Saturday, June 2, 2012
A virion is a virus in its extracellular stage. It contains nucleic acid which is surrounded and protected by a protein coat or capsid. All virions are extremely small, but there is a considerable range of sizes among them. One of the largest virions is that of the cowposx, which was used in vaccination against smallpox. The virion is a cube about 20 nanometers on a side. On the other hand, a small virion may be only 2 nanometers in diameter. Virions have very simple morphologies, since most of them are composed not of millions of molecules of thousands of different kinds, as are cells, but of only a few hundred molecules of several different kinds.