But here was a mutation which allowed cells to survive under hostile conditions and, while surviving, they were able to accumulate further mutations. When a mutation which increases the capacity of a cell to survive, is combined with another mutation which forces it to divide, the result is a rapidly growing tumour. All cells have a suicide program encoded in their genes. This is for the good of the organism. If cells are infected by a virus, for example, it s better for the organism that they die, and that s what they usually do.
Apoptosis is also crucial to keep the balance between cell production and cell death in an organism.
If the longevity of cells wasn t tightly defined, we d blow up with the number of cells we produce every day. Biology over-produces, selects what it wants and discards the rest. This process can go wrong in two ways. Either there s not enough cell death and the result is cancer. Or there s too much cell death and you get a degenerative disease.