o An Electron-Density Map of Tobacco Mosaic Virus at 10 A Resolution

Thirty-five years ago Bernal began investigating diffraction from oriented gels of tobacco mosaic virus (TMV) (Bernal and Fankuchen, 1941). Bernal and Fankuchen realized that the diffraction diagram showed TMV to be composed of an identical array of subunits. This realization was of considerable significance in the development of molecular biology; however, the exact nature of the ordering of the subunits in TMV eluded them. This was in fact elucidated by Watson (1954). Using the theory of diffraction from a helix developed by Cochran, Crick, and Vand (1952), he was able to show that TMV is a helical array of subunits, that the virus repeated exactly after three turns of the helix, and that the number o