From the time an organism is first conceived until the time it dies, it has undergone an estimated 10 quadrillion divisions (that's 1 trillion times 10 thousand, for those keeping score at home). That's a pretty big number. The human genome has roughly 4 billion base pairs. What would happen if human DNA manufacturing & proofreading machinery erred at a rate of 1 in a million? What about 1 in a billion? 1 in a trillion? What number must be reached to be compatible with life & yet not so cumbersome such that the cell has no energy left for any other growth or manufacturing?