The difference between cancer cells and normal cells is profound, not only because of the way they look and the way they behave, but because of the radical difference in their lifespans. Placed in tissue culture, cancer cells can live forever. Normal cells, on the other hand, die after about 50 generations. The best proof of cancer cell immortality comes from HeLa cells, a cultured cancer cell line that was established in 1951 from a cervical tumor that was isolated from a woman named Henrietta Lacks. The HeLa cell line has been growing well ever since, and cultures of these cells are maintained for research purposes by laboratories around to world. Henrietta Lacks, a native of Baltimore, Maryland, was 31 years old when the tumor was discovered. She died of cervical cancer eight months later.
It may seem odd to think that the achievement of immortality is a bad thing. There is a tendency to believe that if a cell becomes immortal, it might immortalize the entire organism. But an animal's body is designed around the principle of regulated cell division for the good of the community. Most cells in an adult's body are postmitotic, a condition that guarantees a stable organ size and shape. Some cells, such as skin and bone marrow, are allowed to divide, but only a limited number of times. The only immortal cells in the body are the germ cells (sperm and eggs), although even they will die out if the individual never has children. This is not to say that such an arrangement can never be tampered with. Stem cells can also proliferate for years in culture (although we do not yet know if they are truly immortal) and their use in medical therapies may make it possible to extend the human lifespan, but for now, the acquisition of immortality by a somatic cell always leads to trouble.
For 40 years scientists struggled to understand the mechanism by which cancer cells are immortalized. Throughout the 1990s attention was drawn to a special DNA sequence called a telomere, located at the
Long arm -
tips of the chromosomes. Each time the DNA is replicated during the S phase (see chapter 3) of the cell cycle, the telomeres shrink, but they are later restored by a special enzyme called telo-merase. By carefully studying the mechanics of DNA replication, scientists have been able to conclude that telomeres are essential for the correct duplication of each chromosome. A failure to duplicate the DNA automatically terminates the cell cycle. Normal cells lack telomerase (that is, the gene for telomerase is turned off in adult cells), and for this reason they cannot proliferate indefinitely in the body (in vivo) or in cell culture (in vitro).
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