Virology

Virus Structure

Viruses are comprised of a heterogeneous group of microorganisms that vary in size, morphology, host range, complexity, replication strategy, and pathogenesis. All viruses are obligate intracellular parasites that depend on a host cell for replication. A single virus particle is termed a virion, and the genome of a given virus can be made up of single or double-stranded RNA or DNA. For example, the West Nile virus genome is a single strand of RNA of positive polarity, while the smallpox virus genome is double stranded DNA.

There are two main classes of viral proteins: structural and non-structural proteins. The structural proteins make up the viral particle. Capsid proteins surround the genome to comprise the nucleocapsid while the matrix and envelope glycoproteins, together with the lipid envelope comprise the outer layer of enveloped viruses. Non-structural proteins [Page 1744]are involved in virus replication and other processes such as host immune evasion.

Virus Taxonomy

Similar to other biological classification schemes, the classification system for viruses includes the order, family, genus, and species. In general, virus families share common characteristics, such as type of nucleic acid, morphology, and replication strategy. Family names end in “–idae”, for example the Paramyxoviridae family. Genus names end in “–virus” and members of the same genus often share serologic cross-reactiv-ity due to common proteins recognized by antibodies. Mumps virus is a species of the Rubulavirus genus and is part of the Paramyxoviridae family.

Virus Replication

The general replication cycle of viruses can be divided into various steps: adsorption, penetration, uncoating, replication, assembly, and egress. During the first step (adsorption or attachment), a specific viral protein binds to a specific receptor on the host cell surface. For example, polio virus surface proteins bind to CD155 (also called the polio virus receptor) on host cell membranes. The attachment step is often essential for virus infection and research in the area of viral pathogenesis frequently focuses on adsorption.

Following adsorption, viruses penetrate and gain entry into a host cell either by translocation across the membrane, pinocytosis, or membrane fusion of the viral and cell envelopes. For example, the causative agent of chickenpox and shingles (varicellazoster virus) enters the host cell through fusion of the viral and host cell membranes, thereby allowing the nucleocapsid to enter the cell. Penetration is followed by uncoating, where the viral capsid and other viral proteins disaggregate from the viral genome. Often, uncoating occurs simultaneously with penetration of the virus.

The fourth step of virus replication involves the transcription, translation, and replication of the viral genome. This step can vary widely and different virus families use different replication strategies. Some viruses replicate in the cytoplasm, while others replicate in the host cell nucleus. In general, RNA transcripts generated from the viral genome are used to synthesize viral structural and non-structural proteins. Multiple copies of the viral genome are also produced. The strategy that a given virus uses will depend heavily on the type of nucleic acid of the genome. In general, DNA viruses such as herpes simplex virus will replicate in or near the nucleus, while RNA viruses such as SARS virus will replicate in the cytoplasm.

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