So, in the same way that if we want to build a building, we have to have the bricks, the building blocks to build that building, to build a polynucleotide chain, we have to have the dNTP molecules, the deoxy nucleotide triphosphates. And there are four different types of dNTP molecules. We have deoxy adenosine, five prime triphosphate. We have deoxy guanosine, five prime triphosphate. We have deoxy pyridine, five prime triphosphate, and we have the thymidine, five prime triphosphate. So for the DNA polymerase to actually build and extend the DNA polynucleotide chain, it has to have those four nucleotides swimming around in that solution.