The sides of the DNA molecule, also known as the sugar-phosphate backbone, are made up of alternating molecules of deoxyribose sugar and phosphate groups. Each nucleotide in DNA consists of a nitrogenous base attached to a deoxyribose sugar, which is in turn linked to a phosphate group. These sugar and phosphate groups are covalently bonded together through phosphodiester bonds, forming the continuous backbone of each DNA strand. The two strands run antiparallel and twist into the characteristic double helix, with the sugar- phosphate backbones on the outside and the nitrogenous bases pairing on the inside as the "rungs" of the ladder
