Virus Replication Cycle

For a virus to multiply it must infect a living cell. All viruses employ a common set of steps in their replication cycle. These steps are: attachment, penetration, uncoating, replication, assembly, maturation, and release.

Attachment and Penetration. A virion surface protein must bind to one or more components of the cell surface, the viral receptors. The presence or absence of receptors generally determines the type of cell in which a virus is able to replicate. This is called viral tropism. For example, the poliovirus receptor is present only on cells of higher primates and then in a limited subset of these, such as intestine and brain cells. While called virus receptors, these are actually used by the cell for its own purposes, but are exploited by the virus for entry.

Entry of the viral genome into the cell can occur by direct penetration of the virion at the cell surface or by a process called endocytosis, which is the engulfment of the particle into a membrane-based vesicle. If the latter, the virus is released when the vesicle is acidified inside the cell. Enveloped cytoplasm the material . . c . . . .. . r . ... .

in a cell excluding the viruses may also fuse with the cellular surface membrane, which results in nucleus release of the capsid into the cytoplasm. Surface proteins of several viruses



|7. Release

Virus attaches to a receptor, enters the cell, and is uncoated, releasing its genetic material. In this example, positive polarity ( + ) DNA is released, which triggers formation of a complementary ( —) DNA strand. Double-stranded DNA is copied in the nucleus to make progeny DNA. It is also transcribed to make messenger RNA, for synthesis of viral proteins at the ribosome.

contain "fusion peptides," which are capable of interacting with the lipid bilayers of the host cell.

Uncoating and Replication. After penetration, viral capsid proteins must be removed, at least partly, to express and replicate the viral genome. In the case of most DNA viruses, the capsid is routed to the nucleus prior to uncoating. An example can be seen in the poxviruses, whose large DNA genomes encode most of the proteins needed for DNA replication. These viruses uncoat and replicate completely in the cytoplasm. RNA viruses typically lose the protective envelope and capsid proteins upon penetration into the cytoplasm. In reoviruses, only an outer protein shell is removed and replication takes place inside a structured subviral particle.

Viral genomes must be expressed as mRNAs in order to be translated into structural proteins for the capsids and, in some cases, as replicative proteins for replicating the virus genome. Viral genomes must also provide templates

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