What is the backbone of DNA in the Watson-Crick model formed by?

The backbone of DNA in the Watson-Crick model is formed by the sugar-phosphate-sugar chain, and the nitrogen bases are projected more or less perpendicular to this backbone but face inside. The heterocyclic compounds in nucleic acids are the nitrogenous bases named adenine, guanine, uracil, cytosine, and thymine. Adenine and Guanine are substituted purines while the rest are substituted pyrimidines. The skeletal heterocyclic ring is called as purine and pyrimidine respectively. A and G of one strand of DNA compulsorily base pair with T and C, respectively, on the other strand.