What type of bond holds the DNA base pairs together across the two strands?

• A. Covalent bonds
• B. Ionic bonds
• C. Van der Waals interactions
• D. Hydrogen bonds

Answer: (D)

Base pairing across the two DNA strands is held together by hydrogen bonds between complementary bases. These non-covalent bonds are individually weak, but many of them together provide enough stability to keep the double helix intact while still allowing the strands to separate when needed for replication or transcription. Adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three hydrogen bonds, which explains why GC-rich regions are more stable. The covalent bonds in DNA form the backbone of each strand rather than the cross-strand links, and while van der Waals interactions help with base stacking inside the same strand, they are not the links that bind the two strands together.