Life would be impossible if the DNA in dividing cells were replicated with anything less than near-perfect precision. Every time a nucleated cell commits to becoming two cells, every “letter” of its genome must be replicated once and only once. In humans, the task boggles the imagination. If unwound, the double helix crammed into each of our cells would measure 6 feet in length. In our bone marrow alone, half a billion new cells are born every minute. These cells alone contain enough DNA to wrap around the earth’s equator 25 times. Within daunting tolerances, each new cell must have a genome identical to that of the cell that gave birth to it. Cancer and other diseases can result when the process goes awry.