In the Watson-Crick model for the DNA double helix, which statement is NOT true?

• A. The 2 strands run anti-parallel to one another
• B. The base-pairing occurs on the inside of the double helix
• C. The double helix (B-form) is right-handed
• D. Two equally sized grooves that run up the sides of the helix

Answer: (D)

Understanding the structure of DNA in the Watson–Crick model: the backbones form the outside of a right-handed double helix, while complementary bases pair on the inside through hydrogen bonds, holding the two strands together in an anti-parallel orientation. In this model, the helix twists to the right and contains two grooves along its length that differ in size: the major groove is wider and the minor groove is narrower. This difference in groove size is important because it affects how proteins can interact with the DNA.

So, the statement claiming the two grooves are equally sized is not true. The other features—anti-parallel strands, base pairing inside the helix, and the right-handed B-form—are all consistent with the Watson–Crick description.