What types of bonds hold the two DNA strands together in the double helix?

• A. Covalent Bonds Along The Sugar-Phosphate Backbone
• B. Ionic Bonds Between Bases
• C. Hydrogen Bonds Between Complementary Base Pairs And Base-Stacking Interactions
• D. Peptide Bonds Linking Nucleotides

Answer: (C)

Non-covalent interactions between the bases hold the two strands together. The pairing of A with T through two hydrogen bonds and G with C through three hydrogen bonds links the strands at each base pair. In addition, base-stacking interactions—the pi-stacking and van der Waals forces between adjacent bases—stabilize the overall helical structure. The sugar-phosphate backbone is held together by covalent phosphodiester bonds within each strand, not between strands. Ionic bonds between bases don’t occur, and peptide bonds connect amino acids in proteins, not nucleotides.