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chapter 24
DNA Replication, Repair, and Mutagenesis
FIGURE 24-9
The basic structure of camptothecin. Analogues of camptothecin that are
used to treat various neoplastic diseases involve substitutions at positions
C-7, C-9, and C-l 1 of the basic molecule. All analogues as well as the
parent molecule bind to topoisomerase 1 and interfere with its functions
during DNA replication.
A variety of antibiotics
and
antineoplastic
drugs
function as topoisomerase I inhibitors; these include
the quinolone antibiotics, anthracyclines (doxorubicin),
epipodophylotoxins (etoposide), and the
camptothecins,
which are active in treating lung, ovarian, and colorectal
cancers. The camptothecins also are used in the treatment
of myelomonocytic syndromes,
chronic myelomono-
cytic leukemia (CMML),
acute leukemia, and
multiple
myeloma.
A variety of synthetic analogues of natural
camptothecins are being tested clinically for efficacy
and safety in the treatment of these aggressive cancers
(Figure 24-9).
The camptothecins were discovered in extracts from
the Chinese tree
Camptotheca acuminata.
Initial stud-
ies showed that camptothecins had antitumor activity, but
clinical trials demonstrated severe side effects and toxic-
ity. Numerous camptothecin analogues have been synthe-
sized; several have received FDA approval (irinotecan and
topotecan), whereas others are still in clinical trials.
24.4 Chromosome Replication
DNA in mammalian cells is organized in complex struc-
tures called chromosomes (prokaryotes do not have a nu-
cleus, do not divide by mitosis, and do not, strictly speak-
ing, have chromosomes). The DNA in the chromosomes of
human and other eukaryotic cells is intimately associated
with two classes of proteins called
histones
and
nonhis-
tones.
Collectively, DNA, histones, and nonhistones con-
stitute
chromatin,
from which the name chromosome is
derived. The DNA in a chromosome is an extremely long,
linear molecule that must be condensed and organized to
fit into the chromosomes in the nucleus. (The DNA in the
46 human chromosomes would be about 1 m long if fully
extended.) Histones are responsible for the structural orga-
nization of DNA in chromosomes; the nonhistone proteins
FIGURE 24-10
Structure of a nucleosome. DNA is looped around a core
of eight histone proteins (pairs of four different histone proteins) and
connected to adjacent nucleosomes by linker DNA and another
histone (HI).
regulate the functions of DNA including replication and
gene expression. The positive charge of histones, due to
the presence of numerous lysine and arginine residues, is
a major feature of the molecules, enabling them to bind
to the negatively charged phosphate groups in DNA. The
electrostatic attraction is an important stabilizing force in
chromatin. If chromosomes are placed in solutions of high
salt that break down electrostatic interactions, chromatin
dissociates into free histones and DNA. Chromatin also
can be reconstituted by mixing purified histones and DNA
in concentrated salt solutions and gradually removing the
salt by dialysis.
Histones share a similar primary structure among eukar-
yotic species. However, they undergo various posttransla-
tional modifications such as phosphorylation, acetylation,
m├ęthylation, and ADP ribosylation.The chemical modifi-
cations of histones can alter their net charge, shape, and
other properties affecting DNA binding.
Pairs of four different histones (H2A, H2B, H3, and H4)
combine to form an eight-protein bead around which DNA
is wound; this bead-like structure is called a
nucleosome
(Figure 24-10). A nucleosome has a diameter of 10 nm and
contains approximately 200 base pairs. Each nucleosome
is linked to an adjacent one by a short segment of DNA
(linker) and another histone (HI). The DNA in nucleo-
somes is further condensed by the formation of thicker
structures called
chromatin fibers,
and ultimately DNA
must be condensed to fit into the metaphase chromosome
that is observed at mitosis (Figure 24-11).
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