So notice that because there are four different types of nitrogenous bases, that means there are four different types of nucleotides. And it is very common to call our nucleotide by the type of nitrogenous base that it contains. So for example, because this nucleotide contains the guanine nitrogenous base, we simply call the nucleotide the guanine. And because this contains the adenine nitrogenous base, this nucleotide can be called simply the adenine nucleotide. So we have one nucleotide bonds to a second nucleotide via this phosphodia ester bond. Now, DNA in cells, specifically in human cells, doesn't actually exist as a single strand of DNA.