Which description best characterizes antiparallel orientation of the DNA strands?

• A. The two strands run in the same direction
• B. The sugar-phosphate backbones face opposite directions but the strands are aligned
• C. The bases pair with identical directionality
• D. The strands run in opposite directions along the sugar-phosphate backbones

Answer: (D)

Antiparallel orientation means the two DNA strands run in opposite directions along their sugar-phosphate backbones. Each strand has a 5' end and a 3' end, so one strand runs 5' to 3' in one direction while the other runs 3' to 5' in that same region. This opposite directionality is essential for proper base pairing (A pairs with T, G with C) and for enzymes like DNA polymerase, which add nucleotides in the 5' to 3' direction using a 3' to 5' template. Descriptions that suggest the strands run in the same direction, or that bases pair with identical directionality, don’t capture how the geometry and chemistry of the double helix actually work.