site (Figure 17.16b), frequently leading to an incorporated error. The incorporated error is then transformed into a replication error at the next round of replication. Depuri-nation is a common cause of spontaneous mutation; a mammalian cell in culture loses approximately 10,000 purines every day.
Another spontaneously occurring chemical change that takes place in DNA is deamination, the loss of an amino group (NH2) from a base. Deamination may occur spontaneously or be induced by mutagenic chemicals.
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