Question

Read the article and use the information to answer the questions that follow.

Discovering the Structure of DNA

Explain how the discoveries by Rosalind Franklin helped Watson and Crick build an accurate model of DNA.

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.
What information did you include in your response?

In 1952, Rosalind Franklin produced an image of DNA that suggested it contained two strands twisted in a double helix with a phosphate backbone and bases inside.

Franklin calculated the diameter of DNA, the distance between strands and bases, the angle of the helix, and the number of bases per coil.

Watson and Crick used this information to build and accurate model of DNA.

Answer 1

I included these specific facts and points:

- Photo 51: Franklin’s X‑ray diffraction image produced the characteristic X‑shaped pattern, showing DNA is a helix and a regular, repeating structure.
- Two‑strand double helix: her work indicated DNA consists of two strands twisted together (a double helix) with the phosphate–sugar backbone on the outside and the bases on the inside.
- Quantitative measurements she derived: about 20 Å (2 nm) diameter, 3.4 Å between stacked bases, 34 Å per helical turn, and roughly 10 bases per turn (and the helix angle/geometry implied by those numbers).
- How Watson & Crick used it: these physical constraints let them place the backbones outside, arrange bases inside in paired, complementary fashion, and build the accurate double‑helix model.

(Also implied: Franklin’s measurements and image supplied the crucial constraints that made the correct model possible.)