Interesting read about telomeres (you will learn this in the later topics) for the layman

http://sg.news.yahoo.com/why-flatworms-may-hold-the-secret-to-immortality.html

"Key to a flatworm’s immortality lays in its telomeres – tiny sections of DNA that cap the ends of chromosomes, protecting them from damage and the loss of cell functions linked to ageing.

Each time a cell divides the protective telomere ‘cap’ gets shorter. When they get too short, the cell loses its ability to renew and divide. According to the study an immortal animal would expect cells to be able to maintain telomere length indefinitely so that they could continue to replicate. "