Viruses are microorganisms that are able to infect living cells of other organisms. They target animals as well as plants archeas or bacterias. Upon infection the virus uses its host internal machinery to replicate. Host cells can produce thousands of copies of the virus. Viruses do not have their own metabolism and use the one of the host.
Virion is the form that the virus takes to survive out of cells. These particles are composed of its genetic material which is DNA or RNA, often in single strand, the capsid which is made of structural proteins and encapsulate the genetic material, and a lipidic envelope. The envelope may also carry surface proteins that enable virus attachment or activity. The shape of the capsid and envelope vary depending on the virus and is directly related to the virus genome. The proteins that compose the capsid are produced within the host cells. Therefore the virions cannot reproduce without infecting a cell. Globally, viruses size range between tens and hundreds of nanometers.
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In order to replicate, the virus undergoes different step which may vary depending on the virus structure and genetic material type. It first attach to cells via specific receptors. Indeed the presence of those receptors on the cells determines the capacity of the virus to infect it and its host range. Thanks to the receptor binding, the virus penetrates in the cell via endocytosis or membrane fusion. The capsid and envelope is removed during uncoating. The genetic material that was encapsulated can now be replicated and translated. The new virus assembles thanks to the newly produced proteins and nucleic acid chains and is further released by the host cell.
The virus infection induces an immune response by the host organism. Inactive viruses have also been used in vaccines to prepare the immunity against those viruses. Viruses are responsible for many diseases and epidemics. The mechanism of viral action may vary but implies the host cell lysis. Their transmissibility and virulence make them a constant research area.