Biology and Chemistry
Cheese! Using Diffraction To Take Pictures of DNA
We remember from biology class that DNA is a double helix, but how do we know? DNA is too small to be able to see it. We may be able to see single cells of bacteria under a microscope, but DNA is much tinier than that.
We took a photo of it. That's how.
Rosalind Franklin was the scientist who took the first photo of DNA, not with a Polaroid or digital camera, but using X-rays, which are very short wavelength light. She knew that shining light on something small causes diffraction, as the light makes its way around and through that object,. It's much like the Double Slit Experiment, but in the case of DNA a more complicated pattern results.
When we observe how light diffracts, we can tell something about the shape of the object changing the path of the light. This is exactly what Rosalind did. She made crystals of DNA and studied how these crystals diffract X-rays.
She didn't use visible light, as Young did in the Double Slit Experiment, because visible light's wavelength is too big to be affected by a crystal of DNA. Imagine a tiny pebble, the size of a grain of sand, lying on the beach. A wave comes and washes over this grain. Does the path of the wave change? By looking from far away, could we tell that the wave encountered the tiny pebble? No, and no. It's the same idea.
What if we replaced that tiny pebble with a rock the size of a Lazyboy chair? Would the path of the wave change? Yes. For an object to alter the path of a wave, the sizes of the wavelength and object have to be really close to each other. A 1 m water wave won't be impacted by a 1 mm pebble, but it sure will interact with a 3 m Lazyboy. When that wave hits that recliner, we'll definitely see a diffraction pattern.
The same is true for light. DNA is so tiny that visible light would not interact with it enough to generate a diffraction pattern. Rosalind used light of a smaller wavelength, X-rays, which are near in size to the width of DNA.
Biologists (and physicist and chemists) use this method of taking pictures, called X-ray crystallography, for very small things. Biologists use X-ray crystallography to snap shots of not only DNA but also proteins. By taking photos of the proteins, we can learn things about how they work, and just as importantly, why some faulty proteins cause syndromes and conditions.