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NH 2

Purines

1

2

5

4

6

3

CH

C

N CNC

N N

HC

adenine guanine cytosine

Pyrimidines

7

9

8

H

H 2 N

CH

NH 2

3

2

5

6

4

(^1) CH
N C CH
N
C
H
O
uracil
(RNA only)
O
CH
HN C CH
N
C
H
O
thymine
(DNA only)
O
CH
HN C C
N
C
H
O
CH 3
O
C
HN CNC
N N
C
H
Pentoses Nucleosides Nucleotides
5 
4  1 
3  2 
O OH
OH OH
HO CH 2
ribose
O OH
OH
HO CH 2
deoxyribose
OH
O
OH
HO CH 2 O
OH
HO CH 2
OH
O
OH
O CH 2
adenosine deoxyadenosine
adenosine
monophosphate
NH 2
CH
C
N CNC
HC N N
NH 2
CH
C
N CNC
HC N N
NH 2
CH
C
N CNC
HC N N
PO
O
O
Fig. 5.1Structure of bases, nucleosides and nucleotides.
OH
O
O
O CH 2
Phosphodiester
linkage
PO
O
O
5  Base
OH
O
O
CH 2
P
O
O
Base
(5end)
O
OH
O
O
CH 2
P
O
O
O Base
(3end)
Repeated many times
to give polynucleotide
3 
Fig. 5.2Polynucleotide structure.
140 Molecular biology, bioinformatics and basic techniques