128
UNIT ONE
G
C
G
A
T
G
A
T
C
C
T
A
A
C
G
C
T
T
G
A
G
C
G
C
A
P
GC
P
T
P
A
P
C
G
P
G
P
C
P
A
P
T
P
P
Thymine
(T)
Cytosine
(C)
Adenine
(A)
Guanine
(G)
Hydrogen
bonds
Nucleotide strand
Globular
histone
proteins
Metaphase
chromosome
Segment
of DNA
molecule
Chromatin
(a)
(b)
(c)
A
T
FIGURE 4.19
DNA and chromosome
structure. (
a
) The two polynucleotide
chains of a DNA molecule point in
opposite directions (antiparallel) and
are held together by hydrogen bonds
between complementary base pairs.
(
b
) The molecular “ladder” of a DNA
molecule twists into a double helix.
(
c
) Histone proteins enable the long
double helix to assume a compact form
(chromosome) and move when sections
of the DNA are accessed for
gene expression.
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