Deamination

Deamination is another chemical change that occurs naturally in DNA. It’s
what happens when an amino group (composed of a nitrogen atom and two
hydrogens, NH2) is lost from a base. Figure 13-4 shows the before and after
stages of deamination. When cytosine loses its amino group, it’s converted to
uracil. Uracil normally isn’t found in DNA at all because it’s a component of
RNA. If uracil appears in a DNA strand, replication replaces the uracil with a
thymine, creating a substitution error. Until it’s snipped out and replaced
during repair (see “Evaluating Options for DNA Repair” later in this chapter),
uracil acts as a template during replication and pairs with adenine.
Ultimately, what was a C-G pair transitions into an A-T pair instead.