So in Thiamine we have the methyl group here, but in uracel we have the H atom that is replaced. So we have this methyl group that is replaced by the H atom. Now, what's the importance of these bases? Well, one importance is a double helix structure. These bases are used to basically hydrogen bond with respect to one another and they form that double helix as we'll discuss eventually. But the more important reason, the more important fact about these bases is that because it's these bases that essentially change as we go from one nucleotide to another, it's the sequence of these bases that essentially determine the type of genetic information that is stored within that DNA molecule.