DNA stands for deoxyribonucleic acid. The nucleus of each cell contains forty-six structures called chromosomes that together "package" all our genetic information or genes. This information is coded by a series of four bases -Adenine, Guanine, Cytosine and Thymine. These are linked together in a specific sequence or code. In addition, each strand of code has a complementary strand in which the bases are paired: adenine pairs with thymine and cytosine pairs with guanine. The base pairs are like rungs in long, twisting, zipper-like genetic ladders. These base pairs create the sequences, or instructions needed to form our bodies.

Genes are portions of this genetic material critical to growth and reproduction. They also have important day-to-day functions. For example, genes create proteins, enzymes and others substances that in turn carry out many cellular processes, for example, energy generation and hormone creation. All of our cells, except sperm and eggs, contain two copies of each gene. That is, all the genes we need are encoded by 23 chromosomes, but our cells contain a duplicate copy of each chromosome-or a total of 46.

DNA can be damaged in several ways. For example, energy production in cells can produce toxic molecules called reactive oxygen species, a class of "free radicals." These substances can react with and modify the bases in DNA and prevent the code from being used properly. If DNA is like a zipper, free radicals produce stray threads or distorted teeth that can jam and break the zipper. Exposure to toxins, like ultraviolet light or cigarette smoke, can also damage DNA. And the enzymes that make new DNA (which happens just before a cell divides) can make mistakes causing improper base pairs. DNA damage or errors in newly replicated DNA are quite common. In fact, the DNA in each cell of our bodies probably sustains at least 10,000 injuries or errors each day!

Of course, those mistakes in our DNA (DNA damage) need to be repaired. And our cells have evolved a sophisticated system of recognizing DNA errors and fixing them. To return to the analogy of DNA as a zipper, picture our cells as having tiny scavenger proteins that spend all of their time searching for and eliminating stray threads and foreign matter and broken teeth that have made their way into our DNA zippers. This process of weeding out such mistakes is DNA repair.