Science Questions

What limits the length of the human life? Simon in Norwich

There are a number of things that can determine how long we live. The first thing is the rate of your metabolism. If you compare a mouse with a human, then we're very very genetically similar. But if you look at the heart rate of a mouse it's running along at several hundred beats a minute, whereas our heart's beating at 50 times a minute. So a mouse, in order to survive because it's so small, has to run it's body very very fast. It's almost like you being whipped along by a slave driver. In a mouse, the cells are growing faster, they're dividing more, they're doing everything more quickly, and so the mouse burns itself out more quickly than a bigger animal, like a human. If you compare animals like tortoises, which are cold-blooded, they're big, they're cold blooded, and their metabolism is running much more slowly. Giant tortoises can live for 100, maybe 200 years. So it's down to the speed of your metabolism. Scientists have found whales in the ocean which might even be several hundred years old. There are harpoon tips which have been recovered from the blubber of whales that date back historically to the types of harpoons that haven't been used for several hundred years. Leading scientists to conclude that they really are quite old. The second thing is something called a telomere.These are structures on the ends of each of your chromosomes that are almost like the pieces of cellophane you find around the end of your bootlaces to keep the lace from fraying. Every time a cell divides, it erodes a little piece off of the telomere so it gets shorter and shorter and shorter. If you look at cancer cells that seem to live forever, they have switched on an enzyme that makes telomeres get longer and longer and longer. That seems to be what gives the cells the ability to divide forever. But in a normal cell, the problem is that eventually the telomeres get so short that they run out, and that seems to determine how many times a cell can actually divide. It's called the Hayflick number so there's a limited number of times a cell can divide. But that's not why someone dies. They usually die because something has damaged their DNA and that causes the cells to work less well, and so you get things like cancer.

December 2006