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

CHROMOSOMAL DNA AND ITS PACKAGING IN THE CHROMATIN FIBER
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(B)
10 µm
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frequently done at the stage in the cell cycle called mitosis, when chromosomes
are especially compacted and easy to visualize (see below).
Another more traditional way to distinguish one chromosome from another
is to stain them with dyes that reveal a striking and reproducible pattern of bands
along each mitotic chromosome (Figure 4–11). These banding patterns presumably
reflect variations in chromatin structure, but their basis is not well understood.
Nevertheless, the pattern of bands on each type of chromosome is unique,
and it provided the initial means MBoC6 to n4.444/4.10 identify and number each human chromosome
reliably.
17
X
18
X
Figure 4–10 The complete set of human
chromosomes. These chromosomes,
from a female, were isolated from a cell
undergoing nuclear division (mitosis)
and are therefore highly compacted.
Each chromosome has been “painted” a
different color to permit its unambiguous
identification under the fluorescence
microscope, using a technique called
“spectral karyotyping.” Chromosome
painting can be performed by exposing
the chromosomes to a large collection of
DNA molecules whose sequence matches
known DNA sequences from the human
genome. The set of sequences matching
each chromosome is coupled to a different
combination of fluorescent dyes. DNA
molecules derived from chromosome 1 are
labeled with one specific dye combination,
those from chromosome 2 with another,
and so on. Because the labeled DNA can
form base pairs, or hybridize, only to the
chromosome from which it was derived,
each chromosome becomes labeled
with a different combination of dyes. For
such experiments, the chromosomes are
subjected to treatments that separate
the two strands of double-helical DNA in
a way that permits base-pairing with the
single-stranded labeled DNA, but keeps
the overall chromosome structure relatively
intact. (A) The chromosomes visualized as
they originally spilled from the lysed cell.
(B) The same chromosomes artificially
lined up in their numerical order. This
arrangement of the full chromosome
set is called a karyotype. (Adapted from
N. McNeil and T. Ried, Expert Rev. Mol.
Med. 2:1–14, 2000. With permission from
Cambridge University Press.)
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50 million
nucleotide pairs
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1 µm
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Figure 4–11 The banding patterns of
human chromosomes. Chromosomes
1–22 are numbered in approximate order
of size. A typical human cell contains two
of each of these chromosomes, plus two
sex chromosomes—two X chromosomes
in a female, one X and one Y chromosome
in a male. The chromosomes used to
make these maps were stained at an early
stage in mitosis, when the chromosomes
are incompletely compacted. The
horizontal red line represents the position
of the centromere (see Figure 4–19),
which appears as a constriction on
mitotic chromosomes. The red knobs on
chromosomes 13, 14, 15, 21, and 22
indicate the positions of genes that code
for the large ribosomal RNAs (discussed
in Chapter 6). These banding patterns are
obtained by staining chromosomes with
Giemsa stain, and they can be observed
under the light microscope. (Adapted from
U. Francke, Cytogenet. Cell Genet. 31:24–
32, 1981. With permission from the author.)