Biology Reference
In-Depth Information
(a) Deoxynucleotide 5
′
-triphosphate
(b) Deoxyribose
O
O
O
O
P
O
O
P
O
O
P
O
CH
2
C5
Base
HOH
2
C
O
O
OH
C1
H
H
C4
H
H
H
H
H
H
HO
H
C2
C3
HO
H
(c) Nitrogenous bases
O
N
N
O
N
N
N
N
N
N
N
N
N
O
N
N
N
N
O
Adenine (A)
Cytosine (C)
Guanine (G)
Thymine (T)
Figure 2.1
The DNA molecule is built up of deoxynucleotide 5' triphosphates (a). The sugar (b)
contains five carbon atoms (labelled C1-C5); one of four different types of nitrogenous base (c) is
attached to the 1' carbon, a hydroxyl group to the 3' carbon and the phosphate group to the
5' carbon. Adenine and guanine both have a double ring and are purines, whereas cytosine and
thymine have a single ring and are pyrimidines
5
′
O
P
O
O
O
5
′
3
′
O
H
2
C
H
2
C
N
O
O
O
N
G
C
Guanine
O
N
N
H
2
C
O
P
O
O
N
O
P
O
P
O
O
O
O
O
O
1 bp
O
H
2
C
H
2
C
N
O
N
O
N
A
T
Adenine
O
N
N
H
2
C
O
P
O
O
O
O
P
O
P
O
O
O
O
O
O
H
2
C
H
2
C
N
O
O
N
Cytosine
C
G
N
O
O
H
2
C
O
O
O
3
′
5
′
3
′
(a) (b)
Figure 2.2
Nucleotides are joined together by phosphodiester bonds to form a single-stranded
molecule (a). The DNA molecule in the cell is double-stranded (b) with two complementary single-
stranded molecules held together by hydrogen bonds. Adenine and thymine form two hydrogen
bonds while guanine and cytosine form three bonds
