On complete hydrolysis of DNA, the following fragments are formed a pentose sugar ($\mathrm{\beta }$-D-2-deoxyribose) phosphoric acid (${\mathrm{H}}_3{\mathrm{PO}}_4$) and bases (nitrogen-containing heterocyclic compounds) Structures

A unit formed by the attachment of a base to 1-position of sugar is called nucleoside. When nucleoside links to phosphoric acid at 5-position of the sugar moiety, a nucleotide is formed. Nucleotides are joined together by phosphodiester linkage between 5' and 3 carbon atoms of the pentose sugar.

In DNA two nucleic acid chains are wound about each other and held together by hydrogen bonds between pairs of bases.

The two strands are complementary to each other because hydrogen bonds are formed between specific pair of base adenine form hydrogen bonds with thymine whereas cytosine form hydrogen bonds with guanine.