Describe the DNA sugar-phosphate backbone.

• A. A repeating pattern of sugar-phosphate-sugar-phosphate
• B. A chain of nitrogenous bases
• C. A sequence of base pairs
• D. A region outside the backbone

Answer: (A)

The backbone of DNA is formed by alternating deoxyribose sugar and phosphate groups, linked together by phosphodiester bonds to create a repeating sugar-phosphate pattern along each strand. This structural scaffold runs the length of the molecule, with the nitrogenous bases projecting inward to pair with the opposite strand. Describing the backbone this way highlights its role as the repeating framework that supports the DNA structure. The other ideas describe components not part of the backbone: nitrogenous bases form the interior pairs, base pairs are the rung-like connections between strands, and a region outside the backbone isn’t the backbone itself.