Why don’t our cells go on dividing forever?
It is important to have just the right amount of cell division, so the process has lots of checks and balances. If the controls fail, there is a final limit, in order to prevent unregulated cell proliferation (multiplication). Each time a chromosome is copied, a repetitive stretch of DNA at its end (the telomere) gets shorter. The telomere is needed for the proteins that copy DNA during DNA replication to work properly, so when it is all gone there is no more chromosome copying and no more cell division. In effect, telomeres count cell divisions.
Human cells can normally manage between 40 and 60 divisions – this is called the Hayflick limit, named after Leonard Hayflick’s 1965 discovery. Stem cells can get around this limit using the enzyme telomerase, which rebuilds the ends of chromosomes, allowing for continual cell division. This system is also used by many cancer cells, which activate telomerase to allow them to quickly multiply without dying owing to shortened telomeres. Research is now investigating whether inhibiting telomerase in order to limit the lifespan of cancer cells could be used as a cancer treatment.Lead image: