Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

210 Chapter 4: DNA, Chromosomes, and Genomes
X chromosome
of the genome (see Figure 4–40). Such regions, where all of the chromatin has
a similar structure, are separated from neighboring domains by barrier proteins
(see Figure 4–41). At low resolution, the interphase chromosome can therefore
be considered as a mosaic of chromatin structures, each containing particular
nucleosome modifications associated MBoC6 m4.58/4.49
with a particular set of non-histone proteins.
Polytene chromosomes allow us to see details of this mosaic of domains in
the light microscope, as well as to observe some of the changes associated with
gene expression.
There Are Multiple Forms of Chromatin
By staining Drosophila polytene chromosomes with antibodies, or by using a
more recent technique called ChIP (chromatin immunoprecipitation) analysis
(see Chapter 8), the locations of the histone and non-histone proteins in chromatin
can be mapped across the entire DNA sequence of an organism’s genome.
Such an analysis in Drosophila has thus far localized more than 50 different chromatin
proteins and histone modifications. The results suggest that three major
types of repressive chromatin predominate in this organism, along with two major
types of chromatin on actively transcribed genes, and that each type is associated
with a different complex of non-histone proteins. Thus, classical heterochromatin
contains more than six such proteins, including heterochromatin protein 1 (HP1),
(A)
right arm of
chromosome 3
normal mitotic
chromosomes at
same scale
region
where two
homologous
chromosomes
are separated
2 µm
interbands
chromosome 4
chromocenter
(B)
right arm of
chromosome 2
left arm of
chromosome 2
left arm of
chromosome 3 20 µm
1 µm
bands
Figure 4–50 The entire set of polytene
chromosomes in one Drosophila salivary
cell. In this drawing of a light micrograph,
the giant chromosomes have been
spread out for viewing by squashing them
against a microscope slide. Drosophila
has four chromosomes, and there are four
different chromosome pairs present. But
each chromosome is tightly paired with
its homolog (so that each pair appears
as a single structure), which is not true
in most nuclei (except in meiosis). Each
chromosome has undergone multiple
rounds of replication, and the homologs
and all their duplicates have remained in
exact register with each other, resulting
in huge chromatin cables many DNA
strands thick.
The four polytene chromosomes
are normally linked together by
heterochromatic regions near their
centromeres that aggregate to create
a single large chromocenter (pink
region). In this preparation, however, the
chromocenter has been split into two
halves by the squashing procedure used.
(Adapted from T.S. Painter, J. Hered.
25:465–476, 1934. With permission from
Oxford University Press.)
Figure 4–51 Micrographs of polytene
chromosomes from Drosophila salivary
glands. (A) Light micrograph of a portion of
a chromosome. The DNA has been stained
with a fluorescent dye, but a reverse image
is presented here that renders the DNA
black rather than white; the bands are
clearly seen to be regions of increased
DNA concentration. This chromosome
has been processed by a high-pressure
treatment so as to show its distinct pattern
of bands and interbands more clearly.
(B) An electron micrograph of a small
section of a Drosophila polytene
chromosome seen in thin section. Bands
of very different thickness can be readily
distinguished, separated by interbands,
which contain less condensed chromatin.
(A, adapted from D.V. Novikov, I. Kireev
and A.S. Belmont, Nat. Methods 4:483–
485, 2007. With permission from
Macmillan Publishers Ltd; B, courtesy
of Veikko Sorsa.)