DNA is the nucleic acid that forms the genetic material in all organisms. The structure of deoxyribose nucleic acid was elucidated by Watson and Crick. The DNA is a double helical structure having two anti-parallel strands of polynucleotide chains joined to each other by hydrogen bonding formed between the N-bases. In a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one; this observation was given by Erwin Chargaff. Thus, $\mathrm{A}+\mathrm{G}=\mathrm{T}+\mathrm{C}\mathrm{ }\&\mathrm{ }\mathrm{A}=\mathrm{T}\mathrm{ };\mathrm{ }\mathrm{G}=\mathrm{C}$ . Therefore, percentage of Adenine and Thymine are same and that of Guanine and Cytosine is same. $\mathrm{G}=\mathrm{C}=20\mathrm{\%};\mathrm{ }\mathrm{A}=\mathrm{T}=30\mathrm{\%}$ .