Excellent question. Normal cells only undergo a limited number of divisions, because they have a ‘counter’ in their DNA, which gets decreased by one every time the cell divides – basically a bit of DNA is removed on each division. Once this counter has run out, the cell stops dividing. This is one of our bodies natural anti-cancer mechanisms, and also one of the reasons we grow old.
However, sometimes this clock needs to be reset, or our bodies need to add to this counter instead of take away from it. For this, our body has a molecular machine called telomerase. In cancer, there is a mutation which switches telomerase on all the time, so the cell can keep dividing without stopping – it basically becomes immortal, and that is why the cell doesn’t have a hayflick limit.
In fact, cancer researchers often use cells called HeLa cells, which were taken from a women with cancer in 1951, when she died. As long as these cells had nutrients, they keep on growing and dividing. In a sense, these cells are immortal, even though they killed the women from which they were taken.
The Hayflick limit is the number of times a cell can divide, before it just stops or dies. We think that cells have this limit because it’s hard to replicate the ends of chromosomes. Every time a cell divides, it has to copy its DNA, but because it’s hard to copy the ends, the chromosomes get shorter and shorter with each division.
Ends of chromosomes have sequences called telomeres, which are just repetetive ‘junk’ DNA, so it doesn’t really matter if these get shortened with each cell division. But once the chromosomes get so short that you reach genes, the cell can’t keep dividing.
Some cells, including cancer cells, have an enzyme called “telomerase” that allows them to elongate the telomeres after the DNA has replicated, so the chromosome shortening doesn’t happen, and the cells can go on dividing indefinitely.