STOCKHOLM, Oct 05(AP): Americans Elizabeth H. Blackburn, Carol W. Greider and Jack W. Szostak won the 2009 Nobel Prize in medicine on Monday for discovering a key mechanism in the genetic operations of cells, an insight that has inspired new lines of research into cancer.

It was the first time two women have been among the winners of the medicine prize.

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 - features 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, done in the late 1970s and 1980s, 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.

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 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!'"