Bases are linked by ________ bonds.

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

Answer: (A)

Bases in nucleic acids pair through hydrogen bonds. These are the relatively weak attractions between a hydrogen atom on one base and an electronegative atom on the facing base, which allow complementary pairing to occur without forming strong covalent links. In DNA, adenine pairs with thymine using two hydrogen bonds, and cytosine pairs with guanine using three hydrogen bonds. This specific hydrogen-bonding pattern provides the recognition needed for accurate replication and transcription, while also letting the two strands separate when needed. Covalent bonds anchor each base to its sugar in the backbone, and peptide bonds link amino acids — but the pairing between bases across strands is not covalent. Ionic bonds are not how bases pair either.