BIOINORGANIC CHEMISTRY A Short Course Second Edition

NUCLEIC ACIDS 49

Base a Nucleoside Nucleotide Example Sugar

NH

HN

O

O

2

4

1

5

Thymine, T
5-methyl uracil

Deoxythymidine

(DNA)

Deoxythymidine 5 ′ -

diphosphate (5 ′ - dTDP)

O

OH H

H H

H H

NH

N

O

PO O

O

O–

• OP

O

O–

O

β - D - 2 -

Deoxyriboseb

in DNA

NH

NH

O

(^1) O
2
5 4
Uracil, U
Uridine Uridine 3 ′ - monophosphate
(3′ - UMP)
O
OH
H H
H H
HN
N
O
O
O
O P
O–
HO
O–
β - D - Ribose c in
RNA
a Arrows indicate common metal - binding sites
b H at 2 ′ position
c OH at 2 ′ position
TABLE 2.2 Continued
One type of hydrogen bonding is shown in Figure 2.14. Watson – Crick pairing
is shown; however, other hydrogen bonding conformations are found. Some
of these are known as Hoogsteen pairs, reversed Hoogsteen pairs, and reversed
Watson – Crick pairs. Note that in Watson – Crick base pairing A – T pairs form
two hydrogen bonds while C – G pairs form three. RNA usually exists in the
single - stranded (ss) form but may fold into secondary and tertiary structures
through the formation of specifi c base pairings (A – U and C – G). Double -
stranded RNA is also known.
Double - stranded DNA exhibits complementarity in forming the double
helix. The complementary sequences have opposite polarity; that is, the two
chains run in opposite directions as in the following illustration:
53...′′ATCCGAGTG ...
35...′′TAGGCTCAC ...