Essential Cell Biology 5th edition

216 CHAPTER 6 DNA Replication and Repair
Figure 6–24 Depurination and
deamination are the most frequent
chemical reactions known to create
serious DNA damage in cells.
(A) Depurination can remove guanine
(or adenine) from DNA. (B) The major
type of deamination reaction converts
cytosine to uracil, which, as we have seen,
is not normally found in DNA. However,
deamination can occur on other bases as
well. Both depurination and deamination
take place on double-helical DNA,
and neither break the phosphodiester
backbone.
(A) DEPURINATION
P
O
N
N
H
N N
O guanine
DNA strand
H
N
H
H
H
H 2 O
O
N
N N
H
N
H
N
H
H
P
O
DNA strand
OH
sugar phosphate
after depurination
(B)
DEAMINATION
cytosine
H H
N
H
N
H 2 O
H
uracil
O
N
H
H
N
O
H
N
O
P
O
NH 3
P
O
DNA strand
DNA strand
These are only a few of many chemical changes that can occur in our
DNA. Others are caused by reactive chemicals produced as a normal part
of cell metabolism. If left unrepaired, DNA damage leads either to the
substitution of one nucleotide pair for another as a result of incorrect
base-pairing during replication (Figure 6–26A) or to deletion of one or
more nucleotide pairs in the daughter DNA strand after DNA replication
(Figure 6–26B). Some types of DNA damage (thymine dimers, for example)
can stall the DNA replication machinery at the site of the
ECB5 e6.23-6.24
damage.
In addition to this chemical damage, DNA can also be altered by replication
itself. The replication machinery that copies the DNA can—albeit
rarely—incorporate an incorrect nucleotide that it fails to correct via
proofreading (see Figure 6–14).
For each of these forms of DNA damage, cells possess a mechanism for
repair, as we discuss next.
P
O
P
O
DNA strand
O
N
O
N
thymine
H
C N
C
H
C
C
H
C
H
N
C
thymine
C O
UV radiation
CH 3
C O
CH 3
P
O
P
O
DNA strand
O
N
O
N
H
C N
C C
H
C N H
C C
H
thymine dimer
C O
CH 3
C O
CH 3
Figure 6–25 The ultraviolet radiation in sunlight can cause the formation of thymine
dimers. Two adjacent thymine bases have become covalently attached to each other to
form a thymine dimer. Skin cells that are exposed to sunlight are especially susceptible to
this type of DNA damage.
ECB5 e6.24/6.25