So how did they know it was there all along?
James Watson and Francis Crick in 1953 suggested a double helical
structure for the DNA molecule based on informed guesses arising from interpretation of vague X-ray diffraction
image of the deoxyribonucleic acid (DNA) molecule.
But after more than half a century after Watson and Crick, imaging techniques, using X-ray diffraction analysis (or crystallography
), have improved and scientists have acquired over the decades, an improved and clearer visualization of the molecule.
But unlike a plain photograph of an object that shows you what it looks like exactly, X-ray diffraction analysis is based on interpretation of the manner in which the molecule deflects, or more appropriately, scatters X-rays, creating what technicians call an X-ray diffraction pattern. Thus, as the The Huffington Post
points out, an X-ray diffraction pattern is not really a photograph of the structure under study, but patterns that can be used to build a picture of what the structure looks like.
According to the New Scientist
, X-ray diffraction analysis of DNA structure involves X-rays scattering off atoms in "crystallized arrays of DNA to form a complex pattern of dots on photographic film. Interpreting the images requires complex mathematics to figure out what crystal structure could give rise to the observed patterns."
explains that X-ray diffraction/crystallography "remains a workable, and powerful, technique for visualizing DNA strands. But crystallography creates its own kind of rendering: It's a technology whose imaging power relies on diffracted light. When we look at those now-iconic images of the double helix... we're not seeing the DNA itself so much as we're seeing x-rays deflected from its atoms. "
But now for the first time, scientists in a new paper published in the journal Nanoletters
, have succeeded in capturing actual photographs of DNA. According to Discovery News
, Enzo di Fabrizio, researcher at the University of Genoa, Italy, has developed a technique that stretches out strands of DNA between silicone pillars, and then photographs them using an electron microscope
The image below shows a thread of DNA suspended on a bed of nanoscopic silicon pillars using a process developed by Enzo di Fabrizio
and his team. According to the New Scientist
, the researchers recovered strands of DNA from a dilute solution using water-repellent silicon nanopillars that cause moisture to evaporate quickly leaving dry strands, much like you may spread out mushy material to dry in the sun. They drilled nano-holes at the base of the beds and though the holes passed beams of electrons that captured reasonably high resolution images of the DNA thread.
The New Scientist
explains that the technique was used to capture the image of seven strands of DNA wrapped in a "cord" because the electrons emitted by the microscope were too powerful to be used to capture an image of a single DNA strand without destroying it in the process.
The Huffignton Post
reports that Di Fabrizio has suggested that the technical difficulties encountered in capturing the image of a single DNA strand could possibly be solved by using a lower-power electron microscope. He said: "With improved sample preparation and better imaging resolution, we could directly observe DNA at the level of single bases."
The team has published the results of their work in the journal Nanoletters
. New Scientists
says the new technique "represents a significant step forward for nanobiology and all the fields connected to it, giving scientists a new way to understand DNA. Particularly when it comes to its structure."
The authors wrote
: "Direct imaging becomes important when the knowledge at few/single molecule level is requested and where the diffraction does not allow to get structural and functional information."
Scientists are excited at the development because further refinement will allow them to literally watch proteins, RNA and other biomolecules interact with DNA.