Many people associate mutagens with materials made by humans, but many mutagenic substances are found in nature. An example of a naturally occurring mutagen is aflatoxin, which is made by the mold Aspergillus. When mammals ingest the mold, the aflatoxin is converted by the smooth endoplasmic reticulum in liver cells into a product that, like benzopyrene from cigarette smoke, binds to guanine; this also causes mutations. Plants (and to a lesser extent animals) make thousands of small molecules with a variety of functions, some of which are mutagenic and potentially carcinogenic. Examples of human-made mutagens are nitrites, which are used to preserve meats. Once in mammals, nitrites get converted by the smooth endoplasmic reticulum (SER) to nitrosamines, which are strongly mutagenic because they cause deamination of cytosine (see Figure 15.4).

Radiation can also be human-made or natural. Some of the isotopes made in nuclear reactors and nuclear bomb explosions are certainly harmful. For example, extensive studies have shown increased mutations in the survivors of the atom bombs dropped on Japan in 1945. As previously mentioned, natural ultraviolet radiation in sunlight also causes mutations.

By now, you may be getting worried about all the ways your DNA can be mutated. But for us, and most organisms, DNA repair mechanisms have evolved so that most changes in DNA do not get passed on to either the daughter cells or the next generation (see Figure 13.18). Biochemists have estimated how much DNA damage occurs in the human genome under normal circumstances: among the genome’s 3.2 billion base pairs, there are about 16,000 DNA-damaging events per cell per day, of which 80 percent are repaired.