The Origin of Icosahedral Symmetry in Viruses
Certainly one of the most intriguing facts about viruses is that the large majority of them display full icosahedral symmetry, arguably the highest and also the most esthetically-pleasing symmetry shown in Nature. The elements of icosahedral symmetry involve 6 five-fold rotation axes, 10 three-fold, and 15 two-fold. The figure to the right shows a number of examples, including the 60nm-diameter human papilloma virus at one end and 28nm CCMV near the other; similar image reconstructions for still larger viruses, up to the 100nm-diameter herpes simplex virus, are available from cryo-EM and X-ray work (Figure from Review by Baker et al.). Before the brilliant conjectures of Crick and Watson in 1956, and the pioneering structural virology principles laid out by Casper and Klug in 1962, the large majority of viruses (indeed, all apart from the minority that were cylindrical) were believed to be spherical. But spherical symmetry gave way to icosahedral as soon as sufficiently high resolution X-ray and EM studies became available.
