In addition to spontaneous mutations that arise in replication, mutations also result from spontaneous chemical changes in DNA. One such change is depurination, the loss of a purine base from a nucleotide. Depurination results when the covalent bond connecting the purine to the 1′-carbon atom of the deoxyribose sugar breaks (Figure 13.14a), producing an apurinic site, a nucleotide that lacks its purine base. An apurinic site cannot act as a template for a complementary base in replication. In the absence of base-pairing constraints, an incorrect nucleotide (most often adenine) is incorporated into the newly synthesized DNA strand opposite the apurinic site (Figure 13.14b), frequently leading to an incorporated error. The incorporated error is then transformed into a replication error at the next round of replication. Depurination is a common cause of spontaneous mutation; a mammalian cell in culture loses approximately 10,000 purines every day. Loss of pyrimidine bases also occurs, but at a much lower rate than depurination.

Another spontaneously occurring chemical change that takes place in DNA is deamination, the loss of an amino group (NH₂) from a base. Deamination may be spontaneous or may be induced by mutagenic chemicals.

Deamination can alter the pairing properties of a base: the deamination of cytosine, for example, produces uracil (Figure 13.15a), which pairs with adenine in replication. After another round of replication, the adenine will pair with thymine, creating a T • A pair in place of the original C • G pair (C • G → U • A → T • A); this chemical change is a transition mutation. This type of mutation is usually prevented by enzymes that remove uracil whenever it is found in DNA. Their ability to recognize the product of cytosine deamination may explain why thymine, not uracil, is found in DNA. In mammals, including humans, some cytosine bases in DNA are naturally methylated and exist in the form of 5-methylcytosine (5mC). When deaminated, 5mC becomes thymine (Figure 13.15b). Because thymine pairs with adenine in replication, the deamination of 5-methylcytosine changes an original C • G pair to T • A (C • G → 5mC • G → T • G → T • A). Consequently, C • G → T • A transitions are frequent in mammalian cells, and 5mC sites are mutation hotspots in humans. TRY PROBLEM 20

CONCEPTS