Final answer:
When tautomers replace normal bases in DNA, they can change base pairing and potentially cause mutations.
Step-by-step explanation:
Base pairing in DNA occurs between a purine base and a pyrimidine base. Adenine (A) is complementary to thymine (T), and cytosine (C) is complementary to guanine (G). Tautomers are isomers of DNA bases that differ in the placement of a hydrogen atom. When the tautomeric form of cytosine (C) replaces the normal form of cytosine (C) in DNA, it forms a base pair with adenine (A) instead of guanine (G). This altered base pairing can lead to a mutation in the DNA sequence and potentially affect how genes are expressed.