Within the next several years we and our colleagues learned much about how telomerase works. Like all polymerases and virtually all enzymes, it consists mainly of protein, and it requires that protein to function. Uniquely, though, it also includes a single molecule of RNA (close cousin to DNA) that contains the critical nucleotide template for building telomeric subunits. Telomerase places the tip of one strand of DNA on the RNA, positioning itself so that the template lies adjacent to that tip. Then the enzyme adds one DNA nucleotide at a time until a full telomeric subunit is formed. When the subunit is complete, telomerase can attach another by sliding to the new end of the chromosome and repeating the synthetic process.
In 1988 Greider left Berkeley for Cold Spring Harbor Laboratory, and later our groups and others found telomerase in ciliates distinct from Tetrahymena, as well as in yeast, frogs and mice. In 1989 Gregg B. Morin of Yale also discovered it, for the first time, in a human cancer cell line—that is, in malignant cells maintained for generations in culture dishes. Today it is evident that telomerase is synthesized by nearly all organisms with nucleated cells. The precise makeup of the enzyme can differ from species to species, but each version possesses a species-specific RNA template for building telomeric repeats.
The importance of telomerase in many single-cell organisms is now indisputable. Such organisms are immortal in that, barring accidents or geneticists meddling in their lives, they can divide indefinitely. As Guo-Liang Yu in Blackburn's research group demonstrated in 1990, Tetrahymena needs telomerase in order to retain this immortality. When the enzyme is altered, telomeres shrink and cells die. Blackburn's team and others have similarly demonstrated in yeasts that cells lacking telomerase undergo telomere shortening and perish. But what role does telomerase play in the human body, which consists of a myriad of cell types and is considerably more complex than Tetrahymena or yeast?
Surprisingly, many human cells lack telomerase. Greider and others made this discovery in the late 1980s, when they pulled together the threads of research that investigators in Philadelphia had initiated more than 25 years earlier. Before the 1960s, human cells that replicated in the body were thought to be capable of dividing endlessly. But then Leonard Hayflick and his co-workers at the Wistar Institute demonstrated unequivocally that this notion was incorrect. Today it is known that somatic cells (those not part of the germ line) derived from human newborns will usually divide 80 to 90 times in culture, whereas those from a 70-year-old are likely to divide only 20 to 30 times. When human cells that are normally capable of dividing stop reproducing—or, in Hay- flick's words, become "senescent"—they look different and function less eÛciently than they did in youth, and after a while they die.
In the 1970s a Soviet scientist named A. M. Olovnikov linked this programmed cessation of cell division to the end-replication problem. He proposed that human somatic cells might not correct the chromosomal shortening that occurs when cells replicate their DNA. Perhaps division ceased when cells discerned that their chromosomes had become too short.
We were unaware of Olovnikov 's ideas until 1988, when Calvin B. Harley, then at McMaster University, brought them to Greider's attention. Intrigued, Greider, Harley and their collaborators decided to see if chromosomes do get shorter in human cells over time.
Sure enough, most normal somatic cells they examined lost segments of their telomeres as they divided in culture, a sign that telomerase was not active. Similarly, they and Nicholas D. Hastie's group at the Medical Research Council (MRC) in Edinburgh found that telomeres in some normal human tissues shrink as people age. (Reassuringly, Howard J. Cooke, also at the MRC in Edinburgh, had shown that telomeres are kept intact in the germ line.) These results indicated that human cells might "count" divisions by tracking the number of telomeric repeats they lose, and they might stop dividing when telomeres decline to some critical length. But definitive proof for this possibility has not yet been obtained.