Viruses range from 20 to 300 nm in diameter; this corresponds roughly to a range of sizes from that of the largest protein to that of the smallest cell (see Figure 2–2). Their shapes are frequently referred to in colloquial terms (e.g., spheres, rods, bullets, or bricks), but in reality they are complex structures of precise geometric symmetry (see later). The shape of virus particles is determined by the arrangement of the repeating subunits that form the protein coat (capsid) of the virus. The shapes and sizes of some important viruses are depicted in Figure 28–1.
Shapes and sizes of medically important viruses. (Reproduced with permission from Fenner F, White DO. Medical Virology. 4th ed. Academic Press. Copyright 1994 Elsevier.)
The anatomy of two representative types of virus particles is shown in Figure 28–2. The viral nucleic acid (genome) is located internally and can be either single- or double-stranded DNA or single- or double-stranded RNA.1
Only viruses have genetic material composed of single-stranded DNA or of single-stranded or double-stranded RNA. The nucleic acid can be either linear or circular. The DNA is always a single molecule; the RNA can exist either as a single molecule or in several pieces. For example, both influenza virus and rotavirus have a segmented RNA genome. Almost all viruses contain only a single copy of their genome (i.e., they are haploid). The exception is the retrovirus family, whose members have two copies of their RNA genome (i.e., they are diploid).
The nucleic acid is surrounded by a protein coat called a capsid made up of subunits called capsomers. Each capsomer, consisting of one or several proteins, can be seen in the electron microscope as a spherical particle, sometimes with a central hole.
The structure composed of the nucleic acid genome and the capsid proteins is called the nucleocapsid. The arrangement of capsomers gives the virus structure its geometric symmetry. Viral nucleocapsids have two forms of symmetry: (1) icosahedral, in which the capsomers are arranged in 20 triangles that form a symmetric figure (an icosahedron) with the approximate outline of a sphere; and (2) helical, in which the capsomers are arranged in a hollow coil that appears rod-shaped. The helix can be either rigid or flexible. All human viruses that have a helical nucleocapsid are enclosed by an outer membrane called an envelope (i.e., there are no naked helical viruses). Viruses that have an icosahedral nucleocapsid can be either enveloped or naked (see Figure 28–2).