Where are the nitrogenous bases located in DNA?

• A. On the outside of the double helix.
• B. Floating in the sugar-phosphate backbone.
• C. Attached to lipids.
• D. On the inside of the double helix.

Answer: (D)

The nitrogenous bases in DNA are located on the interior of the double helix, facing inward toward the center between the two strands. Each base is attached to a deoxyribose sugar, and the sugar-phosphate backbone wraps around the outside of the molecule, forming the stable exterior. The bases pair across the two strands (A with T, C with G) through hydrogen bonds, creating the rungs of a ladder-like structure inside the helix. This arrangement—bases inside, backbone outside—allows specific base pairing and stable replication of the genetic material.