The two molecules forming the sides of the DNA ladder are phosphates and a sugar called deoxyribose. Deoxyribose binds to the phosphate group by a bond called a phosphodiester linkage.
The phosphate groups alternate with deoxyribose to form the sides of the DNA ladder. The phosphate groups are also called inorganic phosphates. Phosphate groups comprise one phosphorus atom bound to four oxygen atoms. One of the oxygen atoms on the phosphate forms a bond with one of the carbons on deoxyribose. Deoxyribose and the closely related ribose sugar of RNA molecules are called pentose sugars because they each contain a ring with five carbon atoms.
The rungs of DNA are made by bases that contain nitrogen. These "rungs" connect to the deoxyribose portions of the sides of the DNA molecule. The phosphate group, deoxyribose and a base together form a nucleotide.