What was the significance of Franklin's X-ray diffraction images for DNA?

• A. They provided key evidence of a helical, uniform structure and helped determine dimensions consistent with the double helix.
• B. They showed DNA was a protein.
• C. They proved DNA was a lipid.
• D. They demonstrated DNA is a carbohydrate.

Answer: (A)

The main idea being tested is how X-ray diffraction images reveal DNA’s shape and dimensions. Franklin’s images produced a distinctive pattern that signals a helix, showing that DNA is a long, uniform fiber with a constant diameter. From the spacing in the pattern, scientists could infer important numbers: a repeat distance of about 3.4 Å per base pair and roughly 10 base pairs per turn, giving a pitch of about 34 Å. This combination of a regular, helical structure and specific dimensions strongly supports a two-stranded helical model, i.e., the double helix. Those diffraction features also help rule out models based on proteins, lipids, or carbohydrates, which would not produce the same clear helical pattern with those spacings. So Franklin’s X-ray images provided crucial, concrete evidence for a helical, uniform DNA and the dimensions compatible with the double helix.