Deamination alters DNA bases

Deamination may alter the pairing properties of a base: the deamination of cytosine, for example, produces uracil (Figure 17.17a), which pairs with adenine during 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 repaired by enzymes that remove uracil whenever it is found in DNA. The ability to recognize the product of cyto-sine deamination may explain why thymine, not uracil, is found in DNA. Some cytosine bases in DNA are naturally methylated and exist in the form of 5-methylcytosine (5mC; see p. 000 in Chapter 10 and Figure 10.19), which when deaminated becomes thymine (Figure 17.17b). Because thymine pairs with adenine in replication, the deamination of 5-methylcytosine changes an original C-G pair to T-A (C-G:5mC-A:T-A). This change cannot be detected by DNA repair systems, because it produces a normal base. Consequently, C-G:T-A transitions occur frequently in eukaryotic cells.

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