Figure 2. Crystals Consist of Identical Objects with a Known Spatial and Angular Relationship.

(A and B) A 2D protein crystal is made up o f a layer of molecules that, in this cartoon, all have the same orientation and w hose respective locations are specified by points on a regular lattice (A). Henderson and Unwin (1975) took advantage of 2D crystals to facilitate merging data from many thousands of bacteriorhodopsin molecules, thus building up sufficient statistical definition of the signal without exposing any one molecule to more radiation than it could tolerate. Such images had to then be recorded for different crystals, tilted by different amounts, in order to obtain the data needed for a 3D density map. Rather than relying on crystalline forms, however, single-particle cryo-EM uses an ensemble of randomly dispersed macromolecules (B). Saxton and Frank (1977) demonstrated that, in principle, information about the locations and orientations of these molecules could be obtained computationally from their images. In doing so, “virtual crystals” would effectively be grown in silico, in the sense that it is then possible to merge data from many thousands of identical molecules.