Here, a model of DNA is installed. The DNA consists of four types of basic units called nucleotides. Each nucleotide is made up of a pentose sugar (deoxyribose type), a phosphate group and a nitrogenous base. A subunit composed of only sugar and the nitrogen base is known as nucleoside. The four nucleosides differ from each other in the type of base, which could be Adenine(A), Guanine(G), Thymine(T) or Cytosine(C). Adenine and Guanine are Purines. Thymine and Cytosine are Pyrimidines.