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Introduction

Viruses are infectious agents that are obligate intracellular parasites because they must replicate inside a host cell, utilizing its macromolecular machinery and energy supplies for their replication process. The infectious form of a virus, the viral particle or virion, replicates itself by entering a host cell, disassembling itself and copying its components, which are then assembled into progeny virus These progeny virus particles can then infect additional cells. Survival of the virus requires transport of the genetic material from an infected cell to an uninfected cell in either the same or a new host organism. To accomplish this, viruses have evolved mechanisms of packaging their genomic nucleic acids, ribonucleic acid (RNA) or deoxyribonucleic acid (DNA), along with any other components necessary for replication, within protein coats composed of repeating protein subunits. The protein coat is called the capsid, and the complex of the genome plus capsid is called the nucleocapsid. Capsids can be classified into three general classes based on the symmetry of the protein arrangement within the capsid. The first form of symmetry is helical, where the subunits are wrapped around a central axis (Figure 1a). The second is icosahedral, resulting in a spherical particle with 2-, 3- and 5-fold axes of symmetry (Figure 1b). The third general class contains more In some viruses this complex is surrounded by a lipid membrane and associated proteins, a structure called the viral envelope. Thus, we can broadly divide viruses into five classes based on structure: helical nonenveloped virions (e.g. tobacco mosaic virus), helical enveloped virions (e.g. rabies virus: Figure 1a), icosahedral non-enveloped virions (e.g. adenovirus), icosahedral enveloped virions (e.g. herpes simplex virus), and other more complex structures (e.g. pox viruses such as smallpox and vaccinia virus) (Sander et al., 2002).

Despite the fact that these viral structures are made from only a few different proteins (in some cases only one), they must be able to perform a wide variety of functions. virus must successfully package all of the viral nucleic acids and proteins necessary for a stable, infectious viral particle. This particle must exit the cell and survive in the extracellular environment until it encounters a suitable host cell. The virion must then bind to and enter a new host cell for replication. Thus, the viral particle must be stable extracellularly but readily disassembled upon entry into a new host cell. Some of the best-understood examples of these structure-function relationships will be reviewed in this article.

Source: https://onlinelibrary.wiley.com/doi/abs/10.1002/9780470015902.a0001091.pub2