The human genome consists of 3 billion base pairs, and when a cell divides, it takes about seven hours to complete making a copy of its DNA. That's almost 120,000 base pairs per second. At that breakneck speed, one might expect errors to occur, and they do, at a rate of about two per second in every dividing cell. But cells have a "DNA repair kit" of enzymes that can correct those errors at a rate matching that at which they occur.