section 23.1
Components of Nucleic Acids
523
C:
I
N.
y
D e o x y rib o s e
C:
N .
y
D e o x y rib o s e
N — H - -
-0
C H ,
S=N \
/
/
3
C —
c
c
— c
^ J /
/
\
c
N - - - H —
N
c —
\
/
\
/
N =
c
C -— N
V
/ /
\
H
0
D e o
A d e n in e
T h y m in e
H
/
0 - - - H —
N
H
^ N \
/
\
/
C —
c
C -— C
\
/
c
N — H - - -N
c —
\
V
\
/
G u a n in e
H'
\
/ /
N — H - - - 0
/
\
D e o x y rib o s e
C y to s in e
F I G U R E 2 3 -3
The normal base pairs in DNA. Adenine in one polynucleotide chain pairs
with thymine in the complementary chain; guanine pairs with cytosine.
A-T base pairs are joined by two hydrogen bonds; G-C base pairs are
joined by three hydrogen bonds.
DNA which require that A pair with T and G pair with
C in the double-stranded DNA molecule (Figure 23-3).
Because of the base pairing rules it follows that [purines] =
[pyrimidines] in all DNA molecules.
The overall base composition of DNA varies consider-
ably among organisms.
Base composition
is expressed as
the fraction of all bases in DNA that are GC pairs divided
by the total number of base pairs, such as ([G] + [C])/[all
bases]. This fraction is termed the GC content or percent
GC. For human beings and other primates, the value of
the GC content is approximately 0.5. For lower organisms
the value can vary widely; the most extreme variation is
found in bacteria where the GC content varies from 0.27
to 0.76 from one genus to another;
E. co li
DNA has a
GC value of 0.5, which may reflect its close association
and evolutionary history with human beings. The higher
the GC content of DNA, the more stable is the double-
stranded helical molecule. This is because GC base pairs
share three hydrogen bonds whereas AT base pairs share
only two (Figure 23-3).
Tautomerization of Bases
Although the bases are chemically quite stable, certain
hydrogen atoms bound to the bases are able to undergo
tautomerization
in which they change their locations on
the bases (Figure 23-4). The preferred tautomeric forms of
adenine and cytosine are the
amino
configurations, how-
ever, with low probability each can assume the
imino
con-
figuration. The preferred tautomeric forms of guanine and
cytosine are the
keto
configuration. However, again with
low probability, each can assume the
enol
configuration.
A d e n in e
G u a n in e
A m in o
Im in o
K e to
En ol
F I G U R E 2 3 -4
Tautomerization of bases in DNA. The most stable forms of adenine and cytosine are the amino conformations. With
low probability these bases can tautomerize into the imino form; if this occurs during replication, an incorrect base pair
(a point mutation) may result. The stable forms of guanine and thymine are the keto conformations; the enol
conformations also can result in mistakes in base pairing during replication.
