In the DNA double helix, which bonds hold the strands together and which bonds compose the backbone?

• A. Hydrogen bonds connect strands; Covalent bonds connect the sugar‑phosphate backbone
• B. Covalent bonds connect strands; Hydrogen bonds connect the backbone
• C. Hydrogen bonds connect the backbone; Covalent bonds connect base pairs
• D. Ionic bonds connect strands; Hydrogen bonds hold the backbone

Answer: (A)

DNA’s stability comes from two different kinds of bonds serving two roles. The two strands stay together through hydrogen bonds forming between complementary bases: A pairs with T via two hydrogen bonds, and G pairs with C via three hydrogen bonds. These non-covalent interactions are weak enough to allow the strands to separate during replication and transcription, yet specific enough to keep the strands aligned.

The sugar–phosphate backbone that runs along each strand is held together by covalent phosphodiester bonds between the sugar of one nucleotide and the phosphate of the next. These strong covalent links give the backbone its rigid, continuous structure and define the 5′ to 3′ directionality.

So, hydrogen bonds connect the two strands, while covalent bonds compose the sugar–phosphate backbone.