The backbone of DNA is formed by linking sugar and phosphate groups through which type of bond?

• A. Hydrogen bonds
• B. Ionic bonds
• C. Peptide bonds
• D. Phosphodiester bonds

Answer: (D)

DNA’s backbone is built by linking sugars and phosphate groups through phosphodiester bonds. This covalent linkage connects the 5' phosphate of one nucleotide to the 3' hydroxyl of the adjacent sugar, creating a strong sugar-phosphate chain that forms the framework of the molecule. The bases themselves stick out from this backbone and pair with their partners on the opposite strand via hydrogen bonds, which stabilize the double helix but do not form the backbone. Other bond types—like peptide bonds that join amino acids, or ionic or hydrogen bonds between bases—do not create the main sugar–phosphate backbone.