This introductory chapter begins with an appreciation of the unique position of electron microscopy in biological research as it bridges a wide gap between X-ray crystallography and light microscopy. The scope of this book is defined to cover three-dimensional imaging of molecular assemblies that exist in an in vitro sample in large numbers with identical or near-identical structure. Only with such samples it is possible to collect large numbers of projection images suitable for averaging and three-dimensional reconstruction. The fact that molecules can be imaged as single, isolated particles embedded in ice (“crystallography without crystals”) makes the techniques described in this book uniquely suited to image molecular machines in their various processing states. In sharp contrast, electron tomography, not covered in this book, is concerned with the three-dimensional imaging of “unique” objects that may be an organelle or slice of a cell. The vision of a unified structural analysis of macromolecules is articulated, which would lead to an integration of results from cryo-EM, X-ray crystallography and NMR, and a cross-fertilization among these disciplines. The chapter concludes by making the point that the development of single-particle reconstruction would not have been possible without the vast increase in computer power seen in the past decades.
Keywords: cryo-electron microscopy, crystallography without crystals, electron tomography, light microscopy, molecular asemblies, single-particle reconstruction, X-ray crystallography, three-dimensional reconstruction
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