>>507817945 (OP)Serious answer.
Each time a cell divides, the complete genome inside also divides in half. Each daughter cell gets half the DNA and an enzyme completes the missing part. Cells divide when they cannot detect their neighbors anymore, like healing a cut. But environmental damage can cause the cell to be deaf and begin to divide when there is no damage.
Two main lines of defense for cells that go rogue like that.
Step one is the body orders the cell to die. Cells are capable of suiciding when ordered (apoptosis) and have zero reluctance. This can fail because the signal isn't detectable by the cell or it's otherwise unable to comply. Both are caused by damage. (radiation, chemicals, injury)
Step two relies on telomeres. Each time the DNA divides, a piece is torn off both ends. This would be catastrophic if information was being lost, but there are around 100 repeating TG sequences that don't code for anything (no start code at the beginning) so when they are torn off, it isn't fatal. But when the cell detects it has only a few telomeres left, it will refuse to divide. It is perfectly functional otherwise, it just isn't capable of repairing damage.
Incidentally, this is the reason we grow old and die. Eventually most of our cells are unable to repair damage. Our skin wrinkles, our eyesight dims, our hair goes gray, bones become brittle, then the flu takes us out.
Chemo and radiation therapy are designed to destroy cells in the target area as precisely as they can manage. The hope is that new, undamaged cells will replace them. This is called 'remission'.