All of this is shown as happening in two dimensions in Figure 7, but the reality is probably a lot more complex and beautiful, forming a three-dimensional tangle of proteins, DNA, RNA and nucleotides.

In humans, this process occurs at a rate of 100 base pairs per second per fork, and there’s probably about 100,000 replication sites, or replicons, within the human genome, each with two forks. That’s a lot of DNA synthesis. There are bound to be errors. Every now and then—about once in every 100 base pairs--polymerase is going to screw up and stick in a C where it should have put in a T or an A where it should have put a G, or whatever. But among the other enzymatic activities in its bag of tricks, polymerase also has a proofreading activity. When polymerase screws up it pauses, excises the incorrect base, and recopies the site.