Which type of bond links the sugar-phosphate backbone of DNA?

• A. Covalent bonds
• B. Hydrogen bonds
• C. Ionic bonds
• D. Metallic bonds

Answer: (A)

The backbone of DNA is held together by covalent bonds called phosphodiester linkages. These covalent bonds connect the sugar of one nucleotide to the phosphate of the next, creating a continuous sugar–phosphate chain that forms each strand’s backbone. Because covalent bonds are strong and stable, they keep the backbone intact under normal cellular conditions and give the strand its directionality (5′ to 3′).

Hydrogen bonds, by contrast, form between complementary bases on opposite strands to hold the two strands together, not the backbone itself. Ionic or metallic bonds aren’t the primary links in the DNA backbone, either. So the correct explanation is that covalent bonds via phosphodiester linkages connect the sugar-phosphate backbone.