The backbone of DNA is made up of alternating sugar and phosphate groups. Specifically, the sugar in DNA is deoxyribose, a five-carbon (pentose) sugar. These sugar and phosphate units are linked together by phosphodiester bonds, forming a sugar-phosphate backbone that provides structural support to the DNA molecule

. In the double helix structure of DNA, two sugar-phosphate backbones run antiparallel to each other and twist around to form the characteristic helical shape. The nitrogenous bases (adenine, thymine, guanine, and cytosine) attach to the sugar molecules and form the "rungs" of the DNA ladder, while the sugar-phosphate backbone forms the "sides"

. To summarize:

• The backbone consists of alternating deoxyribose sugars and phosphate groups.
• These are connected by strong phosphodiester bonds.
• The backbone is negatively charged and hydrophilic, allowing interaction with water.
• The directionality of DNA strands is defined by the 5' phosphate end and 3' hydroxyl end of the sugar molecules

This sugar-phosphate backbone is essential for the stability and integrity of the DNA molecule.