One of the reasons our cells die is because they are inherently programmed to have only 60 to 70 doublings. That’s it. After that, they die.
Why is that?
Turns out, we have things called telomeres that protect our DNA. Telomeres are highly repetitive DNA sequences that protect the end of the chromosome during replication. During cell division, the enzymes that duplicate the chromosome and its DNA can’t continue their duplication all the way to the end of the chromosome. If there wasn’t a fix for this problem, then every time the cell divided, it would lose a bit of DNA at the end of the chromosome! Bad idea.
Telomeres protect the DNA by allowing the enzymes to travel all the way to the end of the important DNA at the end of the chromosome. The telomere doesn’t have any important genetic data in it; it’s just a disposable lengthener that is tacked onto the end of the chromosome. Each time the cell divides, the telomere gets a bit shorter. Eventually, it’s so short that it may as well not exist, and the original DNA starts getting chopped off during replication. The cell realizes that there are errors in DNA replication, and the cell cycle is arrested (via p53 and RB), leading to apoptosis.
So how come stem cells and cancer cells are able undergo an unlimited number of doublings? It turns out that they are able to replenish their telomeres after DNA replication. An enzyme called telomerase restores the telomeres to their original length, avoiding the progressive shortening and eventual cell cycle arrest that normal cells undergo.
As you might expect, the length of one’s telomeres is intimately related to the aging process. Several age-related diseases, including heart disease, are related in part to shortened telomeres. In fact, you can estimate a person’s age from the length of their telomeres! Not much use in every day life, but very useful in forensic pathology.
Perhaps Ponce de Leon’s fountain of youth was really just a bath spiked with telomerase.
Photo credit: Storm Crypt (http://www.flickr.com/photos/storm-crypt/3321856585/), under cc license.