Which technique provided crucial evidence for the helix-form DNA?

• A. X-ray Diffraction
• B. Electron Microscopy
• C. UV-Vis Spectroscopy
• D. Gel Electrophoresis

Answer: (A)

The key idea being tested is how scientists can reveal the actual three-dimensional shape of DNA. X-ray diffraction does this by directing X-rays at a sample and observing the pattern that the X-rays produce when they bounce off the regular, repeating structure of the molecule. For DNA, the diffraction pattern showed an X-shaped arrangement, which is a hallmark of a helical structure, and it allowed precise measurements of the molecule’s geometry. From the pattern, scientists could infer that base pairs stack about 0.34 nanometers apart, that there are roughly 10 base pairs per turn, giving a pitch of about 3.4 nanometers per turn, and that the helix has a consistent diameter. These specific, periodic spacings and the distinct helical pattern are what made X-ray diffraction so decisive in establishing DNA as a double helix.

Other techniques don’t provide this kind of structural detail. Electron microscopy can visualize larger-scale shapes but, at the time, didn’t resolve the fine, periodic features of the DNA helix with enough precision. UV-Vis spectroscopy tells us about concentration and base stacking phenomena, not the molecule’s three-dimensional geometry. Gel electrophoresis separates molecules by size and charge, not by their spatial arrangement.