Which statement best describes the geometry of DNA's two strands?

Which statement best describes the geometry of DNA's two strands?

• A. Parallel double helix
• B. Antiparallel double helix
• C. Single strand
• D. Triple helix

Answer: (B)

DNA's two strands form an antiparallel double helix: they run in opposite directions, with one strand oriented 5' to 3' and the other 3' to 5'. The sugar-phosphate backbones are on the outside, while complementary bases pair in the interior (A with T, G with C), creating a uniform width and stable structure. This arrangement is crucial for replication and enzyme activity, since polymerases add nucleotides to the 3' end and rely on opposite-ward orientation to read one strand while building the other. A parallel double helix would misalign base pairing and the geometry, a single strand cannot form the double-helix shape, and a triple helix is not the standard form of cellular DNA.

DNA's two strands form an antiparallel double helix: they run in opposite directions, with one strand oriented 5' to 3' and the other 3' to 5'. The sugar-phosphate backbones are on the outside, while complementary bases pair in the interior (A with T, G with C), creating a uniform width and stable structure. This arrangement is crucial for replication and enzyme activity, since polymerases add nucleotides to the 3' end and rely on opposite-ward orientation to read one strand while building the other. A parallel double helix would misalign base pairing and the geometry, a single strand cannot form the double-helix shape, and a triple helix is not the standard form of cellular DNA.