we now realize the base pairs come to join each other up together as the system unravels and forms a new pair of DNA molecules well up to a point it does and that point is known to be accurate to about one in 10,000 base pairs now if you and I wrote an article and there was only one typo in a 10,000w article we'd be very pleased but this is nowhere near enough for a DNA sequence of three billion base pairs there would be half a million at least of Errors