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Elizabeth Blackburn, Carol Greider, and Jack Szostak.

Source: Nobel organization website.

Maintaining the information encoded within DNA is essential for the health of cells and organisms, and since the 1930s it had been speculated that the ends of chromosomes, called the telomeres, must be highly specialized to protect DNA strands from being degraded. In the late 1970s Elizabeth Blackburn, studying the model organism tetrahymena, discovered that the ends of chromosomes contain repeats of the sequence CCCCAA. Together with colleague Jack Szostak she also found that a minichromosome injected into cells would be rapidly degraded, unless this CCCCAA sequence was located at its ends – in which case the minichromosome was protected. Later, a PhD student named Carol Greider working in Blackburn's lab discovered the enzyme telomerase, which is responsible for depositing the CCCCAA repeats at chromosome ends. It is now understood that loss of telomeres contributes to the loss of cells' ability to divide and, conversely, that the unwanted ability of many cancer cells to over‐proliferate is linked to the overactivity of telomerase. Blackburn, Szostak, and Greider together won the Nobel Prize in Physiology and Medicine in 2009 for explaining “how chromosomes are protected by telomeres and the enzyme telomerase.”