The story of the photograph that revealed the geometry upon which all life is based
Enzo di Fabrizio via New Scientist
There are few mysteries that even approach that of life -- where it comes from, how it replicates -- in their greatness. What could be more basic, more beautiful, and more awe-inducing than the double helix, those interlocking, winding strands that give way to all life, that we can now see an image of for the first time?
But of course we knew what DNA looked like before we could see it so directly with an electron microscope as in the top photo, which was released this week.
We've known about the double helix for 60 years, actually. And that's because James Watson and Francis Crick saw this, an X-ray image taken by Rosalind Franklin in 1952:
There is something so stunning about that moment when Watson first saw this picture and became, suddenly, the first person to understand the structure of genetic material. He later wrote:
The instant I saw the picture my mouth fell open and my pulse began to race. The pattern was unbelievably simpler than those obtained previously. Moreover, the black cross of reflections which dominated the picture could only arise from a helical structure.
Although in that image it might appear that you are seeing a little snippet of DNA, the X-ray crystallography technique that Franklin used does not, as an electron microscope does, show an image of what you would see with a naked eye if only you could see something so small. Rather, as NOVA explained in a 2003 special report, "X-rays can create pictures of minuscule structures like DNA because their wavelengths are so short that X-rays actually bounce off atoms. As if inside a microscopic pinball machine, X-rays passing through the DNA molecule ricochet off molecular structures in their path and scatter, or diffract, in different directions. As the X-rays exit the DNA, they leave behind a pattern on a piece of photographic film." In photograph 51, as the image is called, you're seeing more the shadow of the DNA than the DNA itself. An interactive on the NOVA site explains how the pattern of an X indicated a helix, and, crucially, how missing "smears" in the image at the fourth layer indicated a double helix. With that insight, Watson and Crick were able to finally build a model of DNA, and their paper, published in Nature in 1953, established the modern field of genetics.