Why does for example smoking cause cancer? How do various toxins in general make your body start growing cells uncontrollably?

Question

Why does for example smoking cause cancer? How do various toxins in general make your body start growing cells uncontrollably?

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Answer 1

A carcinogen is anything that can cause mutations, i.e. damage your DNA. That can be anything from soot particles in your lungs to UV light breaking up molecules. The overwhelming vast majority of the resulting mutations are irrelevant, as they are either fixed, target unnecessary parts of the genome, or simply cause the individual cell to die.

Cancer happens if you get extremely unlucky and the thing that gets damaged is precisely the mechanism which controls the life cycle of the cell. Normal cells have a life cycle where they divide and then die off, but if that mechanism is broken without the cell dying, it just keeps dividing uncontrollably.