This nuclein preparation was DNA complexed with protein. Miescher and others who continued to work on nuclein devised ways to remove the protein, and the term nucleic acid was coined in 1889. The chemical composition of nucleic acids from many different sources was studied by hydrolysis and analysis of the resulting chemical products. The composition included pentose sugars, phosphoric acid and purine and pyrimidine bases. The chemical compounds purine and pyrimidine are shown on p. 42.
Purine is a double ring that is pyrimidine with an imidazole ring fused to it.
The five bases of RNA and DNA are just modifications of these basic structures. One easy way to remember which compounds are purines is to remember that purine has two syllables and the purine bases have two rings. It is very common to refer to purine nucleotides or pyrimidine nucleotides, so you need to know which is which. There is another type of base used in nucleotides called pyridine. It is very similar to pyrimidine except it has only one nitrogen in the ring.