What makes up the backbone (sides) of a DNA molecule?

• A. Hydrogen bonds
• B. Adenine and Thymine
• C. Ribose sugar and a phosphate group
• D. Deoxyribose sugar and a phosphate group

Answer: (D)

The backbone of DNA is the sugar-phosphate framework that forms the rails of the double helix. It is made up of deoxyribose sugar molecules linked to phosphate groups by phosphodiester bonds, giving the long, repeating sugar–phosphate chain that provides structure and stability. The bases (adenine, thymine, cytosine, and guanine) hang off this backbone and pair across strands via hydrogen bonds, which hold the two strands together inside the helix. The sugar in DNA is deoxyribose, not ribose, which is found in RNA. Adenine and thymine are bases, not components of the backbone, and hydrogen bonds connect base pairs rather than forming the backbone itself.