54
UNIT 1
Organization of the Body
2
molecule is coiled into a spiral staircase–like structure called
a
double helix
.
Bonding of the bases is very specific: A always bonds to T,
and G always bonds to C. A and T are therefore called
comple-
mentary bases
, as are C and G. According to these base-pairing
rules, ATGA on one DNA nucleotide strand would necessarily
be bonded to TACT (a complementary base sequence) on the
other strand.
DNA is a long, double-stranded polymer—a double chain
of nucleotides (Figure 2.22b and c). ±e bases in DNA are A,
G, C, and T, and its pentose sugar is
deoxyribose
(as reflected
in its name). Its two nucleotide chains are held together by
hydrogen bonds between the bases, so that a ladderlike mol-
ecule is formed. Alternating sugar and phosphate compo-
nents of each chain form the
backbones
or “uprights” of the
“ladder,” and the joined bases form the “rungs.” ±e whole
C
T
C
T
P
P
(b)
Deoxyribose
sugar
Phosphate
Sugar-phosphate
backbone
(a)
Adenine nucleotide
Hydrogen
bond
Thymine nucleotide
Phosphate
Sugar:
Deoxyribose
Phosphate
Sugar
Thymine (T)
Base:
Adenine (A)
H
N
H
H
O
O
H
N
N
H
N
N
H
O
O
O
O
CH
2
CH
3
OH
H
H
H
H
H
O
O
O
O
O
H
2
C
OH
H
H
H
H
H
N
N
O
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
(c)
A
A
G
A
T
T
T
C
G
C
G
C
A
A
G
G
A
Figure 2.22
Structure of DNA. (a)
The unit of DNA is the nucleotide, which is composed of a deoxyribose sugar
molecule linked to a phosphate group, with a base attached to the sugar. Two nucleotides, linked by hydrogen
bonds between their complementary bases, are illustrated.
(b)
DNA is a coiled double polymer of nucleotides
(a double helix). The backbones of the ladderlike molecule are formed by alternating sugar and phosphate units.
The rungs are formed by the binding together of complementary bases (A-T and G-C) by hydrogen bonds
(shown by dotted lines).
(c)
Computer-generated image of a DNA molecule.
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