What type of bonds connect complementary bases across the two DNA strands?

• A. Covalent bonds
• B. Ionic bonds
• C. Hydrogen bonds
• D. Metallic bonds

Answer: (C)

Hydrogen bonding between complementary bases across the two DNA strands stabilizes the double helix. Adenine pairs with thymine using two hydrogen bonds, and guanine pairs with cytosine using three hydrogen bonds. These are noncovalent interactions—strong enough to hold the strands together yet weak enough to separate during replication and transcription. Covalent bonds link nucleotides within each strand (the sugar–phosphate backbone), not between the strands. Ionic and metallic bonds do not participate in standard base pairing.