Who proposed the double-helix structure of DNA and what evidence supported it?

• A. Linus Pauling Proposed the Double Helix, Supported by Base-Pairing Rules
• B. Erwin Chargaff Proposed The Double Helix, Supported by X-Ray Evidence
• C. Rosalind Franklin Proposed The Double Helix, Based on X-Ray Data
• D. Watson and Crick Proposed the Double Helix, Supported by Chargaff's Rules and Rosalind Franklin's X-Ray Diffraction Data

Answer: (D)

Watson and Crick are the scientists who proposed the double-helix structure. The evidence backing their model came from two crucial lines of data. Chargaff’s rules showed that the amounts of adenine and thymine are about equal and the amounts of cytosine and guanine are about equal, implying A pairs with T and C pairs with G—a foundation for complementary base pairing and faithful replication. Rosalind Franklin’s X-ray diffraction results revealed a helical shape with a consistent diameter and a regular pattern along the length, providing the geometric constraints (two antiparallel strands with a sugar–phosphate backbone on the outside and base pairs inside) that the model had to fit. The combination of these insights allowed Watson and Crick to construct the accurate double-helix model. The other options point to pieces of the story (Pauling’s incorrect triple-helix idea, Chargaff’s rules alone, Franklin’s data alone) but do not capture the proposed structure and the corroborating evidence together.