Question: How do we know what DNA and genes really look like?
We see images in books that look like this, but each individual atom is only a nanometer (1 x 10 -10 m) wide.
No visible light microscope can view objects made with such small pieces.
So the real way that we figured out the atom-by-atom structure of DNA is through a technique called X-ray crystallography.
Our molecule of interest – in this case, DNA – is concentrated and crystallized.
It is placed in front of an X-ray source.
The X-rays scatter off the DNA’s atoms. We capture this diffraction pattern on film (or on a digital X-ray detector.)
This diffraction pattern is beautiful but doesn’t directly look like the original molecule.
There is a mathematical relationship between the placement of the atoms, and where the atoms deflect – just like there is a relationship between hitting pool balls and how they deflect:
When you know how a pool table is set up, what balls are made of, and see how the balls move after being it, you could use math to work backwards to figure out where the balls originally where.
The same is true here: We can use math to figure out where each individual atom in the DNA is! Let’s follow the steps below:
On the left, we see X-rays leave a source. Some of these x-rays pass through a lead screen.
The X-rays hit a crystallized DNA sample.
The X-rays bounce off of the molecules, like how pool balls bounce off of each other.
Some of the x-rays bounce onto a film plate. This makes an image.
We end up with a diffraction pattern on film.
Once we have a diffraction pattern, we then use math to work backwards, and figure out where the atoms must have been.
The result is an electron density map which almost exactly traces out the shape of the molecule.
Can we image DNA more directly?
Yes. One can use a scanning tunneling microscope (STM).) It shows detail at the the atomic level. Along with the following image please read Livescience: DNA directly-photographed-for-first-time.html
Here is another STM image of DNA. You can see how closely it matches the model from X-ray crystallography.