It is true that smoking cigarettes,
harmful chemicals in our environment, and nuclear radiation all cause cancer.
Nonetheless, all of these factors first act by causing mutations. Because
cancers are primarily the result of mutations in somatic cells, those chemicals
that cause mutations, that is, mutagens, can also cause cancer. Radiation that
causes DNA damage similarly leads to mutations and cancers. Not every mutation
will actually cause cancer. Most mutations do not even affect transcribed
regions of DNA, and even if they do affect a particular gene, it is not likely
to be involved in controlling cell division.
In practice, some cancer-causing
mutations are due to environmental factors, whereas others occur spontaneously
as a result of mistakes made during replication of the cell’s DNA.
Cancer-causing agents are often called carcinogens.
Almost all carcinogens are also mutagens. Occasional discrepancies occur due to
metabolism within the body, usually in the liver. Certain chemicals that do not
react with DNA themselves may be altered by the body, giving rise to
derivatives that do react with DNA. In this case, the original compound is by
definition a carcinogen but not, strictly speaking, a mutagen.
Approximately 80% of cancers are
derived from the epithelial cells that form the outer covering of tissues.
Epithelial cells are the surface cells of the skin and are also found lining
the intestines and the lungs. Because the outermost layers are constantly worn
away, the underlying layers must keep dividing. Cells from tissues where cell
division is rare only occasionally become cancerous. (Nerve and muscle cancers
account for only 3% to 4% of the total.) In addition, the surface cells are
much more likely to suffer exposure to dangerous chemicals and harmful