Rosalind Franklin used which technique to provide evidence for the DNA double helix?

• A. X-ray diffraction
• B. Electron microscopy
• C. NMR spectroscopy
• D. Ultraviolet spectroscopy

Answer: (A)

X-ray diffraction of DNA crystals can reveal the three-dimensional shape of the molecule. When X-rays pass through regularly arranged atoms in a crystal, the waves scatter and form a pattern that depends on how the atoms are spaced and organized. For a helical molecule like DNA, this scattering creates a distinctive cross-shaped pattern and specific spacings that indicate a uniform, right-handed helix with a consistent width.

Rosalind Franklin’s X-ray diffraction images showed these telltale features clearly, including evidence for about 2 nanometers in diameter and a repeating pattern every 3.4 angstroms along the strand, consistent with ten base pairs per turn. From these diffraction hints, scientists could infer that DNA is a double helix with two strands wound around each other.

Other techniques don’t provide that same direct structural signature. Electron microscopy can suggest overall shape but doesn’t produce the precise diffraction pattern that reveals helical symmetry; NMR spectroscopy works best with smaller molecules in solution and provides different kinds of structural details; ultraviolet spectroscopy measures light absorption related to bases, not the 3D arrangement of the strands.