What is the opposite arrangement of the sugar-phosphate backbones in a DNA double helix?

• A. Parallel
• B. Complementary
• C. Anti-parallel
• D. Antiparallel

Answer: (D)

The arrangement being tested is how the two DNA strands orient their backbones. In DNA, the sugar-phosphate backbones run in opposite directions along the helix: one strand goes 5' to 3' while the other goes 3' to 5'. This antiparallel orientation is essential for proper base pairing and for enzymes like DNA polymerase, which add nucleotides to the 3' end during synthesis. If the strands were parallel (same direction), the geometry wouldn’t support the standard Watson-Crick pairing in the helix and would disrupt replication and transcription. Complementary describes which bases pair, not the overall backbone direction, so it doesn’t describe the orientation of the backbones. So the term antiparallel best captures how the backbones are arranged.