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36 37 death and other helpful illnesses Death is something everyone can be sure of. However, cells can theoretically reproduce infinitely they have been kept alive for decades in laboratories, keeping the body young and strong, so why are they programmed to stop Each cell can only replace itself a certain number of times. The protective tip of each chromosome the telomere, opposite top shortens on each copying, like a fuse, and when it is gone, decay and death follow. As we get older our DNA accumulates errors, and death stops these being passed on. Sex and death drive evolution. Thus, while women safely create all their ova before they are born, mens sperm become ever more errorridden as they age. Even long lived tortoises 150 years and crocodiles 100 years occasionally need to be refreshed from their gene pools. Some illnesses also confer advantage. A famous example is sickle cell disease, which protects against malaria lower opposite. Above At the tip of every chromosome are regions of repetitive DNA called telomeres that protect the ends. In vertebrates, fungi and even some slime molds, the repeated motif is TTAGGG. In insects it is TTAGG. The number of repeats varies around 1000 in humans. Every time a cell divides, the copying mechanism misses some repeats, and shortens the chromosome. When is it all gone, the cell cannot reproduce, only die. Death is part of the program. Left Sicklecell disease is a genetically inherited condition which reduces life expectancy because the deformed sickle shaped red blood cells it produces inhibit gaseous exchange in the lungs. The disease reduces the life expectancy of those who have it, so why it has persisted The answer is that it confers a resistance to malaria, transmitted by mosquitos above. So in places where people are more likely to die from malaria than sicklecell, populations tend to have unusually high instances of the disease.
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