ABSTRACT
Viruses show a remarkable diversity in structure from simple nucleic acid threads, particles containing either of the two nucleic acids with structures behaving like organic molecules to highly complex enveloped multi-layered particles. Th e simplest form is infectious nucleic acid or viroid. True viruses are particulate and structured. Structures are either helically or cubically symmetrical. Helically symmetrical viruses are anisometric particles. Th ey are of diff erent forms and sizes. Forms vary from short or medium length rods, to long or very long fl exuous particles (Figures 2.1-2.4). Th ese particles oft en form liquid crystals in which rods are regularly arrayed in two dimensions. X-ray crystallography is not possible with fl exuous rods and complex particles. Cubical particles are isometric with icosahedral symmetry having 20 equal sides. Simple particles form true crystals. Th ey normally do not widely diff er in form and size. Th ere is a unique case where particles consist of twinned or geminate icosahedra. Major diff erences among these viruses lie in the patterns of the assembly of protein subunits and their physical, biochemical, genomic and biological properties. All types of particles may be mono-partite or multi-partite depending upon the type of the viruses. Some of the mono-partite viruses are: Dianthovirus, Caulimovirus, Phytoreovirus, Carmovirus, Luteovirus, Potexvirus, Capillovirus, Carlavirus, Potyvirus and Closterovirus. Comovirus, Alphacryptovirus, Betacryptovirus, Tobravirus and Furovirus are bipartite. Tri-partite viruses are: Cucumovirus, Ilarvirus, Pomovirus, Begomovirus and Varicosavirus. Alfamovirus, Ourmiavirus and Benyvirus are quadric-partite whereas Tenuiviruses are pentapartite. Complex particles are basically isometric, helical or both but the pattern of the assembly of the protein subunits is complex and lipo-protein layers envelope these particles. Th e basic symmetry in Rhabdoviruses is helical whereas it is isometric in Tospoviruses.
