In each case the oxygen atom that is added in this reaction with water is colored red. (A) The spontaneous deamination products of A and G are recognizable as unnatural when they occur in DNA and thus are readily recognized and repaired. The deamination of C to U was previously illustrated in Figure 5-47; T has no amino group to deaminate. (B) About 3% of the C nucleotides in vertebrate DNAs are methylated to help in controlling gene expression (discussed in Chapter 7). When these 5-methyl C nucleotides are accidentally deaminated, they form the natural nucleotide T. This T would be paired with a G on the opposite strand, forming a mismatched base pair.