Inside the nucleus of a cell, our genes are arranged along twisted, double-stranded molecules of DNA called chromosomes. At the ends of the chromosomes are stretches of DNA called telomeres, which protect our genetic data, make it possible for cells to divide, and hold some secrets to how we age.
Telomeres have been compared with the plastic tips on shoelaces, because they keep chromosome ends from fraying and sticking to each other, which would destroy or scramble an organism’s genetic information.
Yet, each time a cell divides, the telomeres get shorter. When they get too short, the cell can no longer divide; it becomes inactive or “senescent” or it dies.
Like the rest of a chromosome, including its genes, telomeres are sequences of DNA — chains of chemical code. Like all DNA, they are made of four nucleic acid bases: G for guanine, A for adenine, T for thymine, and C for cytosine.
Telomeres are made of repeating sequences of TTAGGG on one strand paired with AATCCC on the other strand. Thus, one section of telomere is a “repeat” made of six “base pairs.”
In white blood cells, the length of telomeres ranges from 8,000 base pairs in newborns to 3,000 base pairs in adults and as low as 1,500 in elderly people. (An entire chromosome has about 150 million base pairs.) Each time it divides, an average cell loses 30 to 200 base pairs from the ends of its telomeres.
Cells normally can divide only about 50 to 70 times, with telomeres getting progressively shorter until the cells become senescent or die.
Telomeres do not shorten in tissues where cells do not continually divide, such as heart muscle.
An enzyme named telomerase adds bases to the ends of telomeres. In young cells, telomerase keeps telomeres from wearing down too much. But as cells divide repeatedly, there is not enough telomerase, so the telomeres grow shorter and the cells age.
Geneticist Richard Cawthon and colleagues at the University of Utah found shorter telomeres are associated with shorter lives. Among people older than 60, those with shorter telomeres were three times more likely to die from heart disease and eight times more likely to die from infectious disease.
While telomere shortening has been linked to the aging process, it is not yet known whether shorter telomeres are just a sign of aging — like gray hair — or actually contribute to aging.
If telomerase makes cancer cells immortal, could it prevent normal cells from aging? Could we extend lifespan by preserving or restoring the length of telomeres with telomerase? If so, would that increase our risk of getting cancer?
Scientists are not yet sure. But they have been able to use telomerase in the lab to keep human cells dividing far beyond their normal limit, and the cells do not become cancerous.
If we used telomerase to “immortalize” human cells, we may be able to mass produce cells for transplantation, including insulin-producing cells to cure diabetes, muscle cells for treating muscular dystrophy, cartilage cells for certain kinds of arthritis, and skin cells for healing severe burns and wounds. An unlimited supply of normal human cells grown in the laboratory would also help efforts to test new drugs and gene therapies.
Cellular aging is the process by which a cell becomes old and dies. It is due to the shortening of chromosomal telomeres to the point that the chromosome reaches a critical length. Cellular aging is analogous to a wind up clock. If the clock stays wound, a cell becomes immortal and constantly produces new cells. If the clock winds down, the cell stops producing new cells and dies. Our cells are constantly aging. Being able to make the body’s cells live forever certainly creates some exciting possibilities. Telomerase research could therefore yield important discoveries related to the aging process.
Knowing what we have just learned about telomeres and telomerase, it can be said that scientists are on the verge of discovering many of telomerase’s secrets. In the future, their research in the area of telomerase could uncover valuable information to combat aging, fight cancer, and even improve the quality of medical treatment in other areas such as skin grafts for burn victims, bone marrow transplants, and heart disease. Who knows how far this could go?