The DNA double helix seen, for the first time, through an electron microscope

The Istituto Italiano di Tecnologia researchers have been able to capture, by Transmission Electron Microscopy, the direct and background-free image of the DNA Double Helix, paving the way to the direct study of the interaction between DNA and proteins, RNA and other molecules.

The DNA Double Helix has been seen, for the first time, through an electron microscope image, thanks to a technique invented by a group of researchers of the Department of Nanostructures and Nanochemistry of the Istituto Italiano di Tecnologia (IIT), based in Genoa, in collaboration with the University of Magna Grecia (Catanzaro, Italy). The researchers, coordinated by Enzo Di Fabrizio, developed a method that allowed them to stretch DNA filaments, throughout all their double helix structure, on a particular silicon surface, permitting them to directly image it by a transmission electron microscope in a completely background-free way. The results have been published on the International journal Nanoletters, with the title “Direct Imaging of DNA fibers: The Visage of Double Helix”.

The study of single molecules, or of small quantities of molecules, is important for the understanding of fundamental biological mechanisms at the nanoscale level. The technique developed at the Istituto Italiano di Tecnologia will allow us to see how proteins, RNA and other biomolecules interact with DNA. “Our research originated from the awareness that, to deepen the knowledge of the way DNA works, it is necessary to have new tools, allowing us to show in a direct way its structure and its functions, both in the coding and non-coding portions” explains Enzo Di Fabrizio, coordinator of the study and Director of the Nanostructures Department of IIT. These tools originate from the advanced developments of nanotechnologies: a well-established imaging technique such as transmission electron microscopy is now coupled with the novel and improved capability of building and controlling artificial structures on a nanoscale level.

The researchers built a device made up of a silicon surface, with regularly spaced micro-pillars rising from it, interspersed by holes. The micro-pillars confer to the device the characteristic of super-hydrophobicity, whilst the holes allow the electrons to cross the sample to reach the microscope detector undisturbed, that is with no interaction with the silicon surface. This experiment requires a very complex procedure: enclosing the DNA strands in a drop of solution; laying the drop on the device which, thanks to the micro-pillars, sustains its shape leaving the strands intact; allowing the solution to evaporate slowly and using the electron microscope to image the result. In particular, during the evaporation procedure, the connective movements inside the drop stretch the DNA strands, arranging them on the micro-pillars. At the end of the evaporation, DNA is then suspended between the micro-pillars and ready to be irradiated with the electron beam by the microscope.

The result has been obtained on strands made of six molecules, coiled around a seventh one. In the near future, the development of electron detectors 10–100 times more sensitive than those currently available, will allow to directly image single and double DNA helices with the aim to study directly both epigenetic phenomena and the information contained in the non-coding portions of DNA.

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