Let's begin by asking why don't you live forever? Suppose you go through life eating moderately and taking a regular amount of exercise. You might reasonably expect to remain the same always, fluctuating a little in weight and fitness but otherwise unchanged.

Unfortunately, it won't happen. As you get older you will lose teeth or hair, or find that you can't see as well as you did. The old explanation for this was simple and natural: your body is a machine, and it wears out just as a machine wears out.

We are even lucky. We are good for three score years and ten, while you don't see many 1920s cars on the road.

About 150 years ago, that explanation for aging became suspect. We learned that our bodies are made up of cells, endless billions of them. We are multi-cellular creatures. However, there are in the world many creatures consisting of a single cell. They reproduce by a simple splitting into two parts, and they can go on doing so indefinitely. A one-celled animal doesn't get old. It dies only because it encounters an environment where it can't live, or something else eats it.

Aha, said the scientists. That means we grow old because of the way that our different cells interact with each other. Isolate a human body cell, and in the right circumstances it too will live forever.

That's reasonable, but it's wrong. During the 1960s it was found that isolated human body cells are able to reproduce only a certain number of times; after that they reach the "Hayflick limit," grow old, and die. Moreover, cells from a human newborn can divide 80 to 90 times, while cells from a 70-year-old will divide successfully only 20 to 30 times. What is going on here?

Any body cell capable of reproducing itself contains DNA (household initials since the Simpson trial). That DNA is organized into long strands called chromosomes, and when a cell divides the chromosome is copied. However, the copying is not complete. A small piece of the end of the chromosome, known as a telomere, is lost. Over time, as a cell divides again and again, the telomere shortens and shortens. The chromosomes develop the equivalent of split ends, and the cell can't make more copies. It dies.

How, then, is a baby able to "start over" with long and intact telomeres? It can do so because the reproductive process uses a substance called telomerase to rebuild the telomeres to their original length.

If we could introduce telomerase into all the cells of our body, would we be rejuvenated?

That's the hope. Rebuild the telomeres, and tissue repair should take place in the 70-year-old at the same rate as in the newborn. The aging process may be halted, and perhaps even reversed.

When will we know how to do this, and when will we learn if it works? Almost certainly, within 20 years. Plan to stay around until then.

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