560
chapter 24
DNA Replication, Repair, and Mutagenesis
A
BU
A
G
T
BU
/
\
A
G
T
C
G
BU
G
1
A
+
C
BU
/
\
(a)
Error in replication: A • T —»
G • C
(b)
Error in incorporation: G ♦ C —* A «
T
F I G U R E 2 4 - 1 6
Two mechanisms of 5-bromouracil (BU)-induced mutagenesis, (a) During
replication, BU in its usual keto form substitutes for T, and the replica of an
initial A-T pair becomes an A-BU pair. In the first mutagenic round of
replication, BU in its rare enol form pairs with G. In the next round of
replication, the G pairs with a C, completing the transition from an A-T
pair to a G-C pair, (b) During replication of a G-C pair, a BU in its rare
enol form pairs with a G. In the next round of replication, the BU is again
in the common keto form and it pairs with A, so the initial G-C pair
becomes all A-T pair. The replica of the A-BU pair produced in the next
round of replication is another A-T pair.
40-42°C), the mutation is called
a temperature-sensitive
(Ts) mutation.
Sometimes no amino acid corresponds to a
new base sequence (Chapter 25); in that case, termination
of synthesis of the protein occurs at that point, and the
mutation is called a
chain termination mutation
or
non-
sense mutation.
These mutations generate one of the three
nonsense codons: UAA, UAG, or UGA (Chapter 25).
Replication errors can be introduced by base analogues
that can be added to a replicating DNA molecule. Such
a base analogue must be able to pair with the base on
the complementary strand being copied, or the 3' —>• 5'
editing function would remove it. However, if it can tau-
tomerize (Chapter 23), or if it has two modes of hydro-
gen bonding, it will be mutagenic. The substituted base
5-bromouracil (BU) is an analogue of thymine inasmuch
as the bromine atom has about the same van der Waals
radius as the methyl group of thymine. BU forms a nu-
cleoside triphosphate, and DNA polymerases will add BU
to a DNA strand opposite an adenine. However, BU tau-
tomerizes at high frequency to a form that can pair with
guanine. Thus, if BU replaces a thymine, in subsequent
rounds of replication it occasionally generates a guanine
in the complementary strand, which in turn specifies cy-
tosine, resulting in formation of a GC pair (Figure 24-16).
A chemical mutagen is a substance that can alter a base
that has already been incorporated into DNA and thereby
change its hydrogen bonding specificity.
Nitrous acid,
a
A - T
A - T
G - C
A c rid in e
1
-------- 1
T • A
G • C
C • G
T • A
C • G
T y r
G lu
T h r
G ly
lie
T A C G A A T C G G G T A T T
A T G C T T A G C C C A T A A
R e p lic a tio n in t h e
p r e s e n c e o f a n a c r id in e
T A C G A G A T C G G G T A T T
A T G C T C T A G C C C A T A A
(b )
T y r
G lu
lie
Gly
Tyr
F I G U R E 2 4 - 1 7
Mutagenesis by intercalating substances (e.g., acridine), (a) Separation
of two base pairs (shown as a box) by an intercalating agent, (b) A base
addition resulting from replication in the presence of an acridine. The
change in amino acid sequence read from the upper strand in groups of
three bases is also shown.
powerful mutagen, converts amino groups to keto groups
by oxidative deamination. Thus, cytosine, adenine, and
guanine are converted to uracil (U), hypoxanthine (H), and
xanthine (X), respectively, which can form the base pairs
UA, HC, and XC, respectively. As a consequence, in a later
round of replication, GC-to-AT and AT-to-GC mutations
can occur. The chemical mutagen
hydroxylamine
reacts
specifically with cytosine and converts it to a modified base
that pairs only with adenine; thus, a GC pair ultimately
becomes an AT pair.
An interesting class of mutagen are intercalating agents,
e.g., the acridines. These planar molecules insert between
base pairs. When replication occurs in the region of an
intercalated molecule, one or both daughter strands are
synthesized that either lack one or more nucleotides or
have additional ones. These changes alter the reading
frame of the base sequence of a gene and hence are called
frameshift mutations
(Figure 24-17).
Triplet Repeats and Fragile Sites
Inherited and acquired (sporadic) mutations account for an
enormous diversity of human diseases and disorders. At
least 300 genetic disorders include mental retardation as
one of the symptoms, implying that many genes can affect
brain development and mental functions. In the 1970s,
a defect at the tip of the long arm of X chromosomes,
called a
fragile site,
was discovered in a large number
of patients with mental retardation. It is now known that
a specific mutation is responsible for the fragile site on
