A few excerpt from this article given below:
"Inside the cell nucleus, all our genetic information is located on twisted, double stranded molecules of DNA which are packaged into chromosomes. At the end of these chromosomes are telomeres, zones of repeated chains of DNA that are often compared to the plastic tips on shoelaces because they prevent chromosomes from fraying, and thus genetic information from getting scrambled when cells divide.
The telomere is like a cellular clock, because every time a cell divides, the telomere shortens. After a cell has grown and divided a few dozen times, the telomeres turn on an alarm system that prevents further division. If this clock doesn't function right, cells either end up with damaged chromosomes or they become "immortal" and continue dividing endlessly -- either way it's bad news and leads to cancer or disease. Understanding how telomeres function, and how this function can potentially be manipulated, is thus extremely important."
New Telomere Discovery Could Help Explain Why Cancer Cells Never Stop Dividing