But many of these 'immortal' cell lines, like the famous HeLa human cell line, are derived from tumour cells. For normal human cells grow old, just like the bodies that contain them. Freshly isolated human cells will go through about 70-80 cell divisions in culture, after which they just stop dividing and eventually die. In contrast, tumour cells keep growing, but they have undergone quite radical genetic and physiological changes that enable them to do so. This raises the possibility that the results of an experiment using these cell lines may not always reflect the behaviour of completely normal cells.
Cell biologists are therefore always on the lookout for new ways of 'immortalizing' normal cells.
One of the signs of cell ageing is a successive shortening of the ends of the chromosomes at each cell division. The ends of chromosomes are composed of DNA with a simple repetitive sequence. This DNA does not encode genes but forms structures called telomeres that prevent the chromosome ends from sticking to other chromosomes. The DNA in a chromosome is a single, very long, molecule. Every time a cell divides, the DNA molecules in its chromosomes are copied or replicated.