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CASE FILES: BIOCHEMISTRY
Figure 10-4. Schematic representation of nucleosomes showing DNA
wrapped around a core of histones H2A, H2B, H3, and H4. Histone H1 asso-
ciates with the linker region of DNA.
Composed of DNA and histones, nucleosomes are approximately 10 nm
in diameter and are primary structural units of chromatin. Between every two
nucleosomes there is a stretch of DNA, the
linker
region, which varies in
length from 15 to over 55 bp. Histone H1 can associate with the linker region
to aid in folding of DNA into more complex chromatin structures. The ques-
tion of how the highly ordered nucleosome is formed can be explained by the
fact that nucleosome assembly is facilitated by molecular chaperones. In the
presence of nucleoplasmin (an acidic protein) and DNA topoisomerase I
(nicking-closing enzyme) the nucleosome assembly proceeds rapidly without
histone precipitation. Nucleoplasmin binds to histones but not to DNA or
nucleosomes. It functions as a molecular chaperone to bring histones and
DNA together in a controlled fashion and prevents their nonspecific aggrega-
tion through their otherwise strong electrostatic interactions. The nicking-
closing enzyme acts to provide the nucleosome with its preferred level of
supercoiling. Six nucleosomes are packed per turn further into a spiral or sole-
noid to form a regular array of
polynucleosomes
of 30 nm length.
As a 30-nm fiber, the typical human chromosome would be 0.1 cm in length
and could span the nucleus more than 100 times. Clearly, there must be a still
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