The Structure of DNA — X-ray Crystallography and Rosalind Franklin's Photo 51

Discovery

Technical Explanation

1. X-ray diffraction — A hydrated DNA fiber is exposed to a collimated X-ray beam (λ ≈ 0.154 nm, Cu Kα line). The atoms in the molecule diffract X-rays according to Bragg's law (2d·sin θ = nλ). The diffraction pattern, recorded on photographic film, encodes the three-dimensional structure. A helix produces a characteristic "Maltese cross" pattern.

2. Photo 51 (Franklin, May 1952) — Rosalind Franklin obtained Photo 51 of the B-form (hydrated) of DNA with a 62-hour exposure. The X-shaped cross unambiguously indicates a helical structure. The layer spacing (meridional reflection at 3.4 Å) reveals the distance between base pairs. The absence of the 4th layer line indicates two strands (not one, not three). The 20 Å diameter is deduced from the angle of the cross.

3. Chargaff's rules — Erwin Chargaff showed (1950) that in any DNA, the amount of adenine equals thymine (A=T), and guanine equals cytosine (G=C). Watson and Crick understood that these pairings (2 hydrogen bonds for A-T, 3 for G-C) explain the complementarity of the strands and enable replication: each strand serves as a template.

4. Molecular modeling — Watson and Crick built physical models (metal rods and plates) respecting bond angles, interatomic distances (data from Jerry Donohue on the correct tautomeric forms of the bases), and the constraints of Photo 51. The final structure simultaneously satisfied stereochemistry, the diffraction data, and Chargaff's rules.

Why It Worked

The convergence of three independent approaches: crystallography (Franklin), quantitative biochemistry (Chargaff), and three-dimensional modeling (Watson-Crick). No single method sufficed. Photo 51 provided the geometric parameters; Chargaff's ratios dictated the base pairing; physical modeling verified stereochemical consistency. The structure immediately explained the replication mechanism — Watson and Crick noted in their paper: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

This was not just the discovery of a molecular structure — it was the birth of molecular biology. A single one-page paper in Nature unlocked the logic of heredity, gene expression, and ultimately the entire biotechnology revolution.

Causal Chain

Discovery of DNA (Miescher, 1869) → DNA carries genetic information (Avery, 1944) → Chargaff's rules A=T, G=C (1950) → Franklin's Photo 51 (1952) → Watson-Crick double helix (1953) → Genetic code (Nirenberg, 1961) → Restriction enzymes (1970s) → Sanger sequencing (1977) → PCR (1983) → Human Genome Project (2003) → CRISPR-Cas9 (2012)

Anecdote

Watson and Crick never obtained Franklin's consent to use Photo 51. Maurice Wilkins showed them the photograph without her knowledge in January 1953. Franklin, who was independently working on the same structure, was never informed. She died of ovarian cancer in 1958 at age 37 — possibly linked to X-ray exposure. The 1962 Nobel Prize was awarded to Watson, Crick, and Wilkins.

Sources

References verified during the August 2026 fact-checking audit: these are the pages
against which this bulletin's claims were checked.

1. Rosalind Franklin's famous Photo 51 (mai 1952, avec Raymond Gosling) — UKRI
2. Cliché 51 — Science History Institute, collections numériques
3. Milestone: 20 000 structures deposited in the PDB in 2025 — PDBe