Telomeres are bits of DNA on the ends of our chromosomes-think of the hard ends of your shoelaces. While they do not contain genes, telomeres act as starting points for replication or duplication of the chromosomes during cell division. Each time a normal cell divides, its telomeres are cut just a bit shorter, until eventually they are so short that no further cell division can occur. Cells with critically short telomeres become senescent, unable to divide further, and eventually malfunction and die. While some have likened this to a genetic biological clock, others have described telomeres as a fuse that becomes shorter and shorter, until it sets off a kind of cellular time bomb that wreaks havoc on the cell's internal workings.

When we are born, we don't have every cell our bodies will ever need. As we grow, we need new skin, bone, blood and many other kinds of cells. Even as adults, we need to make new cells. For example, skin cells and those cells that line our intestines are constantly replaced. All of these reproducing cells use their telomeres to get cell division started. Without their telomeres, our cells would be unable to reproduce at all.

The process of cell division is called mitosis. Each time mitosis occurs, the telomeres of the dividing cells get just a bit shorter. Once a cell's telomeres have reached a critically short length, that cell can no longer divide. Its structure and function begins to fail, and ultimately, the cell dies. As our cells age, so do we. Some scientists have suggested an association between aging cells with shorter telomeres and wrinkled skin or immune system decline. Current research seeks to learn whether older cells and shortened telomeres actually cause aging in these and other parts of the human body. Some researchers, however, caution that the entire aging process cannot be explained solely by telomere shortening. They point out that no relationship exists between starting telomere length and a specie's life span. Mice, for example, have much longer telomeres than humans, and live only two years or so.

Almost 90% of all cancer cells have been found to possess telomerase. Telomerase is an enzyme that "rewinds" our mitotic or cellular clocks. Telomerase strengthens and lengthens our shortened telomeres, replacing the bits of DNA lost in ordinary cell division. If telomerase stops telomere shortening, then in theory, those cells with telomerase can live forever. Since most cancer cells contain telomerase, researchers believe it is a critical factor in conferring immortality upon these cells.

The shortening of telomeres has been associated with a number of diseases, many of them age-related. Shortened telomeres have been identified in aging skin, blood, muscle, central nervous system and cardiovascular cells. And cells from people with a variety of diseases-from atherosclerosis to hepatitis to blood disorders-have been found to have shortened telomeres.