From Nature:
Three US scientists have won the Nobel Prize in Physiology or Medicine for discovering the structure of molecular caps called telomeres and working out how they protect chromosomes from degradation. Their discoveries in cell biology during the 1980s and 1990s opened new avenues of work, in ageing and in cancer research, which are still highly active today. The prize, announced on 5 October, is shared equally between Elizabeth Blackburn at the University of California, San Francisco, Carol Greider of the Johns Hopkins Medical School in Baltimore, Maryland, and Jack Szostak at Harvard Medical School in Boston, Massachusetts. The three have already won numerous prizes for their work, including sharing one of the 2006 Lasker awards, often considered to be a forerunner of the Nobel prize.
Their research revealed a fundamental aspect of how DNA, packed into chromosomes, is copied in its entirety by the DNA polymerase enzyme during cell division. The ends of the chromosomes are capped by telomeres, long thought to have a protective function (see 'Chromosome caps'). Without them, the chromosomes would be shortened during each cell division, because DNA polymerase is unable to copy to the very end of one of the two DNA strands it is replicating.
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