As previously mentioned, we now know that the enzyme DNA polymerase is responsible for the replication of DNA.  In reality, DNA polymerase isn’t just a single enzyme. Rather, it is a massive multi-enzyme complex possessed of multiple catalytic activities. Just think for a minute about what it has to do. DNA polymerase has to start reading in the right place. Once it picks a place to start reading, polymerase has to actually unwind the DNA, which is coiled up tightly like a twisted telephone cord. This activity is known as helicase. As the DNA is unwound, the two strands must separate, or melt. Polymerase must then proceed along the DNA, matching “free” nucleotides to  those in the parent strands. Finally, polymerse must link up the new nucleotides to each other, forming the bonds that make up the sugar-phosphate backbone of the daughter strands—an activity known as ligase. Polymerase has to do all this quickly, because cells have a lot to do and can’t wait forever to replicate their chromosomes. It has to do all this efficiently, copying both strands at the same time with a minimum of disruption and energy expenditure. And it has to do all this accurately, because if you snarf it and stick in a G where you should have put a T, the results can be catastrophic.

Figure 12. The Economy Version. This 3-d model of bacterial DNA polymerase, based on x-ray crystallographic data, shows the enzyme wrapped around DNA. The DNA polymerase in your cells is even more elaborate and complex.