Shared the 2009 Nobel Prize in medicine for the discovery of telomeres.
"There was something completely different about the DNA ends."
In the 1930s, the American geneticist Hermann Muller first noticed that the ends of chromosomes had some sort of unique property that helped them maintain their genetic integrity. In experiments with fruit flies irradiated with X-rays, he observed many animals with genetic abnormalities, but he never found flies with mutations in the chromosome tips. “The terminal gene must have a special function, that of sealing the end of the chromosome, so to speak,” Muller told attendees of a 1938 lecture at the Woods Hole Marine Biological Laboratory in Massachusetts. “And that for some reason, a chromosome cannot persist indefinitely without having its ends thus sealed.” He thus named these ends telomeres, from the Greek words telo, meaning “end,” and mere, meaning “part”.
Thanks to the work of Jack Szostak and his colleagues, scientists now know that Muller was only partially correct. (Still, not a bad hypothesis given that Muller’s lecture came 15 years before the discovery of DNA.) Telomeres do stabilize the ends of chromosomes, but, instead of active genes, telomeres contain a string of highly repeated DNA sequences that act as disposable buffers to prevent vulnerable chromosomes from deterioration.
During cell division, enzymes that duplicate DNA cannot reach all the way to the ends of chromosomes. If cells divided without telomeres, necessary genetic information would get lopped off with each round of division. Fortunately, telomeres are nonsense DNA that do not encode essential proteins, so there are no immediate consequences when telomeric sequences are lost. Yet this telomere shortening can eventually lead to cancer or even cell death — which explains why telomeres have been linked to the ageing process.
To maintain telomeres in rapidly growing cells, however, cells contain an enzyme called telomerase that can synthesize telomeric DNA directly onto chromosome ends. In this way, the original length of the telomere sequence is restored, which in turn helps keep chromosomes intact longer. This function has also earned telomerase the nickname ‘the immortalizing enzyme’.
Illustrations and layout: Annika Röhl, Bengt Gullbing, courtesy of The Nobel Committee for Physiology or Medicine at Karolinska Institutet.
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