Structure

Viruses consist of a nucleic acid core containing the viral genome, surrounded by a protein shell called a capsid (Figs 4.1 and 4.2). The entire structure is referred to as the nucleocapsid. This may be ‘naked’, or it may be ‘enveloped’ within a lipoprotein sheath derived from the host cell membrane. In many viruses (e.g. orthomyxoviruses, paramyxoviruses), the ensheathment begins by a budding process at the plasma membrane of the host cell, while others, such as herpesviruses, ensheath at the membrane of the nucleus or endoplasmic reticulum.

Fig. 4.1 Viral structure (schematic).

Fig. 4.2 Scanning electron micrograph of a herpesvirus. Note the extensive outer lipid envelope and the icosahedral nucleocapsid.

The protein shell or capsid consists of repeating units of one or more protein molecules; these protein units may go on to form structural units, which may be visualized by electron microscopy as morphological units called capsomeres (Fig. 4.1). Genetic economy dictates that the variety of viral proteins be kept to a minimum as viral genomes lack sufficient genetic information to code for a large array of different proteins. In enveloped viruses, the protein units, which comprise the envelopes and are visualized electron microscopically, are called peplomers (loosely referred to as ‘spikes’).

Virus symmetry

The nucleocapsids of viruses are arranged in a highly symmetrical fashion (symmetry refers to the way in which the protein units are arranged). Three kinds of symmetry are recognized (Fig. 4.3):

Fig. 4.3 Structural components and symmetry of different viruses. (A) Naked icosahedral; (B) naked helical; (C) enveloped icosahedral; (D) enveloped helical.

• Icosahedral symmetry. The protein molecules are symmetrically arranged in the shape of an icosahedron (i.e. a 20-sided solid, each face being an equilateral triangle). Herpesviruses are an example (Figs 4.1 and 4.2).

• Helical symmetry. The capsomeres surround the viral nucleic acid in the form of a helix or spiral to form a tubular nucleocapsid. Most mammalian RNA viruses have this symmetry, where the nucleocapsid is arranged in the form of a coil and enclosed within a lipoprotein envelope.

• Complex symmetry. This is exhibited by a few families of viruses – notably the retroviruses and poxviruses.

Taxonomy

Vertebrate viruses are classified into families, genera and species. The attributes used in classification are their symmetry, the presence or absence of an envelope, nucleic acid composition (DNA or RNA), the number of nucleic acid strands and their polarity. Classification of some of the recognized families of RNA and DNA viruses is given in Table 4.1. (Note: to memorize which viruses contain DNA, remember the acronym ‘PHAD’: P is for papova and pox, H for herpes and AD for adenoviruses. Most of the remainder are RNA viruses, including the self-evident picornaviruses.)

Table 4.1 Classification of some of the viruses causing human disease

HTLV-I, human T cell leukaemia virus type I.

The following is a concise description of the families of mammalian viruses.