The backbone of the DNA molecule is made up of which parts?

• A. Phosphate groups and deoxyribose sugar
• B. Nitrogen bases
• C. Hydrogen bonds
• D. Water molecules

Answer: (A)

The backbone of DNA is the sugar-phosphate chain that runs along the outside of the molecule. Each nucleotide contributes a deoxyribose sugar and a phosphate group, and these units are linked by phosphodiester bonds to form a repeating sugar-phosphate backbone. This outer framework supports the double-helix structure, while the nitrogenous bases project inward and pair with each other through hydrogen bonds.

So, the correct part is the phosphate groups and deoxyribose sugar. The nitrogen bases sit inside as the pairs that encode genetic information, hydrogen bonds connect those base pairs, and water is just the surrounding solvent—not part of the backbone.