So let's take a look at what these nitrogenous bases actually are. Now, unlike in the backbone, where these essentially repeat throughout the entire nucleic acid, these bases do change when we go from one nucleotide to another nucleotide. And that's exactly what allows these bases and allows the DNA to ultimately store genetic information, as we'll see in just a moment. So although the backbone does not change, the bases in the nucleotides do vary from one monomer, one nucleotide to the next nucleotide. And there are two categories of bases. We have purines and we have pyrimidines.