$DNA$ (deoxyribose nucleic acid) consists of $3$ different molecules-phosphate, $5$carbon deoxyribose sugar and nitrogenous base. The nitrogenous base may be a $9-membered,$ double purine, $i.e.,$ adenine $(A)$ or guanine $(G$), or a $6-membered$, single -ringed pyrimidine, $i.e.$, thymine $(T)$ or cytosine $(C)$.