In DNA molecule the adjacent deoxyribonucleotides are joined in a chain by phosphodiester bridges or bonds, which link the $5^{\prime}$ carbon of deoxyribose of one mononucleotide unit with 3 carbon of deoxyribose of next mononucleotide unit. According to Watson and Crick DNA molecule consists of two such polynucleotide chains wrapped helically around each other, with the sugar phosphate chain on the outside and purine and pyrimidine on the inside of helix. The two strands run antiparallely, ie, one strand has phosphodiester linkage in $3 \rightarrow 5^{\prime}$ direction while other strand has phosphodiester linkage in $5^{\prime} \rightarrow 3$ direction.
Chargaff (1950) suggested that despite wide compositional variations exhibited by different types of DNA the total amount of purines equaled the total amount of pyrimidines $(A+$ $\mathrm{G}=\mathrm{T}+\mathrm{C}$.