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The project has been cancelled, Mech Avalon was destroyed by the Morningstar Concern. |
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:''This article is concerned with virus as a biological infectious particle; for other uses of the term see [[virus (disambiguation)]]. An extensive treatment of the pluralization of the word "virus" in English is found in the article [[Plural of virus]]. A [[list of biological viruses]] has also been prepared.'' |
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[[Image:virus-types.png|thumb|350px|Three types of viruses: a bacterial virus, otherwise called a [[phage|bacteriophage]] (left center); an animal virus (top right); and a [[retrovirus]] (bottom right). Viruses depend on the host cell that they infect to reproduce. When found outside of a host cell, viruses consist of genomic [[nucleic acid]], either [[DNA]] or [[RNA]] (depicted as blue), surrounded by a protein coat, or [[capsid]], with or without a [[glycoprotein]] [[envelope]]. Retroviruses contain RNA and reverse transciptase]] |
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A '''virus''' is a small particle that [[Infectious diseases|infect]]s cells in biological organisms. Viruses are [[parasites|obligate intracellular parasites]]; they can reproduce only by invading and taking over other cells as they lack the cellular machinery for self reproduction. The term ''virus'' usually refers to those particles which infect [[eukaryote]]s (multi-celled organisms and many single-celled organisms), whilst the term ''bacteriophage'' or ''[[phage]]'' is used to describe those infecting [[prokaryote]]s ([[bacterium|bacteria]] and bacteria-like organisms). Typically these particles carry a small amount of nucleic acid (either [[DNA]] or [[RNA]]) surrounded by some form of protective coat consisting of [[protein]]s, [[lipid]]s, and [[glycoprotein]]s. Importantly, viral [[genome]]s code not only for the [[protein]]s needed to package its [[genetic material]], but for proteins needed by the virus during [[lysogenic cycle|lysogenic]] and [[lytic cycle]]s, the reproductive cycles. |
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== Etymology == |
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The original word comes from the [[Latin]] ''virus'' referring to [[poison]] and other noxious things. Today it is used to describe the biological viruses discussed above and also as a metaphor for other parasitically-reproducing things, such as [[meme]]s or [[computer virus]]es. The word ''virion'' or ''viron'' is used to refer to a single infective viral particle. |
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The English plural form of ''virus'' is ''viruses''. No reputable dictionary gives any other form, including such "reconstructed" Latin plural forms as ''viri'' (which actually means ''men''). (No plural form appears in ''any'' extant Latin manuscript). (See [[plural of virus]]). |
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==Viruses: non-living or alive?== |
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A virus makes use of existing [[enzymes]] and other molecules of a [[host (biology)|host]] cell to create more virus particles. Viruses are neither [[unicellular organism|unicellular]] nor [[multicellular organism]]s; they are somewhere between being living and non-living. Viruses have genes and show inheritance, but are reliant on host cells to produce new generations of viruses. Many viruses have similarities to complex molecules. Like DNA, viruses undergo molecular [[replication]] and they can often be [[crystal]]lized. Because viruses are dependent on host cells for their replication they are generally not classified as "living". Whether or not they are "alive", they are obligate parasites, and have no form which can reproduce independently of their host. Like most parasites, they have a specific host range, sometimes specific to one species (or even limited cell types of one species) and sometimes more general. |
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Viruses form when molecules are assembled from [[organic compound]]s providing complex, microscopic structures which have the potential for [[self-assembly]], and thusly they have large implications in the study of the [[origin of life]]. In the debate of whether viruses are alive or not, if the requirement for autonomous self-reproduction is abandoned, it can be strongly argued that viruses are indeed alive. Some small viruses are more efficient than most cellular life forms as their ratio of functions to working parts is so high. If viruses are alive then the prospect of creating [[artificial life]] is enhanced or at least the standards required to call something artificially alive are reduced. |
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==Study and applications of viruses== |
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===Viruses as tools for exploring basic cellular processes=== |
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Viruses are important to the study of [[molecular biology|molecular]] and [[cellular biology]] because they provide simple systems that can be used to manipulate and investigate the functions of cell types. Below, this entry discusses how viral replication depends on the [[metabolism]] of the host. Therefore, the study of viruses can provide fundamental information about aspects of cell biology and metabolism. The rapid growth and small [[genome]] size of [[bacteria]] make them excellent tools for experiments in biology. Bacterial viruses have also further simplified the study of bacterial [[genetics]] and have deepened our understanding of the basic mechanisms of [[molecular genetics]]. Because of the [[complexity]] of an animal cell genome, viruses have been even more important in studies of animal cells than in studies of bacteria. Numerous studies have demonstrated the utility of animal viruses as probes for investigating different activities of eukaryotic cells. Other examples in which animal viruses have provided important models for biological research of their host cells include studies of [[DNA replication]], |
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[[transcription]], [[RNA processing]], and [[protein transport]]. |
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===Viruses as tools for genetic engineering=== |
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[[Genetic]]ists regularly use viruses as [[vector (biology)|vectors]] to introduce DNA into cells that they are studying. Attempts to treat human diseases through genetic engineering have also made use of viruses in similar ways. Deaths have occurred through virus infections caused by virus vectors used in [[gene therapy]], so their application to human subjects is still nascent. |
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== Viral size, structure, and anatomy == |
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Virus particles comprise a nucleic acid [[genome]] that may be either [[DNA]] or [[RNA]], single- or double-stranded, and positive or negative sense. This is surrounded (encapsidated) by a protective coat of [[protein]] called a [[capsid]]. The viral capsid may be either spherical or helical and is composed of proteins encoded by the viral genome. In helical viruses, the capsid protein (frequently called the nucleocapsid protein) binds directly to the viral genome. For example, in the case of the measles virus, one nucleocapsid protein binds every six bases of RNA to form a helix approximately 1.3 micrometers in length. This complex of protein and nucleic acid is called the nucleocapsid, and, in the case of the [[measles]] virus, is enclosed in a lipid "envelope" acquired from the host cell, in which virus-encoded glycoproteins are embedded. These are responsible for binding to and entering the host cell at the start of a new infection. Spherical virus capsids completely enclose the viral genome and do not generally bind as tightly to the nucleic acid as helical capsid proteins do. These structures can range in size from less than 20 nanometers up to 400 nanometers and are composed of viral proteins arranged with [[icosahedral]] symmetry. Icosahedral architecture is the same principle employed by R. Buckminster-Fuller in his [[geodesic dome]], and it 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" whereby 60t proteins are necessary. In the case of the hepatitis B virus, the T-number is 4, therefore 240 proteins assemble to form the capsid. As in the helical viruses, the spherical virus capsid may be enclosed in a lipid envelope, although frequently spherical viruses are not enveloped, and the capsid proteins themselves are directly involved in attachment and entry into the host cell. The complete virus particle is referred to as a '''virion'''. A virion is little more than a gene transporter, and components of the envelope and capsid provide the mechanism for injecting the viral genome into a host cell{{ref|Gelderblom}}. |
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==Viral replication== |
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Because viruses are acellular and do not have their own metabolism, they must utilize the machinery and metabolism of the host to reproduce. For this reason, viruses are called obligate intracellular [[parasite]]s. Before a virus has entered a host cell, it is called a '''virion''' — a package of viral genetic material. Virions can be passed from host to host either through direct contact or through a [[vector (biology)|vector]], or carrier. Inside the organism, the virus can enter a cell in various ways. [[Bacteriophage]]s—bacterial viruses—attach to the cell wall surface in specific places. Once attached, enzymes make a small hole in the cell wall, and the virus injects its DNA into the cell. Other viruses (such as [[HIV]]) enter the host via [[endocytosis]], the process whereby cells take in material from the external environment. After entering the cell, the virus's genetic material begins the |
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destructive process of taking over the cell and forcing it to produce new viruses. |
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There are three different ways genetic information contained in a viral genome can be reproduced. The form of genetic material contained in the viral [[capsid]], the protein coat that surrounds the [[nucleic acid]], determines the exact replication process. Some viruses have DNA, which once inside the host cell is replicated by the host along with its own DNA. |
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There are two different replication processes for viruses containing RNA. In the first process, the viral RNA is directly copied using an enzyme |
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called [[RNA replicase]]. This enzyme then uses that RNA copy as a template to make hundreds of duplicates of the original RNA. A second group of RNA-containing viruses, called the [[retrovirus]]es, uses the enzyme [[reverse transcriptase]] to synthesize a complementary strand of DNA so that the virus's genetic information is contained in a molecule of DNA rather than RNA. The viral DNA can then be further replicated using the resources of the host cell. |
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===Steps associated with viral reproduction=== |
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#Attachment, sometimes called absorption: The virus attaches to receptors on the host cell wall. |
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#Injection: The nucleic acid of the virus moves through the plasma membrane and into the cytoplasm of the host cell. The capsid of a [[phage]], a bacterial virus, remains on the outside. In contrast, many viruses that infect animal cells enter the host cell intact. |
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#Replication: The viral genome contains all the information necessary to produce new viruses. Once inside the host cell, the virus induces the host cell to synthesize the necessary components for its replication. |
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#Assembly: The newly synthesized viral components are assembled into new viruses. |
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#Lysis: Assembled viruses are released from the cell and can now infect other cells, and the process begins again. |
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When the virus has taken over the cell, it immediately directs the host to begin manufacturing the proteins necessary for virus reproduction. The host produces three kinds of proteins: [[early protein]]s, enzymes used in nucleic acid replication; [[late protein]]s, proteins used to construct the virus coat; and [[lytic protein]]s, enzymes used to break open the cell for viral exit. The final viral product is assembled spontaneously, that is, the parts are made separately by the host and are joined together by chance. This [[self-assembly]] is often aided by [[chaperone|molecular chaperone]]s, or proteins made by the host that help the capsid parts come together. |
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The new viruses then leave the cell either by [[exocytosis]] or by [[lysis]]. Envelope-bound animal viruses instruct the host's [[endoplasmic reticulum]] to make certain proteins, called [[glycoprotein]]s, which then collect in clumps along the cell membrane. The virus is then discharged from the cell at these exit sites, referred to as exocytosis. On the other hand, bacteriophages must break open, or [[lyse]], the cell to exit. To do this, the phages have a gene that codes for an enzyme called [[lysozyme]]. This enzyme breaks down the cell wall, causing the cell to swell and burst. The new viruses are released into the environment, killing the host cell in |
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the process. |
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==Origins == |
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The origin of viruses is not entirely clear, but the currently favoured explanation is that they are derived from their host organisms, originating from transferrable elements like [[plasmid]]s or [[transposon]]s. It has also been suggested that they may represent extremely reduced microbes, which appeared separately in the [[primordial soup]] that gave rise to the first cells, or that the different sorts of viruses appeared through different mechanisms. |
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Other infectious particles which are even simpler in structure than viruses include [[viroid]]s, [[virusoid]]s, and [[prion]]s. |
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==Human viral diseases== |
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Examples of diseases caused by viruses include the [[common cold]], which is caused by any one of a variety of related viruses; [[smallpox]]; [[AIDS]], which is caused by [[HIV]]; and cold sores, which are caused by [[herpes simplex]]. Recently it has been shown that cervical cancer is caused at least partly by [[papillomavirus]] (which causes papillomas, or warts), representing the first significant evidence in humans for a link between cancer and an infective agent. There is current controversy over whether [[borna virus]], previously thought of primarily as the causative agent of [[neurology|neurological]] disease in horses, could be responsible for [[psychiatry|psychiatric]] illness in humans. The relative ability of viruses to cause disease is described in terms of [[virulence]]. |
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The ability of viruses to cause devastating [[epidemic]]s in human societies has led to concern that viruses will be weaponized for [[biological warfare]]. Further concern was raised by the successful recreation of a virus in a laboratory. Much concern revolves around the smallpox virus, which has devastated numerous societies throughout history, and today is extinct in the wild. In fact, smallpox has been used in a crude form of biological warfare by British colonists against a tribe of [[Native American]]s. |
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This episode of biological warfare was part of a larger phenomenon of Native American populations being devastated by contagious diseases, particularly smallpox, brought to the Americas by European colonists. It is unclear how many Native Americans were killed by smallpox after the arrival of Columbus in the Americas, but it may have been very large. The damage done by this disease may have significantly aided European attempts to displace or conquer the native population. [[Jared Diamond]] argued in his book [[Guns, Germs, and Steel]] that highly contagious diseases develop in agricultural societies and regularly aid those societies when they expand into the territories of non-agricultural peoples. |
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Of all types of virus, the most deadly are known as [[filoviridae|filovirus]]. The Filovirus group consists of [[Marburg_virus|Marburg]], first discovered in 1967 in Marburg Germany, and [[ebola]]. Filovirus are long, worm-like virus particles that, in large groups, resemble a plate of noodles. As of [[April 2005]], the Marburg virus is attracting widespread press attention for an outbreak in [[Angola]]. Beginning in [[October 2004]] and continuing into [[2005]], the outbreak, which now appears to be coming under control, is the world's worst epidemic of any kind of hemorrhagic fever. |
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===Laboratory diagnosis of pathogenic viruses=== |
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Detection and subsequent isolation of viruses from patients is a very specialised laboratory subject. Normally it requires the use of large facilities, expensive equipment, and highly trained specialists such as technicians, [[molecular biologist]]s, and [[virologist]]s. Often, this effort is undertaken by state and national governments and shared internationally through organizations like [[WHO]]. |
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===Prevention and treatment of viral diseases=== |
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Because they use the machinery of their host cells, viruses are difficult to kill. The most effective [[medicine|medical]] approaches to viral diseases, thus far, are [[vaccination]] to provide resistance to infection, and drugs that treat the symptoms of viral infections. Patients often ask for [[antibiotics]], which are useless against viruses, and their misuse against viral infections is one of the causes of [[antibiotic resistance]] in [[bacterium|bacteria]]. That said, sometimes the prudent course of action is to begin a course of antibiotic treatment while waiting for test results to determine whether the patient's symptoms are caused by a virus or a bacterial infection. |
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==See also== |
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*[[Viral plaque]] |
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*[[Virus classification]] |
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*[[List of viruses]] |
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*[[Microbiology]] |
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*[[Horizontal gene transfer]] |
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*[[Computer virus]] |
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==References== |
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* [http://www.virology.net/ All the Virology on the WWW] |
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* Radetsky, Peter (1994). ''The Invisible Invaders: Viruses and the Scientists Who Pursue Them.'' Backbay Books, ISBNs 0316732168 (hc), 0316732176 (pb). |
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* Theiler, Max and Downs, W. G. (1973). ''The Anthropod-Borne Viruses of Vertebrates: An Account of the Rockerfeller Foundation Virus Program 1951-1970''. Yale University Press. |
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* {{NCBI-scienceprimer}} |
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===Numbered references=== |
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# {{note|Gelderblom}} Gelderblom, Hans R. (1996). [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.chapter.2252 41. Structure and Classification of Viruses] in ''[http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed Medical Microbiology] 4th ed.'' Samuel Baron ed. The University of Texas Medical Branch at Galveston. ISBN 0963117211 |
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{{Wiktionary}} |
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[[Category:Viruses| ]] |
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[[Category:Virology]] |
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[[als:Virus (Medizin)]] |
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[[ca:Virus]] |
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[[cs:Virus]] |
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[[de:Virus (Biologie)]] |
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[[eo:Viruso]] |
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[[es:Virus]] |
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[[fi:Virus]] |
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[[fr:Virus]] |
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[[id:Virus]] |
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[[he:נגיף]] |
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[[hu:Vírus (biológia)]] |
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[[it:Virus (biologia)]] |
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[[ja:ウイルス]] |
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[[ko:바이러스]] |
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[[la:virus biologicum]] |
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[[lt:Virusas]] |
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[[lv:Vīruss]] |
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[[nl:Virus (biologie)]] |
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[[no:Virus]] |
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[[pl:Wirus (biologia)]] |
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[[ru:Вирус (биология)]] |
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[[simple:Virus]] |
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[[sl:Virus]] |
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[[sv:virus]] |
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[[ta:அதி நுண் நச்சுயிர்]] |
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[[vi:Virus]] |
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[[zh:病毒]] |
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Revision as of 15:22, 3 July 2005
The project has been cancelled, Mech Avalon was destroyed by the Morningstar Concern.