Which are the weakest bonds in DNA?

• A. Sugar-phosphate
• B. Sugar-base
• C. Base-base
• D. All bonds have the same strength.

Answer: (C)

In DNA, two main types of bonds hold the molecule together: strong covalent bonds along the sugar-phosphate backbone within each strand, and weaker hydrogen bonds between the bases that pair across strands. The weakest links are these base-base hydrogen bonds that connect complementary bases. They are easily disrupted during processes like replication and heating, allowing strands to separate. Even though GC pairs form three hydrogen bonds and AT pairs form two, all of these base-base interactions are still far weaker than the covalent bonds in the backbone or the covalent bonds linking sugars to bases. The sugar-phosphate covalent bonds and the glycosidic bonds within nucleotides are much stronger, so they are not the weakest.