The natural ends of chromosomes appear alarmingly like broken DNA, much as a snapped spaghetti strand is difficult to distinguish from its intact counterparts. Yet every cell in our bodies must have a way of differentiating between the two because the best way to protect the healthy end of a chromosome also happens to be the worst way to repair damaged DNA.
Consider the enzyme telomerase, which is responsible for maintaining protective telomeres at the natural ends of chromosomes. Were telomerase to seal off a broken strand of DNA with a telomere, it would prevent further repair of that break and delete essential genes.
Now, a new study in Science describes how cells avoid such mishaps. These findings show that telomerase can indeed run amok, adding telomeres to damaged DNA, and would do so were it not for the ATR kinase, a key enzyme that responds to DNA damage.
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