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Some cells have very short lifespans because their jobs are completed quickly, (disease-fighting cells in the human blood stream are an example) but others just seem to "wear out." A part of this "wearing out" results from MITOSIS. Each time DNA divides, it degrades (shortens). Human cells can only divide (go through mitosis) about 50 times. This is called the "Hayflick Limit." Eventually the DNA is no longer able to divide, so the cell dies, and the tissue cannot repair itself, so the organism dies. Here are a few links that go into greater depth Theories of Aging Anti-aging strategies Bacterial Aging Does the Hayflick Limit apply to Viruses? Glossary of Aging Related Terms Senescence How We Age Slowing aging with red wine Testosterone

So how does the cell itself die, after it is no longer able to divide? It blows up and rots (necrosis) necrosis & apoptosis link necrosis wickipedia It kills itself! This is called apoptosis: programmed cell death. Here are some links to apoptosis senescence.info Conquering Aging with Cloning Programmed Cell Death NIH Apoptosis links Apoptosis Dance of Death

Breaking the Hayflick Limit HeLa cells (human cells used in cancer research) have a form of telomerase that prevents the shortening of the chromosomes at mitotic devision.