What DNA Actually Looks Like


We all learned about DNA with graphic illustrations of a colorful double helix. Now we have an actual photograph, from an electron microscope, of a DNA strand. Enzo di Fabrizio and a team at the University of Genoa in Italy developed a technique to isolate and dehydrate a single strand of DNA, seen here stretched between two microscopic silicon pillars. Read about how they did it, and see a zoomed-in image at the Atlantic. Link

(Image credit: Enzo di Fabrizio)

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I went to the Link and like others I found the top comment to be more interesting than the actual article there.

I am copying said comment, authored by A_Lee here:
"Not to be a downer, but while you are indeed looking at DNA, you aren't looking at a single strand of DNA. Whoever told you that is mistaken.

If any one is interested in the details, the first image (which looks more like a photograph), is a scanning-electron micrograph (SEM) image. The second, which looks more 2D and grainy, is a transmission-electron micrograph (TEM) image.

First, a description of what you are looking at. You are looking a a bundle of DNA fibers. Their are seven DNA strands, six in a hexagonal wrapping around one core. The reason we know this is because (a) the paper says so, and (b) the strand is far too thick to be one single strand. It's about 20 nanometers thick, which is too thick to be single-strand DNA.

Now, the images.

The SEM image shows the DNA strand suspended over two pillars. It's almost certainly coated with some metal, probably gold-palladium. It's taken at a fairly low resolution, the scale bar is probably in the 1-10 micron range, and the electron beam energy is very low, probably around 1-5keV. At this magnification and this energy level, you cannot actually "see" the DNA strand, anymore than we can "see" stars in the sky. We simply perceive a bright object against a dark background, but we don't have the resolution to distinguish between one star and two stars right next to each other.

The second image is a TEM image. TEM works like a slide projector, shining electrons through the object, and projecting them on the other side. It works with interference patterns (remember, quantum effects dominate here). So it is very good at finding tight periodic structures. In this case, the aligned "curls" of the DNA. What you are seeing is the change in atomic density along the DNA bundle. Imagine a super-long gummy worm that alternates between very short red and green segments. Now put seven of them together in a bundle, making sure that each colored segment matches up. Then step away, and you'll perceive a single, thick, gummy worm with alternating red and green lines. That's what you're seeing here.

Still, imaging organic molecules is always difficult with electron beams, because they tend to fry them. So this is still quite an achievement."
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