A DNA molecule is made up of nucleotides.Each nucleotide is made up of a nitrogen base, a sugar molecule and a phosphate group. There are 4 nitrogen bases such as adenine and guanine known as purines, thine and cytosine are known as pyramidine. These nitrogenous base are arranged in a sequence attached to each other by hydrogen bonding into anti-parallel strands. Therefore, the structure of DNA is termed as the double helical model.