what forms the backbone of dna

The backbone of DNA is formed by an alternating chain of sugar and phosphate groups. Specifically, the sugar in DNA is deoxyribose, and the phosphate groups link the sugars together through strong phosphodiester bonds. This sugar-phosphate backbone provides structural support for the DNA molecule, with the nitrogenous bases (adenine, thymine, cytosine, and guanine) attached to the sugars forming the "rungs" of the DNA ladder. The backbone is on the outside of the DNA double helix, while the bases face inward, pairing with complementary bases on the opposite strand to hold the double helix together.