacted into chromatin, the chromo-
somes themselves are arranged within the
nucleus in a defned pattern. This nuclear
organization is set up and maintained by
interactions between chromosomes and
the nuclear envelope (NE) and possibly an
internal nuclear matrix. However, chro-
mosomes are not absolutely fxed, but are
able to undergo a certain degree oF con-
strained motion, which allows interactions
between chromosomes to take place. This
combination oF specifc positioning and
constrained movement leads to a dynamic
view oF nuclear organization in which
large-scale chromosome organization can
play a decisive role in regulating gene ex-
pression and chromosome interactions.
2
Chromosome Positioning
2.1
Analysis of Polytene Chromosomes
Determining the arrangement oF inter-
phase
chromosomes
is
a
challenging
problem, because during interphase the
chromosomes decondense and thus be-
come
diFfcult
to
visualize.
Early
dra-
matic evidence For nonrandom chromo-
some arrangement came From analysis oF
Drosophila
polytene chromosomes in three
dimensions. Polytene chromosomes are
large and compact and easy to image in-
side living cells. Moreover, the well-known
banding pattern oF polytene chromosomes
allowed the position oF each locus, de-
fned cytologically by banding pattern, to
be determined in three-dimensional space,
thus projecting the one-dimensional ge-
netic map onto a three-dimensional map oF
the nucleus. These studies were among the
frst studies ever to use three-dimensional
fluorescence microscopy. On the basis oF
this type oF analysis, it was Found that
the arrangement oF chromosomes was diF-
Ferent in diFFerent nuclei, indicating that
the position oF a locus was not rigidly
predetermined. But although the arrange-
ments were variable, they were not entirely
random either. ±or one thing, the chromo-
somes Followed a polarized orientation in
the nucleus, with centromeres at one end
oF the nucleus, and telomeres at the other.
Both the polarized arrangement oF chro-
mosomes and their separation into distinct
domains probably reflects a persistence oF
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