Telomere. Along the chromosomes, which are long pieces of DNA...when you look at them as a picture, they look like lines. Well, the hard part is how to protect the ends of this line. Because you could imagine that if you didn't protect them they would become ragged, and maybe there'd be little parts of them that would be lost. So the telomeres are special DNA that sit at the end of the chromosome that have repetitive sequences that are recognized as the end of the chromosome, but they keep the chromosome from becoming frazzled or damaged. And every time the cell divides, the telomeres also divide. But sometimes they can become shorter. And as they become shorter, that's a clock that the cell is counting to know how old it is, and that will limit how many times the cell can divide without losing some of the important DNA on the chromosome. And one of the interesting features that's understood now about telomeres is that in cancer cells, which have a more infinite capacity for self-division, one of the important changes that they make is that they keep their telomeres long, so that molecular clock goes away and those cells can keep dividing, even though they should get to the end of their lifespan. And that's one of the ways in which the cancer cells basically trick the human body into thinking that they should still keep replicating.- Julie A. Segre, Ph.D.