The deamination of DNA
If you look back to Figure 4a, you will notice that cytosine carries an –NH2 group. This group is liable to undergo what is called a deamination reaction, resulting in the formation of a carbonyl group. What you might notice is that deamination of cytidine generates the nucleoside uridine (Table 3b). Since uridine is commonly found in RNA, its presence in DNA is easily detected by the cell, and the damage is repaired. In contrast, deamination of 5-methylcytidine results in the formation of thymidine (Table 3c). Deamination occurs much more readily to 5-methylcytidine than to cytidine. The thymidine that results from deamination of 5-methylcytidine is not detected by the cell as DNA damage and is likely to remain and lead to mutation. Such deamination events are common sources of mutation on the human X chromosome, since this chromosome is subject to inactivation through extensive cytidine methylation (X-inactivation, see Section 7.3). As a consequence, many C → T mutations occur. Such mutations give rise to X-linked diseases such as haemophilia, which is caused by mutations in the factor IX gene.