How does a cell know how old it is?

Every time a cell multiplies to make two new cells, special zones at the ends of its chromosomes, called telomeres, become shorter. Once the telomeres reach a certain length, the cell stops dividing and eventually dies. The only cells to escape this fate are those that divide to make eggs and sperm. In these cells, a substance called telomerase builds the telomeres up again, so they remain the same length.

Each chromosome end - or telomere - is shown by brightly coloured dots. Jill Brown, Lyndal Kearney, Veronica Buckle, Institute of Molecular Medicine, Oxford