What type of bond links the sugar and phosphate groups along the DNA backbone?

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

Answer: (B)

The backbone is held together by covalent phosphodiester bonds, linking the sugar of one nucleotide to the phosphate of the next to form a continuous sugar–phosphate chain that runs 5' to 3'. This strong covalent connection gives the backbone its stability while the bases pair through hydrogen bonds across strands to form the double helix. Hydrogen bonds are what hold base pairs together, not the backbone, so they sit between the two strands. Peptide bonds connect amino acids in proteins, not nucleotides, and ionic bonds do not form the DNA backbone.