Each time a cell divides, its chromosome tips are cut off, until finally it dies.
But some cells--including some cancerous ones--are immortal.
Each time a cell in your body divides, a clock ticks: there’s a limit to the number of times the average body cell can replicate before it dies. Stick an infant’s skin cells in a petri dish and they’ll divide a hundred times or so before the culture peters out. A 60-year-old’s skin cells won’t divide more than 20 times. Biologists call this cellular senescence.
But what mechanism drives the clock? How can a cell count its own divisions and know when to stop splitting in two? Biologists think they may have found the clock at the end of a cell’s chromosomes. And now they think they know what happens when human cells are able to ignore or even stop the clock: it seems the cells turn cancerous.
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