VIRUSES WITH HELICAL SYMMETRY

Abstract

This chapter discusses the structure and dimensions of virus with helical symmetry, tobacco. These viruses are mainly grouped in rod-shaped viruses (Rigidoviridales) and filamentous viruses (Flexiviridales). Both of these groups include viruses devoid of any envelope or additional membrane system and many are associated with plant diseases. Although the rod-shaped and filamentous particles of the type appear to fall neatly into a helical structural form, there are some other rod-shaped or elongated viruses that raise pertinent questions about whether or not they should be considered as helical structures or extended icosahedra. One of these controversial examples is the multicomponent virus, alfalfa mosaic virus. Other viruses have certain structural characteristics that can be considered to belong to the quasi-helical symmetry group, but they are enclosed in an envelope that is itself complex. The influenza and parainfluenza viruses are the most important examples of this form and they are considered under the heading of myxoviruses. The myxoviruses are separated into two primary subgroups, namely, influenza and parainfluenza, because of the different biological and structural characteristics.