5-Bromouracil, a highly mutagenic compound (that is, a compound that causes changes in DNA), is used in cancer chemotherapy. When administered to a patient, it is converted to the triphosphate and incorporated into DNA in place of thymine, which it resembles sterically. Why does it cause mutations?

There are four nitrogenous bases that compose DNA. The nitrogenous base is adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine is paired with thymine with two hydrogen bonding while the cytosine is paired with guanine with three hydrogen bonding.

5-Bromouracilis are incorporated into DNA in place of thymine because of their similar size. Thymine pairs with adenine via two hydrogen bonds. 5-Bromouracil exists primarily in the enol form. The enol cannot form any hydrogen bonds with adenine, but it can form two hydrogen bonds with guanine therefore, 5-bromouracil pairs with guanine. Because 5-bromouracil causes guanine to be incorporated instead of adenine in newly synthesized DNA strands, it causes mutations.

Mutation due to 5-bromouracil