DNA is far too small to see, so its shape was worked out from the way it scattered X-rays rather than from any direct picture. The famous double helix was first a pattern of smudges on film that had to be decoded with geometry.1PubMed Central. Understanding biochemistry: structure and function of nucleic acids The decisive images came from Rosalind Franklin’s lab, while Watson and Crick built the model that fit them.
How X-Rays Can Reveal a Shape Too Small to See
X-ray diffraction means shining a beam of X-rays at a material and recording how the rays scatter onto film.2King’s College London. Maurice Wilkins Atoms spaced at regular intervals scatter waves that reinforce each other in certain directions, and that produces spots and streaks. The distances between spots on the film can then be converted back into distances between parts of the molecule.
Smaller distances in a structure spread the pattern wider, so wide spacing on film means tiny distances in the molecule. X-rays suit the job because their wavelength is close to the distance between neighbouring atoms, while visible light has waves far too long to register such tiny gaps. What the X-rays actually bounce off are the electrons around atoms, so the pattern reflects where electron density sits. Heavier atoms with more electrons scatter strongly, and hydrogen is nearly invisible.
The pattern can’t simply be turned back into a picture. The film records only how bright each spot is, not the timing of the waves that made it, and that missing timing is what you would need to rebuild an image directly. So scientists work in the other direction: they propose a structure, calculate the pattern it would produce, and check whether it matches the film. That is why Photo 51 could rule shapes in or out, and why a model such as Watson and Crick’s had to be tested against it rather than read off it.
Ordinary crystallography gets its regularity from many molecules packed into a neat 3D lattice.3The Bumbling Biochemist. What the frack is fiber diffraction? And how does a blurry X Xplain DNA structure? In fiber diffraction, the regularity comes instead from the symmetry built into each long molecule itself.4The Bumbling Biochemist. What the frack is fiber diffraction? And how does a blurry X Xplain DNA structure? A fiber of DNA molecules lined up side by side can therefore give a readable pattern without forming a true crystal.5The Bumbling Biochemist. What the frack is fiber diffraction? And how does a blurry X Xplain DNA structure?
DNA needed the fiber method because its long, stringy molecules are hard to stack into a neat crystal. Stretching DNA into a thread lines the molecules up in one direction.
Making DNA Fibers Good Enough to Photograph
At King’s College London, Maurice Wilkins worked with calf thymus DNA supplied by Rudolf Signer.6Wikipedia. Maurice Wilkins Signer’s DNA was much more intact than earlier samples, so its molecules could line up well.7Wikipedia. Maurice Wilkins Wilkins found he could pull thin threads from the concentrated solution containing highly ordered DNA. With Raymond Gosling, he then showed by X-rays that DNA in these threads had a regular, crystal-like structure.8Wikipedia. Maurice Wilkins
Intactness mattered because rough preparation chops DNA into short, tangled pieces that do not line up when drawn into a thread. Long, undamaged molecules can slide alongside each other and align as the thread is pulled, which is what a sharp pattern depends on.
Wilkins had already isolated good fibers and gathered early data before Franklin arrived. She brought expertise in X-ray crystallography and careful experimental control.
Franklin and Gosling used a brass camera about the size of a fist holding small pieces of X-ray film.9Science. How did Rosalind Franklin miss the helix in her iconic DNA image? She didn’t A slender DNA fiber was fixed across a small opening at the front of the camera.10Science. How did Rosalind Franklin miss the helix in her iconic DNA image? She didn’t They aimed a fine X-ray beam into that hole for hours or days to make each image.11Science. How did Rosalind Franklin miss the helix in her iconic DNA image? She didn’t One key image needed three days and nights of exposure.12PubMed Central. Photograph 49 is the Key to Understanding the History of Rosalind Franklin’s DNA Photograph 51
The long exposures were needed because a thin fiber scatters only a tiny fraction of the beam. Those faint scattered rays have to build up on film over time. Any movement or drying during days of exposure could blur the result.
Why Water Changed Everything, The A and B Forms
Franklin built a chamber that controlled humidity around the fiber.13The Biophysical Society. Rosalind Franklin That let her study two forms separately, which are called the A-form when DNA is drier and the B-form when it is wetter.14The Biophysical Society. Rosalind Franklin Adding or removing water makes DNA switch between them.15Embryo Project Encyclopedia. “Molecular Configuration in Sodium Thymonucleate” (1953), by Rosalind Franklin and Raymond Gosling Earlier blurry pictures likely mixed both forms, since their patterns differ.16Embryo Project Encyclopedia. “Molecular Configuration in Sodium Thymonucleate” (1953), by Rosalind Franklin and Raymond Gosling
The chamber was a small enclosed space where the moisture in the air could be set and held steady, for example by passing air over particular salt solutions. Raising the moisture lets the DNA take up water and change form, and lowering it lets the water leave again.
The wet form repeats every 34 ångströms (3.4 nanometres), about 25% longer per repeat than the dry form.17PubMed Central. Photograph 49 is the Key to Understanding the History of Rosalind Franklin’s DNA Photograph 51 One sample became locked in the wet form, so it could be held taut and photographed at 75% humidity.18PubMed Central. Photograph 49 is the Key to Understanding the History of Rosalind Franklin’s DNA Photograph 51 That sample produced an unusually sharp pattern.19PubMed Central. Photograph 49 is the Key to Understanding the History of Rosalind Franklin’s DNA Photograph 51
The wet form also matters biologically. Cells are watery, so it is closer to DNA’s natural state. The drier form is more crystalline and more complex to decode, while the simpler wet-form pattern made the helix easier to recognize.
Reading Photo 51, What the X Pattern Means
Photo 51 is Franklin and Gosling’s famous image of the wet form, a clear cross of spots on film. Mathematicians had already worked out what X-ray pattern a helix should produce,20Acta Crystallographica. The structure of synthetic polypeptides. I. The transform of atoms on a helix so a cross-shaped pattern showed at once that the molecule was helical.21De Boeck Supérieur. Analysis of DNA by X-ray diffraction Steeper or shallower coils make the X more open or narrow, which is why its angle can be turned into a measure of the helix’s shape.
The measurements come straight off the film:
- Spacing between rungs: strong spots at the top and bottom show a repeat of 0.34 nanometres, the gap between stacked rungs.22De Boeck Supérieur. Analysis of DNA by X-ray diffraction
- Full turn: closer-spaced bands show the full turn repeats every 3.4 nanometres.23De Boeck Supérieur. Analysis of DNA by X-ray diffraction
- Rungs per turn: about 10 rungs fit in each turn.
- Angle: the angle of the X measures about 40 degrees.24De Boeck Supérieur. Analysis of DNA by X-ray diffraction
- Width: combining the angle and the turn length gives a radius of about 1 nanometre, a width of about 2 nanometres.25De Boeck Supérieur. Analysis of DNA by X-ray diffraction
Each band running across the picture corresponds to a repeating feature along the fiber’s length. The first band above the centre marks the full-turn repeat, and bands farther out mark smaller fractions of that spacing. Counting how far apart the layers are is how the 3.4-nanometre turn is measured.
The picture could not tell everything on its own. It showed a helix and its size but not for certain how many strands there were, and it did not directly show which chemical parts sat inside or outside. Those answers needed chemistry and model building.
Why Earlier Attempts Missed the Answer
William Astbury took a strikingly similar photograph two years before the double helix was published.26PubMed. William Astbury and the biological significance of nucleic acids, 1938-1951 Historians suggest he did not follow it up, perhaps due to limited interest in biology or a mistaken model of the gene.27PubMed. William Astbury and the biological significance of nucleic acids, 1938-1951 Having a picture is not enough: someone must interpret it with the right geometry and know what question it answers. His images were less sharp and likely mixed forms, so the clear signature of a helix was harder to pick out.
Linus Pauling relied on Astbury’s blurry patterns rather than Franklin’s sharper images.28PaulingBlog. The X-Ray Crystallography that Propelled the Race for DNA: Astbury’s Pictures vs. Franklin’s Photo 51 He proposed three chains with the phosphate backbones at the centre and the bases pointing outward.29Nature. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid But each phosphate is negatively charged, so packing them in the core would push the structure apart.30DNALC. Linus Pauling’s triple DNA helix model, 3D animation with basic narration He also packed those atoms too tightly and added extra hydrogens to cancel the charge.31Proceedings of the National Academy of Sciences. A Proposed Structure for the Nucleic Acids Charged backbones on the outside can sit next to surrounding water and salts, which pointed Watson and Crick toward a backbone-outside design.
From Pattern to Model, How the Double Helix Came Together
Watson and Crick in Cambridge used model building with data from several sources, including Franklin’s pattern and Chargaff’s base ratios.32PubMed Central. Understanding biochemistry: structure and function of nucleic acids Chargaff had found DNA contains equal amounts of A and T, and of C and G.33PubMed Central. The Discovery of the Double Helix, 1951-1953 That suggested these bases were somehow linked, and a structure in which each A always sits opposite a T and each C opposite a G would produce exactly those equal amounts. It was chemical evidence arriving from a different direction, and it fit the pairing idea without being designed for it. Then Jerry Donohue told Watson and Crick to use the more common forms of the bases, which made the pairs fit.34Omic.ly. Solving the structure of the DNA double-helix took a village
Building the models was physical work. They cut or assembled metal and cardboard pieces representing the atoms and bases, then moved them around to see what could fit. The pieces had fixed bond lengths and angles, so a model that looked plausible on paper often could not be assembled at all.
Pairing A with T and C with G explained Chargaff’s ratios.35PubMed Central. The Discovery of the Double Helix, 1951-1953 These base pairs fit neatly between two backbones winding around a shared axis, like rungs on a twisted ladder.36PubMed Central. The Discovery of the Double Helix, 1951-1953 The rungs met the backbones at right angles while the backbones kept a regular shape, as the X-ray evidence demanded.37PubMed Central. The Discovery of the Double Helix, 1951-1953
The 2-nanometre width ruled out wrong pairings. Two large bases would make rungs too wide, and two small ones would make them too narrow. Only one large with one small kept every rung the same width the X-ray showed.
The structure was published in Nature in 1953, alongside Franklin and Gosling’s data on the two forms.38Nature. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid Watson later credited Franklin’s photograph as key to the discovery, and her own notes show she had been working toward a helical structure. The story is now told as a shared effort of King’s and Cambridge.
How Later X-Ray Work Confirmed the Double Helix
Scientists later crystallized short pieces of DNA with chosen sequences.39PubMed. Crystal structure analysis of a complete turn of B-DNA Chemists can build DNA of a set sequence and length, so every molecule in a sample is identical. Identical short pieces pack into a regular lattice far more easily than long natural DNA, which is the condition ordinary crystallography needs. X-ray analysis of one crystal showed more than a full turn of right-handed B-DNA directly.40PubMed. Crystal structure analysis of a complete turn of B-DNA Later images at extremely fine detail even show water molecules and metal ions sitting in DNA’s grooves.41PubMed. Crystal structures of B-DNA reveal sequence-specific binding and groove-specific bending of DNA by magnesium and calcium
True crystals give far sharper detail than fibers, with separate spots instead of smeared patterns. Fibers showed overall shape, while crystals show each atom’s position, including how the bases stack and how the strands twist. They reveal that the helix is not perfectly uniform, since its twist and tilt vary a little along the sequence. The 1953 picture gave an average shape for the whole fiber, and the newer results matched the dimensions Franklin measured.
Why the Discovery Still Matters
Matched pairs mean each strand carries the information to rebuild the other, so the structure suggested how DNA is copied when cells divide. Structural biology and genetics have since enabled reading the genetic instructions of living things and spawned a global biotech industry.42Royal Society. Structural and molecular biology—an origins story featuring memoirs of Fellows of the Royal Society Fiber diffraction is still used today on materials from muscle to plastics.43PubMed Central. The 3D structure of fibrous material is fully restorable from its X-ray diffraction pattern