The trio solved the mystery of how chromosomes, the rod-like structures that carry DNA, protect themselves from degrading when cells divide.
The Nobel citation said the laureates found the solution in the ends of the chromosomes _ structures called telomeres that are often compared to the plastic tips at the end of shoe laces that keep those laces from unraveling.
Blackburn and Greider discovered the enzyme that builds telomeres _ telomerase _ and the mechanism by which it adds DNA to the tips of chromosomes to replace genetic material that has eroded away.
The prize-winners' work set the stage for research suggesting that cancer cells use telomerase to sustain their uncontrolled growth. Scientists are studying whether drugs that block the enzyme can fight the disease. In addition, scientists believe that the DNA erosion the enzyme repairs might play a role in some illnesses.
"The discoveries by Blackburn, Greider and Szostak have added a new dimension to our understanding of the cell, shed light on disease mechanisms, and stimulated the development of potential new therapies," the prize committee said in its citation.
It was the first time that two women have been among the winners of the medicine prize, committee members said.
Blackburn, who holds U.S. and Australian citizenship, is a professor of biology and physiology at the University of California, San Francisco. Greider is a professor in the department of molecular biology and genetics at Johns Hopkins University School of Medicine in Baltimore.
Greider, 48, said she was telephoned by just before 5 a.m. her time with the news that she had won.
"It's really very thrilling, it's something you can't expect," she told The Associated Press by telephone.
People might make predictions of who might win, but one never expects it, she said, adding that "It's like the Monty Python sketch, 'Nobody expects the Spanish Inquisition!'"
Greider described the research as beginning with experiments aimed at understanding how cells work, not with the idea for certain implications for medicine.
"Funding for that kind of curiosity-driven science is really important," she said, adding that disease-oriented research isn't the only way to reach the answer, but "both together are synergistic," she said.
Blackburn, 60, said she was awakened at 2 a.m.
"Prizes are always a nice thing," she told The AP. "It doesn't change the research per se, of course, but it's lovely to have the recognition and share it with Carol Greider and Jack Szostak."
London-born Szostak has been at Harvard Medical School since 1979 and is currently professor of genetics at Massachusetts General Hospital in Boston. He is also affiliated with the Howard Hughes Medical Institute, the citation said.
The award includes a 10 million kronor ($1.4 million) purse divided among the winners, a diploma and an invitation to the prize ceremonies in Stockholm on Dec. 10.
The researchers have already won a series of medical honors for their enzyme research. In 2006, they shared the Lasker prize for basic medical research, often dubbed "America's Nobel."
Some inherited diseases are now known to be caused by telomerase defects, including certain forms of congenital aplastic anemia, in which insufficient cell divisions in the stem cells of the bone marrow lead to severe anemia. Certain inherited diseases of the skin and the lungs are also caused by telomerase defects.
The Nobel Prizes in physics, chemistry, literature and the Nobel Peace Prize will be announced later this week, while the economics award will be presented on Oct. 12.
Prize founder Alfred Nobel a Swedish industrialist who invented dynamite, left few instructions on how to select winners, but medicine winners are typically awarded for a specific breakthrough rather than a body of research.
Nobel established the prizes in his will in 1895. The first awards were handed out six years later.
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