Essential Cell Biology 5th edition

186 CHAPTER 5 DNA and Chromosomes
linker DNA
“beads-on-a-string”
form of chromatin
NUCLEASE
DIGESTS
LINKER DNA
released
nucleosome
core particle
DISSOCIATION
WITH HIGH
CONCENTRATION
OF SALT
histone
octamer
core histones
of nucleosome
nucleosome includes
~200 nucleotide
pairs of DNA
11 nm
147-nucleotide-pair
DNA double helix
Figure 5–20 Nucleosomes contain DNA wrapped around a protein
core of eight histone molecules. In a test tube, the nucleosome core
particle can be released from chromatin by digestion of the linker
DNA with a nuclease, which cleaves the exposed linker DNA but not
the DNA wound tightly around the nucleosome core. When the DNA
around each isolated nucleosome core particle is released, its length is
found to be 147 nucleotide pairs; this DNA wraps around the histone
octamer that forms the nucleosome core nearly twice.
All four of the histones that make up the octamer are relatively small
proteins with a high proportion of positively charged amino acids (lysine
and arginine). The positive charges help the histones bind tightly to the
negatively charged sugar–phosphate backbone of DNA. These numerous
electrostatic interactions explain in part why DNA of virtually any
sequence can bind to a histone octamer. Each of the histones in the
octamer also has a long, unstructured N-terminal amino acid “tail” that
extends out from the nucleosome core particle (see the H3 tail in Figure
5–21). These histone tails are subject to several types of reversible, covalent
chemical modifications that control many aspects of chromatin
structure.
The histones that form the nucleosome core are among the most highly
conserved of all known eukaryotic proteins: there are only two differences
between the amino acid sequences of histone H4 from peas and
cows, for example. This extreme evolutionary conservation reflects the
vital role of histones in controlling eukaryotic chromosome structure.
DISSOCIATION
H2A H2B H3 H4
Chromosome Packing Occurs on Multiple Levels
Although long strings of nucleosomes form on most chromosomal DNA,
chromatin in the living cell rarely adopts the extended beads-on-a-string
form seen in Figure 5–19B. Instead, the nucleosomes are further packed
on top of one another to generate a more compact structure, such as
the chromatin fiber shown in Figure 5–19A and Movie 5.2. This additional
packing of nucleosomes into a chromatin fiber depends on a fifth
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an H3 histone tail
viewed
face-on
viewed
from the
edge
DNA double helix
histone H2A histone H2B histone H3 histone H4
Figure 5–21 The structure of the nucleosome core particle, as determined by x-ray diffraction analysis, reveals how DNA is
tightly wrapped around a disc-shaped histone octamer. Two views of a nucleosome core particle are shown here. The two strands
of the DNA double helix are shown in gray. A portion of an H3 histone tail (green) can be seen extending from the nucleosome core
particle, but the tails of the other histones have been truncated. (From K. Luger et al., Nature 389:251–260, 1997.)
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