Viruses

__**A **virus** (from the [|Latin] //virus// meaning "[|toxin]" or "[|poison]"), is a [|sub-microscopic] infectious agent that is unable to grow or reproduce outside a [|host] [|cell]. Each viral particle, or //virion//, consists of genetic material, [|DNA] or [|RNA], within a protective protein coat called a [|capsid]. The capsid shape varies from simple helical and icosahedral (polyhedral or near-spherical) forms, to more complex structures with tails or an [|envelope]. Viruses infect cellular life forms and are grouped into animal, plant and bacterial types, according to the type of host infected. [|Biologists] debate whether or not viruses are living [|organisms]. Some consider them non-living as they do not meet all the criteria used in the common definitions of [|life]. For example, unlike most organisms, viruses do not have [|cells]. However, viruses have genes and evolve by [|natural selection]. Others have described them as organisms at the edge of life. Viral infections in human and animal hosts usually result in an immune response and [|disease]. Often, a virus is completely eliminated by the [|immune system]. [|Antibiotics] have no effect on viruses, but [|antiviral drugs] have been developed to treat life-threatening infections. [|Vaccines] that produce lifelong [|immunity] can prevent viral infections.
 * __ VIRUSES

A complete virus particle, known as a virion, consists of [|nucleic acid] surrounded by a protective coat of [|protein] called a [|capsid]. Viruses can have a [|lipid] "envelope" derived from the host [|cell membrane]. A capsid is made from proteins encoded by the viral [|genome] and its shape serves as the basis for [|morphological] and antigenic distinction.[|[31]][|[32]] Virally coded protein subunits will self-assemble to form a capsid, generally requiring the presence of the virus genome. However, complex viruses code for proteins which assist in the construction of their capsid.[|[18]] Proteins associated with nucleic acid are known as [|nucleoproteins], and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. In general, there are four main morphological virus types: Icosahedral architecture was employed by [|R. Buckminster Fuller] in his [|geodesic dome], and is the most efficient way of creating an enclosed robust structure from multiple copies of a single protein. The number of proteins required to form a spherical virus capsid is denoted by the T-number,[|[33]] where 60×//t// proteins are necessary. In the case of the [|hepatitis B] virus the T-number is 4, and 240 proteins assemble to form the capsid. || The viral envelope can give a virion a few distinct advantages over other capsid-only virions, such as protection from enzymes and certain chemicals. The proteins in it can include [|glycoproteins] functioning as [|receptor molecules], allowing host cells to recognize and bind these virions, resulting in the possible uptake of the virion into the cell. Most enveloped viruses are dependent on the envelope for infectivity. ||
 * **Helical viruses** ||
 *   Diagram of a helical capsid || Helical capsids are composed of a single type of subunit stacked around a central axis to form a helical structure which may have a central cavity, or hollow tube. This arrangement results in rod-shaped or filamentous virions: these can be anything from short and highly rigid, to long and very flexible. The genetic material, generally single-stranded RNA, but ssDNA in some cases, is bound into the protein helix, by interactions between the negatively-charged nucleic acid and positive charges on the protein. Overall, the length of a helical capsid is related to the length of the nucleic acid contained within it and the diameter is dependent on the size and arrangement of protomers. The well-studied [|Tobacco mosaic virus] is an example of a helical virus. ||
 * **Icosahedron viruses** ||
 * [[image:http://upload.wikimedia.org/wikipedia/en/thumb/9/90/Enteric_Adenoviruses.jpg/200px-Enteric_Adenoviruses.jpg width="200" height="215" caption="Electron micrograph of icosahedral virions" link="http://en.wikipedia.org/wiki/Image:Enteric_Adenoviruses.jpg"]] Electron micrograph of icosahedral virions || Icosahedral capsid symmetry results in a spherical appearance of viruses at low magnification but actually consists of capsomers arranged in a regular geometrical pattern, similar to a [|soccer ball], hence they are not truly "spherical". Capsomers are ring shaped constructed from five to six copies of protomers. These associate via [|non-covalent bonding] to enclose the viral nucleic acid, though generally less intimately than helical capsids, and may involve one or more protomers.
 * **Enveloped viruses** ||
 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/1/16/Varicella_%28Chickenpox%29_Virus_PHIL_1878_lores.jpg/200px-Varicella_%28Chickenpox%29_Virus_PHIL_1878_lores.jpg width="200" height="242" caption="Herpes zoster virus" link="http://en.wikipedia.org/wiki/Image:Varicella_%28Chickenpox%29_Virus_PHIL_1878_lores.jpg"]] Herpes zoster virus || Viruses are able to envelope themselves in a modified form of one of the [|cell membranes] either the outer membrane surrounding an infected host cell, or from internal membranes such as nuclear membrane or endoplasmic reticulum, thus gaining an outer lipid bilayer known as a [|viral envelope]. This membrane is studded with proteins coded for by the viral genome and host genome; however the lipid membrane itself and any carbohydrates present are entirely host-coded. The Influenza virus and HIV use this strategy.
 * **Complex viruses** ||
 *   Diagram of a bacteriophage || These viruses possess a capsid which is neither purely helical, nor purely icosahedral, and which may possess extra structures such as protein tails or a complex outer wall. Some [|bacteriophages] have a complex structure consisting of an icosahedral head bound to a helical tail, the latter of which may have a hexagonal base plate with protruding protein tail fibres. ||
 * [[image:http://upload.wikimedia.org/wikipedia/en/thumb/f/fa/Poxvirus.jpg/200px-Poxvirus.jpg width="200" height="155" caption="Poxvirus" link="http://en.wikipedia.org/wiki/Image:Poxvirus.jpg"]] Poxvirus || The [|Poxviruses] are large, complex viruses which have an unusual [|morphology]. The viral genome is associated with proteins within a central disk structure known as a [|nucleoid]. The nucleoid is surrounded by a membrane and two lateral bodies of unknown function. The virus has an outer envelope with a thick layer of protein studded over its surface. The whole particle is slightly [|pleiomorphic], ranging from ovoid to brick shape.[|[34]] ||