Submitted by Virginia Salazar, Whittier, Calif. and answered by Dr. Paul Sternberg, Professor of Biology, Caltech
In most cases, the sequence of DNA making up a gene is copied accurately when a cell divides. This accurate process ensures that each cell is like its parent cell. DNA consists of a string of DNA bases, the letters in the genetic alphabet.
The bad news is that DNA is under continual attack by chemicals within the cell that are byproducts of the ordinary workings of each cell; by environmental hazards; by radiation; and by the general tendency for things to break down. Environmental hazards include natural plant products as well as human-made chemicals. These attacks result in a range of problems, ranging from changes of a single DNA letter to a break in the string.
The good news is that cells counter these continual attacks by correcting essentially all the damage, using a host of beautiful molecular machines. But a mutation occurs when a cell fails to repair damage to its DNA, or repairs it incorrectly. When such a cell divides, it passes on the mutated gene to its progeny. Eggs and sperm, which join to form an embryo, are themselves the product of cell divisions and thus subject to errors in the copying of DNA. These mutations are passed on to our children.
Other cells in our bodies are subject to mutation, and mutant cells can become cancerous. Particularly pernicious are mutations that disrupt the ability of a cell to repair its own DNA. Such mutations are in the genes that are responsible for making the repair machinery. When this occurs, the mutant cell will more easily continue to mutate, a disaster in the making!