Which statement correctly describes the shape of the DNA double helix?

Which statement correctly describes the shape of the DNA double helix?

• A. A twisted ladder
• B. A straight, untwisted ladder
• C. A circular ring
• D. A flat sheet

Answer: (A)

DNA is best described as a twisted ladder, a right-handed double helix. Two long sugar-phosphate backbones run antiparallel like rails, with base pairs forming the rungs between them. Hydrogen bonds between complementary bases (A with T, G with C) hold the rungs together, while base stacking along the helix drives the whole molecule into a spiral shape. This twist isn’t just cosmetic—it's essential for compacting the molecule and for providing grooves that proteins recognize during processes like replication and transcription. In the common B-form, the helix turns about once every 10.5 base pairs and has a diameter of roughly 2 nanometers, producing characteristic major and minor grooves. If DNA were a straight, untwisted ladder, it wouldn’t match the observed 3D structure or the way DNA interacts with other molecules. A circular ring or a flat sheet doesn’t describe how the strands wind around each other in the double helix, which is the key feature of DNA’s architecture.

DNA is best described as a twisted ladder, a right-handed double helix. Two long sugar-phosphate backbones run antiparallel like rails, with base pairs forming the rungs between them. Hydrogen bonds between complementary bases (A with T, G with C) hold the rungs together, while base stacking along the helix drives the whole molecule into a spiral shape. This twist isn’t just cosmetic—it's essential for compacting the molecule and for providing grooves that proteins recognize during processes like replication and transcription. In the common B-form, the helix turns about once every 10.5 base pairs and has a diameter of roughly 2 nanometers, producing characteristic major and minor grooves. If DNA were a straight, untwisted ladder, it wouldn’t match the observed 3D structure or the way DNA interacts with other molecules. A circular ring or a flat sheet doesn’t describe how the strands wind around each other in the double helix, which is the key feature of DNA’s architecture.