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The most common damage suffered by a DNA molecule is depurination – the loss of an adenine or guanine because the bond between the purine base and the deoxyribose sugar to which it is attached spontaneously hydrolyzes (Figure 4.4). Within each human cell about 5000–10 000 depurinations occur every day.

Deamination is a less frequent event; it happens about 100 times a day in every human cell. Collision of H₃O⁺ ions with the bond linking the amino group to carbon number 4 in cytosine sets off a spontaneous deamination that produces uracil (Figure 4.4). Cytosine base pairs with guanine, whereas uracil pairs with adenine. If this change were not corrected, then a CG base pair would mutate to a UA base pair the next time the DNA strand was replicated, introducing a mutation at this position in one of the two copies of the DNA double helix that are obtained post‐replication.

Ultraviolet light or chemical carcinogens such as benzopyrene, present in cigarette smoke, can also disrupt the structure of DNA. The absorption of ultraviolet light can cause two adjacent thymine residues to link and form a thymine dimer (Figure 4.5). If uncorrected, thymine dimers create a distortion in the DNA helix known as a bulky lesion. This inhibits normal base pairing between the two strands of the double helix and blocks the replication process. Ultraviolet light has a powerful germicidal action and is widely used to sterilize equipment. One of the reasons why bacteria are killed by this treatment is because the formation of large numbers of thymine dimers prevents replication.

Figure 4.4. Spontaneous reactions corrupt the DNA database.