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88-502: Pseudochromis is a genus of fish in the family Pseudochromidae found in Indian and Pacific Ocean . There are currently 71 recognized species in this genus: Genus The composition of a genus is determined by taxonomists . The standards for genus classification are not strictly codified, so different authorities often produce different classifications for genera. There are some general practices used, however, including

176-406: A host cell is often forced to rapidly produce thousands of copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent viral particles, or virions , consisting of (i) genetic material , i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts; (ii) a protein coat,

264-557: A species : see Botanical name and Specific name (zoology) . The rules for the scientific names of organisms are laid down in the nomenclature codes , which allow each species a single unique name that, for animals (including protists ), plants (also including algae and fungi ) and prokaryotes ( bacteria and archaea ), is Latin and binomial in form; this contrasts with common or vernacular names , which are non-standardized, can be non-unique, and typically also vary by country and language of usage. Except for viruses ,

352-425: A virion , consists of nucleic acid surrounded by a protective coat of protein called a capsid . These are formed from protein subunits called capsomeres . Viruses can have a lipid "envelope" derived from the host cell membrane . The capsid is made from proteins encoded by the viral genome and its shape serves as the basis for morphological distinction. Virally-coded protein subunits will self-assemble to form

440-622: A basic optical microscope. In 2013, the Pandoravirus genus was discovered in Chile and Australia, and has genomes about twice as large as Megavirus and Mimivirus. All giant viruses have dsDNA genomes and they are classified into several families: Mimiviridae , Pithoviridae, Pandoraviridae , Phycodnaviridae , and the Mollivirus genus. Some viruses that infect Archaea have complex structures unrelated to any other form of virus, with

528-418: A capsid diameter of 400 nm. Protein filaments measuring 100 nm project from the surface. The capsid appears hexagonal under an electron microscope, therefore the capsid is probably icosahedral. In 2011, researchers discovered the largest then known virus in samples of water collected from the ocean floor off the coast of Las Cruces, Chile. Provisionally named Megavirus chilensis , it can be seen with

616-581: A capsid, in general requiring the presence of the virus genome. Complex viruses code for proteins that assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins , and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. The capsid and entire virus structure can be mechanically (physically) probed through atomic force microscopy . In general, there are five main morphological virus types: The poxviruses are large, complex viruses that have an unusual morphology. The viral genome

704-639: A cellular structure, which is often seen as the basic unit of life. Viruses do not have their own metabolism and require a host cell to make new products. They therefore cannot naturally reproduce outside a host cell —although some bacteria such as rickettsia and chlamydia are considered living organisms despite the same limitation. Accepted forms of life use cell division to reproduce, whereas viruses spontaneously assemble within cells. They differ from autonomous growth of crystals as they inherit genetic mutations while being subject to natural selection. Virus self-assembly within host cells has implications for

792-493: A different DNA (or RNA) molecule. This can occur when viruses infect cells simultaneously and studies of viral evolution have shown that recombination has been rampant in the species studied. Recombination is common to both RNA and DNA viruses. Coronaviruses have a single-strand positive-sense RNA genome. Replication of the genome is catalyzed by an RNA-dependent RNA polymerase . The mechanism of recombination used by coronaviruses likely involves template switching by

880-559: A few species, or broad for viruses capable of infecting many. Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines , which confer an artificially acquired immunity to the specific viral infection. Some viruses, including those that cause HIV/AIDS , HPV infection , and viral hepatitis , evade these immune responses and result in chronic infections. Several classes of antiviral drugs have been developed. The English word "virus" comes from

968-549: A fluid, by Wendell Meredith Stanley , and the invention of the electron microscope in 1931 allowed their complex structures to be visualised. Scientific opinions differ on whether viruses are a form of life or organic structures that interact with living organisms. They have been described as "organisms at the edge of life", since they resemble organisms in that they possess genes , evolve by natural selection , and reproduce by creating multiple copies of themselves through self-assembly. Although they have genes, they do not have

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1056-445: A genome size of only two kilobases; the largest—the pandoraviruses —have genome sizes of around two megabases which code for about 2500 proteins. Virus genes rarely have introns and often are arranged in the genome so that they overlap . In general, RNA viruses have smaller genome sizes than DNA viruses because of a higher error-rate when replicating, and have a maximum upper size limit. Beyond this, errors when replicating render

1144-513: A ladder. The virus particles of some virus families, such as those belonging to the Hepadnaviridae , contain a genome that is partially double-stranded and partially single-stranded. For most viruses with RNA genomes and some with single-stranded DNA (ssDNA) genomes, the single strands are said to be either positive-sense (called the 'plus-strand') or negative-sense (called the 'minus-strand'), depending on if they are complementary to

1232-651: A later homonym of a validly published name is a nomen illegitimum or nom. illeg. ; for a full list refer to the International Code of Nomenclature for algae, fungi, and plants and the work cited above by Hawksworth, 2010. In place of the "valid taxon" in zoology, the nearest equivalent in botany is " correct name " or "current name" which can, again, differ or change with alternative taxonomic treatments or new information that results in previously accepted genera being combined or split. Prokaryote and virus codes of nomenclature also exist which serve as

1320-436: A life form, because they carry genetic material, reproduce, and evolve through natural selection , although they lack some key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life" and as replicators . Viruses spread in many ways. One transmission pathway

1408-472: A limited range of hosts and many are species-specific. Some, such as smallpox virus for example, can infect only one species—in this case humans, and are said to have a narrow host range . Other viruses, such as rabies virus, can infect different species of mammals and are said to have a broad range. The viruses that infect plants are harmless to animals, and most viruses that infect other animals are harmless to humans. The host range of some bacteriophages

1496-628: A long time and redescribed as new by a range of subsequent workers, or if a range of genera previously considered separate taxa have subsequently been consolidated into one. For example, the World Register of Marine Species presently lists 8 genus-level synonyms for the sperm whale genus Physeter Linnaeus, 1758, and 13 for the bivalve genus Pecten O.F. Müller, 1776. Within the same kingdom, one generic name can apply to one genus only. However, many names have been assigned (usually unintentionally) to two or more different genera. For example,

1584-408: A prime target for natural selection. Segmented genomes confer evolutionary advantages; different strains of a virus with a segmented genome can shuffle and combine genes and produce progeny viruses (or offspring) that have unique characteristics. This is called reassortment or 'viral sex'. Genetic recombination is a process by which a strand of DNA (or RNA) is broken and then joined to the end of

1672-409: A reference for designating currently accepted genus names as opposed to others which may be either reduced to synonymy, or, in the case of prokaryotes, relegated to a status of "names without standing in prokaryotic nomenclature". An available (zoological) or validly published (botanical) name that has been historically applied to a genus but is not regarded as the accepted (current/valid) name for

1760-412: A single viral particle that is released from the cell and is capable of infecting other cells of the same type. Viruses are found wherever there is life and have probably existed since living cells first evolved . The origin of viruses is unclear because they do not form fossils, so molecular techniques are used to infer how they arose. In addition, viral genetic material occasionally integrates into

1848-460: A small part of the total diversity of viruses has been studied. As of 2022, 6 realms, 10 kingdoms, 17 phyla, 2 subphyla, 40 classes, 72 orders, 8 suborders, 264 families, 182 subfamilies , 2,818 genera, 84 subgenera , and 11,273 species of viruses have been defined by the ICTV. The general taxonomic structure of taxon ranges and the suffixes used in taxonomic names are shown hereafter. As of 2022,

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1936-427: A taxon; however, the names published in suppressed works are made unavailable via the relevant Opinion dealing with the work in question. In botany, similar concepts exist but with different labels. The botanical equivalent of zoology's "available name" is a validly published name . An invalidly published name is a nomen invalidum or nom. inval. ; a rejected name is a nomen rejiciendum or nom. rej. ;

2024-455: A total of c. 520,000 published names (including synonyms) as at end 2019, increasing at some 2,500 published generic names per year. "Official" registers of taxon names at all ranks, including genera, exist for a few groups only such as viruses and prokaryotes, while for others there are compendia with no "official" standing such as Index Fungorum for fungi, Index Nominum Algarum and AlgaeBase for algae, Index Nominum Genericorum and

2112-605: A wide diversity of sizes and shapes, called ' morphologies '. In general, viruses are much smaller than bacteria and more than a thousand bacteriophage viruses would fit inside an Escherichia coli bacterium's cell. Many viruses that have been studied are spherical and have a diameter between 20 and 300 nanometres . Some filoviruses , which are filaments, have a total length of up to 1400 nm; their diameters are only about 80 nm. Most viruses cannot be seen with an optical microscope , so scanning and transmission electron microscopes are used to visualise them. To increase

2200-553: A wide variety of unusual shapes, ranging from spindle-shaped structures to viruses that resemble hooked rods, teardrops or even bottles. Other archaeal viruses resemble the tailed bacteriophages, and can have multiple tail structures. An enormous variety of genomic structures can be seen among viral species ; as a group, they contain more structural genomic diversity than plants, animals, archaea, or bacteria. There are millions of different types of viruses, although fewer than 7,000 types have been described in detail. As of January 2021,

2288-416: Is a feature of many bacterial and some animal viruses. Some viruses undergo a lysogenic cycle where the viral genome is incorporated by genetic recombination into a specific place in the host's chromosome. The viral genome is then known as a " provirus " or, in the case of bacteriophages a " prophage ". Whenever the host divides, the viral genome is also replicated. The viral genome is mostly silent within

2376-405: Is a major change in the genome of the virus. This can be a result of recombination or reassortment . The Influenza A virus is highly prone to reassortment; occasionally this has resulted in novel strains which have caused pandemics . RNA viruses often exist as quasispecies or swarms of viruses of the same species but with slightly different genome nucleoside sequences. Such quasispecies are

2464-400: Is associated with proteins within a central disc 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 virion is slightly pleomorphic , ranging from ovoid to brick-shaped. Mimivirus is one of the largest characterised viruses, with

2552-439: Is caused by cessation of its normal activities because of suppression by virus-specific proteins, not all of which are components of the virus particle. The distinction between cytopathic and harmless is gradual. Some viruses, such as Epstein–Barr virus , can cause cells to proliferate without causing malignancy, while others, such as papillomaviruses , are established causes of cancer. Some viruses cause no apparent changes to

2640-512: Is correct. It seems unlikely that all currently known viruses have a common ancestor, and viruses have probably arisen numerous times in the past by one or more mechanisms. The first evidence of the existence of viruses came from experiments with filters that had pores small enough to retain bacteria. In 1892, Dmitri Ivanovsky used one of these filters to show that sap from a diseased tobacco plant remained infectious to healthy tobacco plants despite having been filtered. Martinus Beijerinck called

2728-612: Is discouraged by both the International Code of Zoological Nomenclature and the International Code of Nomenclature for algae, fungi, and plants , there are some five thousand such names in use in more than one kingdom. For instance, A list of generic homonyms (with their authorities), including both available (validly published) and selected unavailable names, has been compiled by the Interim Register of Marine and Nonmarine Genera (IRMNG). The type genus forms

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2816-507: Is first recorded in 1728, long before the discovery of viruses by Dmitri Ivanovsky in 1892. The English plural is viruses (sometimes also vira ), whereas the Latin word is a mass noun , which has no classically attested plural ( vīra is used in Neo-Latin ). The adjective viral dates to 1948. The term virion (plural virions ), which dates from 1959, is also used to refer to

2904-607: Is identical in sequence to the viral mRNA and is thus a coding strand, while negative-sense viral ssDNA is complementary to the viral mRNA and is thus a template strand. Several types of ssDNA and ssRNA viruses have genomes that are ambisense in that transcription can occur off both strands in a double-stranded replicative intermediate. Examples include geminiviruses , which are ssDNA plant viruses and arenaviruses , which are ssRNA viruses of animals. Genome size varies greatly between species. The smallest—the ssDNA circoviruses, family Circoviridae —code for only two proteins and have

2992-431: Is limited to a single strain of bacteria and they can be used to trace the source of outbreaks of infections by a method called phage typing . The complete set of viruses in an organism or habitat is called the virome ; for example, all human viruses constitute the human virome . A novel virus is one that has not previously been recorded. It can be a virus that is isolated from its natural reservoir or isolated as

3080-460: Is somewhat arbitrary. Although all species within a genus are supposed to be "similar", there are no objective criteria for grouping species into genera. There is much debate among zoologists about whether enormous, species-rich genera should be maintained, as it is extremely difficult to come up with identification keys or even character sets that distinguish all species. Hence, many taxonomists argue in favor of breaking down large genera. For instance,

3168-474: Is the type species , and the generic name is permanently associated with the type specimen of its type species. Should the specimen turn out to be assignable to another genus, the generic name linked to it becomes a junior synonym and the remaining taxa in the former genus need to be reassessed. In zoological usage, taxonomic names, including those of genera, are classified as "available" or "unavailable". Available names are those published in accordance with

3256-475: Is through disease-bearing organisms known as vectors : for example, viruses are often transmitted from plant to plant by insects that feed on plant sap , such as aphids ; and viruses in animals can be carried by blood-sucking insects. Many viruses spread in the air by coughing and sneezing, including influenza viruses , SARS-CoV-2 , chickenpox , smallpox , and measles . Norovirus and rotavirus , common causes of viral gastroenteritis , are transmitted by

3344-400: The capsid , which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids . The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria. The origins of viruses in

3432-477: The CD4 molecule—a chemokine receptor —which is most commonly found on the surface of CD4+ T-Cells . This mechanism has evolved to favour those viruses that infect only cells in which they are capable of replication. Attachment to the receptor can induce the viral envelope protein to undergo changes that result in the fusion of viral and cellular membranes, or changes of non-enveloped virus surface proteins that allow

3520-621: The International Code of Zoological Nomenclature ; the earliest such name for any taxon (for example, a genus) should then be selected as the " valid " (i.e., current or accepted) name for the taxon in question. Consequently, there will be more available names than valid names at any point in time; which names are currently in use depending on the judgement of taxonomists in either combining taxa described under multiple names, or splitting taxa which may bring available names previously treated as synonyms back into use. "Unavailable" names in zoology comprise names that either were not published according to

3608-555: The International Committee on Taxonomy of Viruses (ICTV) was formed. The system proposed by Lwoff, Horne and Tournier was initially not accepted by the ICTV because the small genome size of viruses and their high rate of mutation made it difficult to determine their ancestry beyond order. As such, the Baltimore classification system has come to be used to supplement the more traditional hierarchy. Starting in 2018,

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3696-824: The International Plant Names Index for plants in general, and ferns through angiosperms, respectively, and Nomenclator Zoologicus and the Index to Organism Names for zoological names. Totals for both "all names" and estimates for "accepted names" as held in the Interim Register of Marine and Nonmarine Genera (IRMNG) are broken down further in the publication by Rees et al., 2020 cited above. The accepted names estimates are as follows, broken down by kingdom: The cited ranges of uncertainty arise because IRMNG lists "uncertain" names (not researched therein) in addition to known "accepted" names;

3784-690: The Latin vīrus , which refers to poison and other noxious liquids. Vīrus comes from the same Indo-European root as Sanskrit viṣa , Avestan vīša , and Ancient Greek ἰός ( iós ), which all mean "poison". The first attested use of "virus" in English appeared in 1398 in John Trevisa 's translation of Bartholomeus Anglicus 's De Proprietatibus Rerum . Virulent , from Latin virulentus ('poisonous'), dates to c.  1400 . A meaning of 'agent that causes infectious disease'

3872-490: The NCBI Virus genome database has more than 193,000 complete genome sequences, but there are doubtlessly many more to be discovered. A virus has either a DNA or an RNA genome and is called a DNA virus or an RNA virus , respectively. Most viruses have RNA genomes. Plant viruses tend to have single-stranded RNA genomes and bacteriophages tend to have double-stranded DNA genomes. Viral genomes are circular, as in

3960-539: The common cold , influenza , chickenpox , and cold sores . Many serious diseases such as rabies , Ebola virus disease , AIDS (HIV) , avian influenza , and SARS are caused by viruses. The relative ability of viruses to cause disease is described in terms of virulence . Other diseases are under investigation to discover if they have a virus as the causative agent, such as the possible connection between human herpesvirus 6 (HHV6) and neurological diseases such as multiple sclerosis and chronic fatigue syndrome . There

4048-407: The evolutionary history of life are still unclear. Some viruses may have evolved from plasmids , which are pieces of DNA that can move between cells. Other viruses may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer , which increases genetic diversity in a way analogous to sexual reproduction . Viruses are considered by some biologists to be

4136-422: The faecal–oral route , passed by hand-to-mouth contact or in food or water. The infectious dose of norovirus required to produce infection in humans is fewer than 100 particles. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a virus can infect is called its host range : this is narrow for viruses specialized to infect only

4224-430: The germline of the host organisms, by which they can be passed on vertically to the offspring of the host for many generations. This provides an invaluable source of information for paleovirologists to trace back ancient viruses that existed as far back as millions of years ago. There are three main hypotheses that aim to explain the origins of viruses: In the past, there were problems with all of these hypotheses:

4312-404: The platypus belongs to the genus Ornithorhynchus although George Shaw named it Platypus in 1799 (these two names are thus synonyms ) . However, the name Platypus had already been given to a group of ambrosia beetles by Johann Friedrich Wilhelm Herbst in 1793. A name that means two different things is a homonym . Since beetles and platypuses are both members of the kingdom Animalia,

4400-457: The polyomaviruses , or linear, as in the adenoviruses . The type of nucleic acid is irrelevant to the shape of the genome. Among RNA viruses and certain DNA viruses, the genome is often divided into separate parts, in which case it is called segmented. For RNA viruses, each segment often codes for only one protein and they are usually found together in one capsid. All segments are not required to be in

4488-447: The three domains . This discovery has led modern virologists to reconsider and re-evaluate these three classical hypotheses. The evidence for an ancestral world of RNA cells and computer analysis of viral and host DNA sequences give a better understanding of the evolutionary relationships between different viruses and may help identify the ancestors of modern viruses. To date, such analyses have not proved which of these hypotheses

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4576-473: The French botanist Joseph Pitton de Tournefort (1656–1708) is considered "the founder of the modern concept of genera". The scientific name (or the scientific epithet) of a genus is also called the generic name ; in modern style guides and science, it is always capitalised. It plays a fundamental role in binomial nomenclature , the system of naming organisms , where it is combined with the scientific name of

4664-527: The ICTV began to acknowledge deeper evolutionary relationships between viruses that have been discovered over time and adopted a 15-rank classification system ranging from realm to species. Additionally, some species within the same genus are grouped into a genogroup . The ICTV developed the current classification system and wrote guidelines that put a greater weight on certain virus properties to maintain family uniformity. A unified taxonomy (a universal system for classifying viruses) has been established. Only

4752-442: The base for higher taxonomic ranks, such as the family name Canidae ("Canids") based on Canis . However, this does not typically ascend more than one or two levels: the order to which dogs and wolves belong is Carnivora ("Carnivores"). The numbers of either accepted, or all published genus names is not known precisely; Rees et al., 2020 estimate that approximately 310,000 accepted names (valid taxa) may exist, out of

4840-489: The basis of similarities. In 1962, André Lwoff , Robert Horne , and Paul Tournier were the first to develop a means of virus classification, based on the Linnaean hierarchical system. This system based classification on phylum , class , order , family , genus , and species . Viruses were grouped according to their shared properties (not those of their hosts) and the type of nucleic acid forming their genomes. In 1966,

4928-410: The contrast between viruses and the background, electron-dense "stains" are used. These are solutions of salts of heavy metals, such as tungsten , that scatter the electrons from regions covered with the stain. When virions are coated with stain (positive staining), fine detail is obscured. Negative staining overcomes this problem by staining the background only. A complete virus particle, known as

5016-431: The extreme of the ssRNA virus case. Viruses undergo genetic change by several mechanisms. These include a process called antigenic drift where individual bases in the DNA or RNA mutate to other bases. Most of these point mutations are "silent"—they do not change the protein that the gene encodes—but others can confer evolutionary advantages such as resistance to antiviral drugs . Antigenic shift occurs when there

5104-446: The filtered, infectious substance a "virus" and this discovery is considered to be the beginning of virology. The subsequent discovery and partial characterization of bacteriophages by Frederick Twort and Félix d'Herelle further catalyzed the field, and by the early 20th century many viruses had been discovered. In 1926, Thomas Milton Rivers defined viruses as obligate parasites. Viruses were demonstrated to be particles, rather than

5192-446: The form "author, year" in zoology, and "standard abbreviated author name" in botany. Thus in the examples above, the genus Canis would be cited in full as " Canis Linnaeus, 1758" (zoological usage), while Hibiscus , also first established by Linnaeus but in 1753, is simply " Hibiscus L." (botanical usage). Each genus should have a designated type , although in practice there is a backlog of older names without one. In zoology, this

5280-399: The form of single-stranded nucleoprotein complexes, through pores called plasmodesmata . Bacteria, like plants, have strong cell walls that a virus must breach to infect the cell. Given that bacterial cell walls are much thinner than plant cell walls due to their much smaller size, some viruses have evolved mechanisms that inject their genome into the bacterial cell across the cell wall, while

5368-737: The generic name (or its abbreviated form) still forms the leading portion of the scientific name, for example, Canis lupus lupus for the Eurasian wolf subspecies, or as a botanical example, Hibiscus arnottianus ssp. immaculatus . Also, as visible in the above examples, the Latinised portions of the scientific names of genera and their included species (and infraspecies, where applicable) are, by convention, written in italics . The scientific names of virus species are descriptive, not binomial in form, and may or may not incorporate an indication of their containing genus; for example,

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5456-406: The host. At some point, the provirus or prophage may give rise to the active virus, which may lyse the host cells. Enveloped viruses (e.g., HIV) typically are released from the host cell by budding . During this process, the virus acquires its envelope, which is a modified piece of the host's plasma or other, internal membrane. The genetic material within virus particles, and the method by which

5544-432: The idea that a newly defined genus should fulfill these three criteria to be descriptively useful: Moreover, genera should be composed of phylogenetic units of the same kind as other (analogous) genera. The term "genus" comes from Latin genus , a noun form cognate with gignere ('to bear; to give birth to'). The Swedish taxonomist Carl Linnaeus popularized its use in his 1753 Species Plantarum , but

5632-517: The infected cell. Cells in which the virus is latent and inactive show few signs of infection and often function normally. This causes persistent infections and the virus is often dormant for many months or years. This is often the case with herpes viruses . Viruses are by far the most abundant biological entities on Earth and they outnumber all the others put together. They infect all types of cellular life including animals, plants, bacteria and fungi . Different types of viruses can infect only

5720-633: The largest component, with 23,236 ± 5,379 accepted genus names, of which 20,845 ± 4,494 are angiosperms (superclass Angiospermae). By comparison, the 2018 annual edition of the Catalogue of Life (estimated >90% complete, for extant species in the main) contains currently 175,363 "accepted" genus names for 1,744,204 living and 59,284 extinct species, also including genus names only (no species) for some groups. The number of species in genera varies considerably among taxonomic groups. For instance, among (non-avian) reptiles , which have about 1180 genera,

5808-485: The lizard genus Anolis has been suggested to be broken down into 8 or so different genera which would bring its ~400 species to smaller, more manageable subsets. Virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism . Viruses infect all life forms , from animals and plants to microorganisms , including bacteria and archaea . Viruses are found in almost every ecosystem on Earth and are

5896-416: The material is replicated, varies considerably between different types of viruses. The range of structural and biochemical effects that viruses have on the host cell is extensive. These are called ' cytopathic effects '. Most virus infections eventually result in the death of the host cell. The causes of death include cell lysis, alterations to the cell's surface membrane and apoptosis . Often cell death

5984-403: The most (>300) have only 1 species, ~360 have between 2 and 4 species, 260 have 5–10 species, ~200 have 11–50 species, and only 27 genera have more than 50 species. However, some insect genera such as the bee genera Lasioglossum and Andrena have over 1000 species each. The largest flowering plant genus, Astragalus , contains over 3,000 species. Which species are assigned to a genus

6072-416: The most numerous type of biological entity. Since Dmitri Ivanovsky 's 1892 article describing a non-bacterial pathogen infecting tobacco plants and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, more than 11,000 of the millions of virus species have been described in detail. The study of viruses is known as virology , a subspeciality of microbiology . When infected,

6160-428: The name could not be used for both. Johann Friedrich Blumenbach published the replacement name Ornithorhynchus in 1800. However, a genus in one kingdom is allowed to bear a scientific name that is in use as a generic name (or the name of a taxon in another rank) in a kingdom that is governed by a different nomenclature code. Names with the same form but applying to different taxa are called "homonyms". Although this

6248-461: The original virus. Their life cycle differs greatly between species, but there are six basic stages in their life cycle: Attachment is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. This specificity determines the host range and type of host cell of a virus. For example, HIV infects a limited range of human leucocytes . This is because its surface protein, gp120 , specifically interacts with

6336-416: The polymerase during genome replication. This process appears to be an adaptation for coping with genome damage. Viral populations do not grow through cell division, because they are acellular. Instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves, and they assemble in the cell. When infected, the host cell is forced to rapidly produce thousands of copies of

6424-541: The provisions of the ICZN Code, e.g., incorrect original or subsequent spellings, names published only in a thesis, and generic names published after 1930 with no type species indicated. According to "Glossary" section of the zoological Code, suppressed names (per published "Opinions" of the International Commission of Zoological Nomenclature) remain available but cannot be used as the valid name for

6512-965: The ranks of subrealm, subkingdom, and subclass are unused, whereas all other ranks are in use. The Nobel Prize-winning biologist David Baltimore devised the Baltimore classification system. The ICTV classification system is used in conjunction with the Baltimore classification system in modern virus classification. The Baltimore classification of viruses is based on the mechanism of mRNA production. Viruses must generate mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. Viral genomes may be single-stranded (ss) or double-stranded (ds), RNA or DNA, and may or may not use reverse transcriptase (RT). In addition, ssRNA viruses may be either sense (+) or antisense (−). This classification places viruses into seven groups: Examples of common human diseases caused by viruses include

6600-422: The regressive hypothesis did not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape hypothesis did not explain the complex capsids and other structures on virus particles. The virus-first hypothesis contravened the definition of viruses in that they require host cells. Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into

6688-484: The result of spread to an animal or human host where the virus had not been identified before. It can be an emergent virus , one that represents a new virus, but it can also be an extant virus that has not been previously identified . The SARS-CoV-2 coronavirus that caused the COVID-19 pandemic is an example of a novel virus. Classification seeks to describe the diversity of viruses by naming and grouping them on

6776-451: The same virion for the virus to be infectious, as demonstrated by brome mosaic virus and several other plant viruses. A viral genome, irrespective of nucleic acid type, is almost always either single-stranded (ss) or double-stranded (ds). Single-stranded genomes consist of an unpaired nucleic acid, analogous to one-half of a ladder split down the middle. Double-stranded genomes consist of two complementary paired nucleic acids, analogous to

6864-497: The specific name particular to the wolf. A botanical example would be Hibiscus arnottianus , a particular species of the genus Hibiscus native to Hawaii. The specific name is written in lower-case and may be followed by subspecies names in zoology or a variety of infraspecific names in botany . When the generic name is already known from context, it may be shortened to its initial letter, for example, C. lupus in place of Canis lupus . Where species are further subdivided,

6952-412: The standard format for a species name comprises the generic name, indicating the genus to which the species belongs, followed by the specific epithet, which (within that genus) is unique to the species. For example, the gray wolf 's scientific name is Canis lupus , with Canis ( Latin for 'dog') being the generic name shared by the wolf's close relatives and lupus (Latin for 'wolf') being

7040-399: The structure-mediated self-assembly of the virus particles, some modification of the proteins often occurs. In viruses such as HIV, this modification (sometimes called maturation) occurs after the virus has been released from the host cell. Release – Viruses can be released from the host cell by lysis , a process that kills the cell by bursting its membrane and cell wall if present: this

7128-457: The study of the origin of life , as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules . The virocell model first proposed by Patrick Forterre considers the infected cell to be the "living form" of viruses and that virus particles (virions) are analogous to spores . Although the living versus non-living debate continues, the virocell model has gained some acceptance. Viruses display

7216-403: The taxon is termed a synonym ; some authors also include unavailable names in lists of synonyms as well as available names, such as misspellings, names previously published without fulfilling all of the requirements of the relevant nomenclatural code, and rejected or suppressed names. A particular genus name may have zero to many synonyms, the latter case generally if the genus has been known for

7304-576: The values quoted are the mean of "accepted" names alone (all "uncertain" names treated as unaccepted) and "accepted + uncertain" names (all "uncertain" names treated as accepted), with the associated range of uncertainty indicating these two extremes. Within Animalia, the largest phylum is Arthropoda , with 151,697 ± 33,160 accepted genus names, of which 114,387 ± 27,654 are insects (class Insecta). Within Plantae, Tracheophyta (vascular plants) make up

7392-446: The viral messenger RNA (mRNA). Positive-sense viral RNA is in the same sense as viral mRNA and thus at least a part of it can be immediately translated by the host cell. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA-dependent RNA polymerase before translation. DNA nomenclature for viruses with genomic ssDNA is similar to RNA nomenclature, in that positive-strand viral ssDNA

7480-824: The viral capsid remains outside. Uncoating is a process in which the viral capsid is removed: This may be by degradation by viral enzymes or host enzymes or by simple dissociation; the end-result is the releasing of the viral genomic nucleic acid. Replication of viruses involves primarily multiplication of the genome. Replication involves the synthesis of viral messenger RNA (mRNA) from "early" genes (with exceptions for positive-sense RNA viruses), viral protein synthesis , possible assembly of viral proteins, then viral genome replication mediated by early or regulatory protein expression. This may be followed, for complex viruses with larger genomes, by one or more further rounds of mRNA synthesis: "late" gene expression is, in general, of structural or virion proteins. Assembly – Following

7568-429: The virus species " Salmonid herpesvirus 1 ", " Salmonid herpesvirus 2 " and " Salmonid herpesvirus 3 " are all within the genus Salmonivirus ; however, the genus to which the species with the formal names " Everglades virus " and " Ross River virus " are assigned is Alphavirus . As with scientific names at other ranks, in all groups other than viruses, names of genera may be cited with their authorities, typically in

7656-503: The virus to enter. Penetration or viral entry follows attachment: Virions enter the host cell through receptor-mediated endocytosis or membrane fusion . The infection of plant and fungal cells is different from that of animal cells. Plants have a rigid cell wall made of cellulose , and fungi one of chitin, so most viruses can get inside these cells only after trauma to the cell wall. Nearly all plant viruses (such as tobacco mosaic virus) can also move directly from cell to cell, in

7744-460: The virus useless or uncompetitive. To compensate, RNA viruses often have segmented genomes—the genome is split into smaller molecules—thus reducing the chance that an error in a single-component genome will incapacitate the entire genome. In contrast, DNA viruses generally have larger genomes because of the high fidelity of their replication enzymes. Single-strand DNA viruses are an exception to this rule, as mutation rates for these genomes can approach

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