Nucleic Acids in Chemistry and Biology

8.10.2 O-Alkylated Lesions

O^6 -Alkylguanines are locked in the enol-tautomericform while guanine in DNA is normally in the keto-
form (Chapter 2.1.2). Base-mismatch and melting studies have suggested that O^6 -methylguanine forms an
O^6 -MeGC base-pair that is morestable in a DNA duplex than an O^6 -MeGT base-mispair. However, in
replication with various DNA polymerases, O^6 -MeG residues do not block the polymerase and have been
shown to direct preferential incorporation of thymine in place of cytosine. This result has led to a sugges-
tion that the less stable O^6 –MeGT base-mispair may have a more Watson–Crick-like geometry and so bet-
ter satisfy the demands of the DNA polymerase while the O^6 -MeGC is a wobble base-pair(Figure 8.38).
NMR and X-ray structures have identified both these pairings while calculations suggest that the anti-
conformation for the O^6 -methyl group (shown) is less stable than the syn-conformation but only by about
1kcalmol^1.^86
The net biological result is a G→A transition and this type of mutation is common in cells exposed to hard
alkylating agents. It is a lethal lesion in human cells and, in particular, it has been identified with a single
base transition for activation of the Ha-ras-1 proto-oncogene in the process of initiation of mammary
tumours in rats with N-methylnitrosourea(MNU, Section 8.5.3), that is a result of the specific conversion
of G^35 into O^6 -MeG.35,87
O^4 -Alkylthymines exist in the enol-tautomeric form and therefore they can base-pair with guanine.
In model studies, both O^4 -methylthymine and O^4 -ethylthymine form base-mispairs with guanine (Figure
8.38) that do not block the replication of a defined DNA sequence in vitro. However, alkylpyrimidines are very
minor products of DNA alkylation and their biological effects appear to be of low significance.
Lastly, the O-alkylation of the phosphate diesters in DNA gives phosphate triesters, but these are
repairable (Section 8.11.1) and do not seem to be important either as cell-killing agents or mutagenic
lesions. The vital processes for the biological repair of these and other types of DNA damage will now be
described.

8.11 DNA Repair

DNA repair is essential for life on earth for the maintenance of genomic integrity. Estimates of the extent
of DNA damage in a human cell range from 10^4 to 10^6 events per day, which calls for some 10^16 to 10^18
repair events per day in an adult person. All living cells possess a range of DNA repair enzymesin order
to correct damage resulting from spontaneous chemical change, radiation and external chemical agents.
Humans appear to be more effective than rodents in repairing DNA and are also better able to resist muta-
genic agents. Moreover, a striking similarity has emerged between repair systems found in many species
from bacteria to humans, although much of our knowledge comes from studies on bacteria or yeasts.
DNA damage has four possible biological consequences each of which is directly linked to DNA repair
(Figure 8.39). First, the cell cycle can be arrested. This allows time for DNA repair prior to replication and
cell division.^88 Where the extent of DNA damage is too large for effective repair, the cell may go into
apoptosis(programmed cell death).^89 Finally, any surviving DNA damage can lead to mutationsand cancer

Covalent Interactions of Nucleic Acids with Small Molecules and Their Repair 325

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Figure 8.38 An O^6 -MeGT base-mispair (left), an O^6 -MeGC base-pair (centre), and a GO^4 -MeT base-mispair
(right)