Mutagens. Physical and chemical agents capable of bringing about mutations are calledmutagens. Chemical mutagens include nitrous acid. This substance converts adenine to hypoxanthine, a molecule that will not pair with thymine, and thus interrupts the genetic code. A base analog is a chemical mutagen that resembles a nitrogenous base and is incorporated by error into a DNA molecule. Such a DNA molecule cannot function in protein synthesis. Certain dyes and fungal toxins (for example, aflatoxin) are known to be mutagens.