Image processing and 3-D reconstruction in electron microscopy

Electron Microscopy (EM) is a powerful tool in structural biology as it is able to derive structural information from specimens in the whole spectrum from the cellular to macromolecular domain. Different approaches to data collection, image processing, and 3-D reconstruction are used depending on the nature of the specimen and structural information sought. For specimens in the macromolecular domain, specific methodologies have been devised: for 2-D crystalline protein arrays (the approach known as electron crystallography (EC)); for specimens assembled in helical structures; for isolated, rather asymmetric specimens (the "single particles" approach; and for highly-symmetrical specimens, such as icosahedral viruses. For complex specimens in the cellular range, the so-called "electron tomography" (ET) approach is used. With these strategies, EM is able to derive structural information from different biological specimens at a wide spectrum of resolutions, from 40-100 Å (ET) to 6-30 Å (single particles, helical, icosahedral, EC), or even atomic resolution (EC, helical).