What type of bond holds nitrogen bases together across the center of the DNA double helix?

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

Answer: (D)

Hydrogen bonds hold the nitrogen bases together across the center of the DNA double helix. These weak, non-covalent interactions form between complementary bases—A with T via two hydrogen bonds and G with C via three—allowing the two strands to unzip for replication and transcription while maintaining overall stability. The backbone of DNA is held together by covalent bonds between sugar and phosphate along each strand, not between the bases. Ionic bonds aren’t used in base pairing, and peptide bonds link amino acids in proteins, not bases in DNA.