Mutation

Mutations also arise naturally during the manufacture of DNA. Thus, the opportunity for error exists every time a cell replicates. Even so, DNA is correctly made almost always. Predictably, cells that divide numerous times are more at risk for errors than cells that divide less frequently. For example, the egg cells are present in a female at birth and never undergo division, while sperm cells in males are constantly being produced. A theory proposes that the cause of birth defects including a type of dwarfism, Marfan syndrome, and myositis ossificans are typically the result of a defective gene contributed by a mutated sperm.

Developing embryos and fetuses are especially at risk for mutation. Their cells divide very rapidly and become increasingly specialized for specific tasks. Pregnant women must be careful to avoid x rays, almost all medication, and even the extreme temperatures of hot tubs and saunas.

Mutations can occur after birth as well. For example, evidence is mounting that environmental influences can trigger genetic changes. A compound that is responsible is termed a mutagen. The known association between some types of cancer and smoking, for example, may result from a mutation in lung cells that overrides the natural controls to cell growth and division. The rampant growth that can occur produces a cancer.