Which scientist used X-ray crystallography to determine the helical shape of DNA?

• A. Watson and Crick
• B. Franklin
• C. Chargaff's Rule
• D. Mendel

Answer: (B)

Rosalind Franklin used X-ray crystallography to study DNA and produced diffraction images that revealed a repeating helical shape. The distinct X-shaped pattern in her X-ray images indicated a regular helix, and careful analysis provided details about the width of the helix and the spacing of its components. This direct structural evidence is what established that DNA has a helical form and guided the later construction of the DNA double-helix model, which Watson and Crick completed using Franklin’s data. The other options relate to different concepts: base-pairing rules don’t describe three-dimensional shape, Mendel studied inheritance patterns in peas, and Watson and Crick built the model but did not use X-ray crystallography themselves to reveal the helix. So the scientist who used X-ray crystallography to reveal the helical shape is Rosalind Franklin.