127
CHAPTER FOUR
Cellular Metabolism
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FIGURE 4.18
DNA is double-stranded, consisting of two
polynucleotide chains. Hydrogen bonds (dotted lines) hold the
nitrogenous bases of one strand to their partners on the other strand.
The sugars point in opposite directions—that is, the strands are
antiparallel.
single organic ring structure. A binds to T and G binds to
C—that is, a purine always binds to a pyrimidine, and this
is what establishes the constant width of the DNA molecule.
These pairs—A with T, and G with C—are called
comple-
mentary base pairs
(f
g. 4.19
a
)
. The sequence of one DNA
strand can always be derived from the other by following
the “base-pairing rules.” If the sequence of one strand of
the DNA molecule is G, A, C, T, then the complementary
strand’s sequence is C, T, G, A.
The double-stranded DNA molecule twists, forming a
double helix, (F g. 4.19
b
). The human genome is 3.2 billion
DNA bases long, dispersed over the 24 types of chromosomes.
A single gene may be thousands or even millions of bases
long. In the nucleus, DNA is wound around octets of proteins
called
histones
to form chromatin (F g. 4.19
b
). Histones and
other molecules come on and off different parts of the genome
as some genes are accessed for their information to make pro-
teins and others are silenced. During mitosis chromatin con-
denses to form chromosomes visible under the microscope
(F g. 4.19
c
). Investigators can use DNA sequences to identify
individuals (±rom Science to Technology 4.1).
Appendix D,
pages 948–949,
has more detailed DNA structures.
DNA Replication
When a cell divides, each newly formed cell must have a
copy of the original cell’s genetic information (DNA) so it
will be able to synthesize the proteins necessary to build cel-
lular parts and metabolize. DNA
replication
(re
plı˘-ka
shun)
is the process that creates an exact copy of a DNA molecule.
It happens during interphase of the cell cycle.
RECONNECT
To Chapter 3, The Cell Cycle, page 100.
As DNA replication begins, hydrogen bonds break
between the complementary base pairs of the double strands.
Then the strands unwind and separate, exposing unpaired
bases. New nucleotides pair with the exposed bases, forming
hydrogen bonds. An enzyme, DNA polymerase, catalyzes
this base pairing. Enzymes then knit together the new sugar-
phosphate backbone. In this way, a new strand of comple-
mentary nucleotides extends along each of the old (original)
strands. Two complete DNA molecules result, each with one
new and one original strand
(f
g. 4.20)
. During mitosis, the
two DNA molecules that form the two chromatids of each
of the chromosomes separate so that one of these DNA mol-
ecules passes to each of the new cells.
±rom Science to Technology 4.2 discusses the polymerase
chain reaction (PCR), a method for mass-producing, or ampli-
fying, DNA. PCR has revolutionized biomedical science.
PRACTICE
21
What is the function of DNA?
22
What is the structure of DNA?
23
How does DNA replicate?
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FIGURE 4.17
A polynucleotide chain consists of nucleotides
connected by a sugar-phosphate backbone.
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