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Discovering the Structure of DNA
Explain how the discoveries by Rosalind Franklin helped Watson and Crick build an accurate model of DNA.
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Rosalind Franklin’s X‑ray diffraction work gave the crucial physical measurements and features that made Watson and Crick’s double‑helix model possible. Key contributions:
- Her X‑ray image “Photo 51” produced the distinctive X-shaped pattern that showed DNA is a helix and indicated it is a regular, repeating structure.
- She measured the helix dimensions: a repeat (pitch) of 34 Å per turn and a spacing of about 3.4 Å between stacked bases, and a diameter of about 20 Å. These numbers fixed the geometry Watson and Crick had to match.
- Her analysis showed the B form was a right‑handed helix with the sugar‑phosphate backbone on the outside and the bases stacked toward the center, ruling out models that placed phosphates internally.
- Knowing the base spacing and helix diameter allowed Watson and Crick to arrange complementary bases (using Chargaff’s rules) as paired, planar “rungs” inside the helix and to set the strands antiparallel so the chemistry and dimensions fit.
In short, Franklin’s precise X‑ray data and interpretation supplied the geometric constraints and structural clues that made the correct double‑helix model possible.
- Her X‑ray image “Photo 51” produced the distinctive X-shaped pattern that showed DNA is a helix and indicated it is a regular, repeating structure.
- She measured the helix dimensions: a repeat (pitch) of 34 Å per turn and a spacing of about 3.4 Å between stacked bases, and a diameter of about 20 Å. These numbers fixed the geometry Watson and Crick had to match.
- Her analysis showed the B form was a right‑handed helix with the sugar‑phosphate backbone on the outside and the bases stacked toward the center, ruling out models that placed phosphates internally.
- Knowing the base spacing and helix diameter allowed Watson and Crick to arrange complementary bases (using Chargaff’s rules) as paired, planar “rungs” inside the helix and to set the strands antiparallel so the chemistry and dimensions fit.
In short, Franklin’s precise X‑ray data and interpretation supplied the geometric constraints and structural clues that made the correct double‑helix model possible.
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