What base pairing rules and bonds stabilize the DNA double helix?

• A. A pairs with T via two hydrogen bonds; G pairs with C via three hydrogen bonds; DNA is stabilized by covalent bonds between bases.
• B. A pairs with T via two hydrogen bonds; G pairs with C via three hydrogen bonds; the sugar-phosphate backbone provides structural support.
• C. A pairs with C; G pairs with T; hydrogen bonds do not stabilize the helix.
• D. A pairs with G; T pairs with C; base pairing is not hydrogen-bonded.

Answer: (B)

Understanding base pairing and the forces that hold DNA together is key. Adenine pairs with thymine using two hydrogen bonds, while cytosine pairs with guanine using three hydrogen bonds. These hydrogen bonds between the two strands help stabilize the double helix, while the sugar-phosphate backbone—held together by covalent bonds—gives the molecule its strong, structural framework. This combination explains why the correct choice mentions A-T with two hydrogen bonds, G-C with three hydrogen bonds, and the backbone providing structural support. It’s not the bases themselves forming covalent bonds across strands, and base pairing is indeed hydrogen-bonded.