Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

272 Chapter 5: DNA Replication, Repair, and Recombination
NATURAL DNA BASES
UNNATURAL DNA BASES
H
N
H
N
H
N
H 2 O
H
N
O
N
H
N
N
H
N
N
H
adenine
NH 3
hypoxanthine
H
N
N
O
N
N
H
N
H
H 2 O
H
N
N
O
N
N
O
H
guanine
H
NH 3
H
xanthine
H
H
N
H
N
H 2 O
H
O
N
H
H
N
O
H
N
O
NH 3
cytosine
uracil
O
H 3 C
N
H
NO DEAMINATION
H
N
O
(A)
thymine
H 3 C
H
N
H
N
H 2 O
H 3 C
O
N
H
H
N
O
H
N
O
NH 3
5-methyl cytosine
thymine
(B)
Figure 5–43 The deamination of DNA nucleotides. In each case, the oxygen atom that is
added in this reaction with water is colored red. (A) The spontaneous deamination products of
A and G are recognizable as unnatural when they occur in DNA and thus are readily found and
repaired. The deamination of C to U was also illustrated in Figure 5–38; T has no amino group to
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remove. (B) About 3% of the C nucleotides in vertebrate DNAs are methylated to help in controlling
gene expression (discussed in Chapter 7). When these 5-methyl C nucleotides are accidentally
deaminated, they form the natural nucleotide T. However, this T will be paired with a G on the
opposite strand, forming a mismatched base pair.