150-727: Polyomaviridae is a family of viruses whose natural hosts are primarily mammals and birds. As of 2024, there are eight recognized genera. 14 species are known to infect humans, while others, such as Simian Virus 40 , have been identified in humans to a lesser extent. Most of these viruses are very common and typically asymptomatic in most human populations studied. BK virus is associated with nephropathy in renal transplant and non-renal solid organ transplant patients, JC virus with progressive multifocal leukoencephalopathy , and Merkel cell virus with Merkel cell cancer . Polyomaviruses are non-enveloped double-stranded DNA viruses with circular genomes of around 5000 base pairs . With such
300-406: 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,
450-453: A host cell . Cellular receptors for polyomaviruses are sialic acid residues of glycans , commonly gangliosides . The attachment of polyomaviruses to host cells is mediated by the binding of VP1 to sialylated glycans on the cell surface. In some particular viruses, additional cell-surface interactions occur; for example, the JC virus is believed to require interaction with the 5HT2A receptor and
600-412: A platinum plate which was used as the anode, the cathode being a platinum wire partially submerged into mercury. Electrolysis then gave calcium–mercury and magnesium–mercury amalgams, and distilling off the mercury gave the metal. However, pure calcium cannot be prepared in bulk by this method and a workable commercial process for its production was not found until over a century later. At 3%, calcium
750-425: 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
900-434: A "steady state" with respect to calcium input and output. This has important climatological implications, as the marine calcium cycle is closely tied to the carbon cycle . Many calcium compounds are used in food, as pharmaceuticals, and in medicine, among others. For example, calcium and phosphorus are supplemented in foods through the addition of calcium lactate , calcium diphosphate , and tricalcium phosphate . The last
1050-606: 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
1200-418: 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
1350-581: 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
1500-639: 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
1650-491: A cross-reactive antibody to SV40 T antigen (commonly Pab419) is used to stain tissues directly for the presence of JC virus T antigen. PCR can be used on a biopsy of the tissue or cerebrospinal fluid to amplify the polyomavirus DNA. This allows not only the detection of polyomavirus but also which sub type it is. There are three main diagnostic techniques used for the diagnosis of the reactivation of polyomavirus in polyomavirus nephropathy (PVN): urine cytology, quantification of
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#17328841227781800-493: 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
1950-423: A dipositive ion with the stable electron configuration of a noble gas , in this case argon . Hence, calcium is almost always divalent in its compounds, which are usually ionic . Hypothetical univalent salts of calcium would be stable with respect to their elements, but not to disproportionation to the divalent salts and calcium metal, because the enthalpy of formation of MX 2 is much higher than those of
2100-559: 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
2250-549: 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
2400-638: A gas had not been recognised by the ancient Romans. In 1789, Antoine Lavoisier suspected that lime might be an oxide of a fundamental chemical element . In his table of the elements, Lavoisier listed five "salifiable earths" (i.e., ores that could be made to react with acids to produce salts ( salis = salt, in Latin): chaux (calcium oxide), magnésie (magnesia, magnesium oxide), baryte (barium sulfate), alumine (alumina, aluminium oxide), and silice (silica, silicon dioxide)). About these "elements", Lavoisier reasoned: We are probably only acquainted as yet with
2550-445: 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
2700-412: A high pressure of oxygen, and there is some evidence for a yellow superoxide Ca(O 2 ) 2 . Calcium hydroxide, Ca(OH) 2 , is a strong base, though not as strong as the hydroxides of strontium, barium or the alkali metals. All four dihalides of calcium are known. Calcium carbonate (CaCO 3 ) and calcium sulfate (CaSO 4 ) are particularly abundant minerals. Like strontium and barium, as well as
2850-506: 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
3000-436: 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
3150-472: 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
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#17328841227783300-508: A part of the metallic substances existing in nature, as all those which have a stronger affinity to oxygen than carbon possesses, are incapable, hitherto, of being reduced to a metallic state, and consequently, being only presented to our observation under the form of oxyds, are confounded with earths. It is extremely probable that barytes, which we have just now arranged with earths, is in this situation; for in many experiments it exhibits properties nearly approaching to those of metallic bodies. It
3450-408: 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
3600-468: A promising genetic marker for human evolution and migration. It is carried by 70–90 percent of humans and is usually transmitted from parents to offspring. This method does not appear to be reliable for tracing the recent African origin of modern humans . Murine polyomavirus was the first polyomavirus discovered, having been reported by Ludwik Gross in 1953 as an extract of mouse leukemias capable of inducing parotid gland tumors. The causative agent
3750-423: A rather high neutron flux to allow short-lived Ca to capture a neutron. Ca is produced by electron capture in the r-process in type Ia supernovae , where high neutron excess and low enough entropy ensures its survival. Ca and Ca are the first "classically stable" nuclides with a 6-neutron or 8-neutron excess respectively. Although extremely neutron-rich for such a light element, Ca is very stable because it
3900-401: A result, when Ca does decay, it does so by double beta decay to Ti instead, being the lightest nuclide known to undergo double beta decay. Ca can also theoretically undergo double beta decay to Ti, but this has never been observed. The most common isotope Ca is also doubly magic and could undergo double electron capture to Ar , but this has likewise never been observed. Calcium is
4050-412: 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
4200-456: 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,
4350-419: A small size, they are ranked among the smallest known double stranded DNA viruses. The genome is packaged in a viral capsid of about 40-50 nanometers in diameter, which is icosahedral in shape (T=7 symmetry). The capsid is composed of 72 pentameric capsomeres of a protein called VP1 , which is capable of self-assembly into a closed icosahedron; each pentamer of VP1 is associated with one molecule of one of
4500-706: A unique domain, called the LT-stabilization domain (LSD), that binds to and inhibits the FBXW7 E3 ligase regulating both cellular and viral oncoproteins. Unlike for SV40, the MCV small T antigen directly transforms rodent cells in vitro. The middle tumor antigen is used in model organisms developed to study cancer, such as the MMTV-PyMT system where middle T is coupled to the MMTV promoter . There it functions as an oncogene , while
4650-605: 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
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4800-553: 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,
4950-472: Is a cosmogenic nuclide , continuously produced through neutron activation of natural Ca. Many other calcium radioisotopes are known, ranging from Ca to Ca. They are all much shorter-lived than Ca, the most stable being Ca (half-life 163 days) and Ca (half-life 4.54 days). Isotopes lighter than Ca usually undergo beta plus decay to isotopes of potassium, and those heavier than Ca usually undergo beta minus decay to isotopes of scandium , though near
5100-438: Is a doubly magic nucleus , having 20 protons and 28 neutrons arranged in closed shells. Its beta decay to Sc is very hindered because of the gross mismatch of nuclear spin : Ca has zero nuclear spin, being even–even , while Sc has spin 6+, so the decay is forbidden by the conservation of angular momentum . While two excited states of Sc are available for decay as well, they are also forbidden due to their high spins. As
5250-497: Is a better conductor by mass than both due to its very low density. While calcium is infeasible as a conductor for most terrestrial applications as it reacts quickly with atmospheric oxygen, its use as such in space has been considered. The chemistry of calcium is that of a typical heavy alkaline earth metal. For example, calcium spontaneously reacts with water more quickly than magnesium and less quickly than strontium to produce calcium hydroxide and hydrogen gas. It also reacts with
5400-416: 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
5550-405: 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
5700-439: Is a very ductile silvery metal (sometimes described as pale yellow) whose properties are very similar to the heavier elements in its group, strontium , barium , and radium . A calcium atom has twenty electrons, with electron configuration [Ar]4s . Like the other elements placed in group 2 of the periodic table, calcium has two valence electrons in the outermost s-orbital, which are very easily lost in chemical reactions to form
5850-440: Is also measure by PCR. Tissue staining using a monoclonal antibody against MCV T antigen shows utility in differentiating Merkel cell carcinoma from other small, round cell tumors. Blood tests to detect MCV antibodies have been developed and show that infection with the virus is widespread although Merkel cell carcinoma patients have exceptionally higher antibody responses than asymptomatically infected persons. The JC virus offers
6000-424: Is also used as a polishing agent in toothpaste and in antacids . Calcium lactobionate is a white powder that is used as a suspending agent for pharmaceuticals. In baking, calcium phosphate is used as a leavening agent . Calcium sulfite is used as a bleach in papermaking and as a disinfectant, calcium silicate is used as a reinforcing agent in rubber, and calcium acetate is a component of liming rosin and
6150-421: Is also used to strengthen aluminium alloys used for bearings, for the control of graphitic carbon in cast iron , and to remove bismuth impurities from lead. Calcium metal is found in some drain cleaners, where it functions to generate heat and calcium hydroxide that saponifies the fats and liquefies the proteins (for example, those in hair) that block drains. Besides metallurgy, the reactivity of calcium
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6300-400: 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
6450-439: 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
6600-494: Is controversy over whether the bornavirus , previously thought to cause neurological diseases in horses, could be responsible for psychiatric illnesses in humans. Calcium Calcium is a chemical element ; it has symbol Ca and atomic number 20. As an alkaline earth metal , calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues strontium and barium . It
6750-512: 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
6900-511: Is distantly related to MCV. Two viruses—HPyV6 and HPyV7—are most closely related to KI and WU viruses, while HPyV9 is most closely related to the African green monkey-derived lymphotropic polyomavirus (LPV). A fourteenth virus has been described. Lyon IARC polyomavirus is related to raccoon polyomavirus. The following 14 polyomaviruses with human hosts had been identified and had their genomes sequenced as of 2017: Deltapolyomavirus contains only
7050-452: Is even possible that all the substances we call earths may be only metallic oxyds, irreducible by any hitherto known process. Calcium, along with its congeners magnesium, strontium, and barium, was first isolated by Humphry Davy in 1808. Following the work of Jöns Jakob Berzelius and Magnus Martin af Pontin on electrolysis , Davy isolated calcium and magnesium by putting a mixture of the respective metal oxides with mercury(II) oxide on
7200-505: Is exploited to remove nitrogen from high-purity argon gas and as a getter for oxygen and nitrogen. It is also used as a reducing agent in the production of chromium , zirconium , thorium , vanadium and uranium . It can also be used to store hydrogen gas, as it reacts with hydrogen to form solid calcium hydride , from which the hydrogen can easily be re-extracted. Calcium isotope fractionation during mineral formation has led to several applications of calcium isotopes. In particular,
7350-493: Is extremely efficient at inducing cellular transformation ; SV40 has an additional capsid protein VP4; some examples have an additional regulatory protein called agnoprotein expressed from the late region. The genome also contains a non-coding control or regulatory region containing the early and late regions' promoters , transcriptional start sites, and the origin of replication . The polyomavirus life cycle begins with entry into
7500-498: 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
7650-607: 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
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#17328841227787800-458: Is less reactive than strontium or barium, the oxide–nitride coating that results in air is stable and lathe machining and other standard metallurgical techniques are suitable for calcium. In the United States and Canada, calcium is instead produced by reducing lime with aluminium at high temperatures. Calcium cycling provides a link between tectonics , climate , and the carbon cycle . In
7950-431: 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
8100-453: Is more complicated and involves the bicarbonate ion (HCO 3 ) that forms when CO 2 reacts with water at seawater pH : At seawater pH, most of the dissolved CO 2 is immediately converted back into HCO 3 . The reaction results in a net transport of one molecule of CO 2 from the ocean/atmosphere into the lithosphere . The result is that each Ca ion released by chemical weathering ultimately removes one CO 2 molecule from
8250-541: Is often confounded by high levels of cross-reactivity for SV40 with widespread human polyomaviruses. Most virologists dismiss SV40 as a cause for human cancers. The diagnosis of polyomavirus almost always occurs after the primary infection as it is either asymptomatic or sub-clinical. Antibody assays are commonly used to detect presence of antibodies against individual viruses. Competition assays are frequently needed to distinguish among highly similar polyomaviruses. In cases of progressive multifocal leucoencephalopathy (PML),
8400-506: Is the fifth most abundant element in the Earth's crust , and the third most abundant metal behind aluminium and iron . It is also the fourth most abundant element in the lunar highlands . Sedimentary calcium carbonate deposits pervade the Earth's surface as fossilized remains of past marine life; they occur in two forms, the rhombohedral calcite (more common) and the orthorhombic aragonite (forming in more temperate seas). Minerals of
8550-458: Is the fifth most abundant element in Earth's crust, and the third most abundant metal, after iron and aluminium . The most common calcium compound on Earth is calcium carbonate , found in limestone and the fossilised remnants of early sea life; gypsum , anhydrite , fluorite , and apatite are also sources of calcium. The name derives from Latin calx " lime ", which was obtained from heating limestone. Some calcium compounds were known to
8700-425: Is the second-most common isotope. The other four natural isotopes, Ca, Ca, Ca, and Ca, are significantly rarer, each comprising less than 1% of all natural calcium. The four lighter isotopes are mainly products of the oxygen-burning and silicon-burning processes, leaving the two heavier ones to be produced via neutron capture processes. Ca is mostly produced in a "hot" s-process , as its formation requires
8850-475: 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
9000-492: Is used to make metallic soaps and synthetic resins. Calcium is on the World Health Organization's List of Essential Medicines . Foods rich in calcium include dairy products such as milk and yogurt , cheese , sardines , salmon , soy products, kale , and fortified breakfast cereals . Because of concerns for long-term adverse side effects, including calcification of arteries and kidney stones , both
9150-431: Is very soluble in water, 85% of extracellular calcium is as dicalcium phosphate with a solubility of 2.00 mM , and the hydroxyapatite of bones in an organic matrix is tricalcium phosphate with a solubility of 1000 μM. Calcium is a common constituent of multivitamin dietary supplements , but the composition of calcium complexes in supplements may affect its bioavailability which varies by solubility of
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#17328841227789300-400: 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
9450-474: 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
9600-540: 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,
9750-675: 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'
9900-484: The Merkel cell virus with heparan sulfate . However, in general virus-cell interactions are mediated by commonly occurring molecules on the cell surface, and therefore are likely not a major contributor to individual viruses' observed cell-type tropism . After binding to molecules on the cell surface, the virion is endocytosed and enters the endoplasmic reticulum - a behavior unique among known non-enveloped viruses - where
10050-484: 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
10200-480: The United States (about 2000 to 4000 tonnes per year). Canada and France are also among the minor producers. In 2005, about 24000 tonnes of calcium were produced; about half of the world's extracted calcium is used by the United States, with about 80% of the output used each year. In Russia and China, Davy's method of electrolysis is still used, but is instead applied to molten calcium chloride . Since calcium
10350-425: The black sea bass and gilthead seabream . A total of fourteen polyomaviruses are known to infect humans. Viruses 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
10500-418: The carboxyl groups of glutamic acid or aspartic acid residues; through interacting with phosphorylated serine , tyrosine , or threonine residues; or by being chelated by γ-carboxylated amino acid residues. Trypsin , a digestive enzyme, uses the first method; osteocalcin , a bone matrix protein, uses the third. Some other bone matrix proteins such as osteopontin and bone sialoprotein use both
10650-539: 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
10800-506: The contraction of muscles , nerve conduction, and the clotting of blood. As a result, intra- and extracellular calcium levels are tightly regulated by the body. Calcium can play this role because the Ca ion forms stable coordination complexes with many organic compounds, especially proteins ; it also forms compounds with a wide range of solubilities, enabling the formation of the skeleton . Calcium ions may be complexed by proteins through binding
10950-407: 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
11100-422: 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
11250-430: 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:
11400-496: The nuclear drip lines , proton emission and neutron emission begin to be significant decay modes as well. Like other elements, a variety of processes alter the relative abundance of calcium isotopes. The best studied of these processes is the mass-dependent fractionation of calcium isotopes that accompanies the precipitation of calcium minerals such as calcite , aragonite and apatite from solution. Lighter isotopes are preferentially incorporated into these minerals, leaving
11550-455: The oxygen and nitrogen in air to form a mixture of calcium oxide and calcium nitride . When finely divided, it spontaneously burns in air to produce the nitride. Bulk calcium is less reactive: it quickly forms a hydration coating in moist air, but below 30% relative humidity it may be stored indefinitely at room temperature. Besides the simple oxide CaO, calcium peroxide , CaO 2 , can be made by direct oxidation of calcium metal under
11700-467: The physiological and biochemical processes of organisms and cells : in signal transduction pathways where they act as a second messenger ; in neurotransmitter release from neurons ; in contraction of all muscle cell types; as cofactors in many enzymes ; and in fertilization . Calcium ions outside cells are important for maintaining the potential difference across excitable cell membranes , protein synthesis, and bone formation. Calcium
11850-454: 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
12000-447: 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
12150-424: The 1997 observation by Skulan and DePaolo that calcium minerals are isotopically lighter than the solutions from which the minerals precipitate is the basis of analogous applications in medicine and in paleoceanography. In animals with skeletons mineralized with calcium, the calcium isotopic composition of soft tissues reflects the relative rate of formation and dissolution of skeletal mineral. In humans, changes in
12300-527: 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
12450-492: The SV40 clade, the avian clade, and the murine polyomavirus clade. A subsequent proposed reclassification by the International Committee on Taxonomy of Viruses (ICTV) recommended dividing the family of Polyomaviridae into three genera: The current ICTV classification system recognises six genera and 117 species, of which five could not be assigned a genus. This system retains the distinction between avian and mammalian viruses, grouping
12600-586: The U.S. Institute of Medicine (IOM) and the European Food Safety Authority (EFSA) set Tolerable Upper Intake Levels (ULs) for combined dietary and supplemental calcium. From the IOM, people of ages 9–18 years are not to exceed 3 g/day combined intake; for ages 19–50, not to exceed 2.5 g/day; for ages 51 and older, not to exceed 2 g/day. EFSA set the UL for all adults at 2.5 g/day, but decided
12750-567: The absence of steric hindrance , smaller group 2 cations tend to form stronger complexes, but when large polydentate macrocycles are involved the trend is reversed. Though calcium is in the same group as magnesium and organomagnesium compounds are very widely used throughout chemistry, organocalcium compounds are not similarly widespread because they are more difficult to make and more reactive, though they have recently been investigated as possible catalysts . Organocalcium compounds tend to be more similar to organoytterbium compounds due to
12900-488: The alkali metals and the divalent lanthanides europium and ytterbium , calcium metal dissolves directly in liquid ammonia to give a dark blue solution. Due to the large size of the calcium ion (Ca ), high coordination numbers are common, up to 24 in some intermetallic compounds such as CaZn 13 . Calcium is readily complexed by oxygen chelates such as EDTA and polyphosphates , which are useful in analytic chemistry and removing calcium ions from hard water . In
13050-423: The ancients, though their chemistry was unknown until the seventeenth century. Pure calcium was isolated in 1808 via electrolysis of its oxide by Humphry Davy , who named the element. Calcium compounds are widely used in many industries: in foods and pharmaceuticals for calcium supplementation , in the paper industry as bleaches, as components in cement and electrical insulators, and in the manufacture of soaps. On
13200-407: The avian subset into the genus Gammapolyomavirus . The six genera are: The following species are unassigned to a genus: Description of additional viruses is ongoing. These include the sea otter polyomavirus 1 and Alpaca polyomavirus Another virus is the giant panda polyomavirus 1. Another virus has been described from sigmodontine rodents. Another - tree shrew polyomavirus 1 - has been described in
13350-484: 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,
13500-515: The calcium isotopic composition of urine have been shown to be related to changes in bone mineral balance. When the rate of bone formation exceeds the rate of bone resorption, the Ca/ Ca ratio in soft tissue rises and vice versa. Because of this relationship, calcium isotopic measurements of urine or blood may be useful in the early detection of metabolic bone diseases like osteoporosis . A similar system exists in seawater, where Ca/ Ca tends to rise when
13650-493: The cell cycle is a powerful force for oncogenic transformation. The small tumor antigen protein is also able to activate several cellular pathways that stimulate cell proliferation. Polyomavirus small T antigens commonly target protein phosphatase 2A ( PP2A ), a key multisubunit regulator of multiple pathways including Akt , the mitogen-activated protein kinase (MAPK) pathway, and the stress-activated protein kinase (SAPK) pathway. Merkel cell polyomavirus small T antigen encodes
13800-410: 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
13950-502: The early mRNA is reduced and expression of the late mRNA, which encodes the viral capsid proteins, begins. As these interactions begin, the LTs belonging to several polyomaviruses, including Merkel cell polyomavirus , present oncogenic potential. Several mechanisms have been described for regulating the transition from early to late gene expression, including the involvement of the LT protein in repressing
14100-432: The early promoter, the expression of un-terminated late mRNAs with extensions complementary to early mRNA, and the expression of regulatory microRNA . Expression of the late genes results in accumulation of the viral capsid proteins in the host cell cytoplasm. Capsid components enter the nucleus in order to encapsidate new viral genomic DNA. New virions may be assembled in viral factories . The mechanism of viral release from
14250-662: The exact mechanisms are unclear. The polyomaviruses are members of group I (dsDNA viruses). The classification of polyomaviruses has been the subject of several proposed revisions as new members of the group are discovered. Formerly, polyomaviruses and papillomaviruses , which share many structural features but have very different genomic organizations, were classified together in the now-obsolete family Papovaviridae . (The name Papovaviridae derived from three abbreviations: Pa for Papillomavirus , Po for Polyomavirus , and Va for "vacuolating.") The polyomaviruses were divided into three major clades (that is, genetically related groups):
14400-431: 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
14550-446: 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
14700-399: The first and the second. Direct activation of enzymes by binding calcium is common; some other enzymes are activated by noncovalent association with direct calcium-binding enzymes. Calcium also binds to the phospholipid layer of the cell membrane , anchoring proteins associated with the cell surface. As an example of the wide range of solubility of calcium compounds, monocalcium phosphate
14850-731: The first type include limestone , dolomite , marble , chalk , and iceland spar ; aragonite beds make up the Bahamas , the Florida Keys , and the Red Sea basins. Corals , sea shells , and pearls are mostly made up of calcium carbonate. Among the other important minerals of calcium are gypsum (CaSO 4 ·2H 2 O), anhydrite (CaSO 4 ), fluorite (CaF 2 ), and apatite ([Ca 5 (PO 4 ) 3 X], X = OH, Cl, or F).gre The major producers of calcium are China (about 10000 to 12000 tonnes per year), Russia (about 6000 to 8000 tonnes per year), and
15000-399: 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
15150-415: The formation of bone by allowing and enhancing the deposition of calcium ions there, allowing rapid bone turnover without affecting bone mass or mineral content. When plasma calcium levels fall, cell surface receptors are activated and the secretion of parathyroid hormone occurs; it then proceeds to stimulate the entry of calcium into the plasma pool by taking it from targeted kidney, gut, and bone cells, with
15300-421: The four human viruses shown in the above table. The Alpha and Beta groups contain viruses that infect a variety of mammals. The Gamma group contains the avian viruses. Clinically significant disease associations are shown only where causality is expected. Antibodies to the monkey lymphotropic polyomavirus have been detected in humans suggesting that this virus - or a closely related virus - can infect humans. All
15450-589: The host cell machinery to replicate the host cell needs to be in s-phase for this to begin. Due to this, large T-antigen also modulates cellular signaling pathways to stimulate progression of the cell cycle by binding to a number of cellular control proteins. This is achieved by a two prong attack of inhibiting tumor suppressing genes p53 and members of the retinoblastoma (pRB) family, and stimulating cell growth pathways by binding cellular DNA, ATPase-helicase, DNA polymerase α association, and binding of transcription preinitiation complex factors. This abnormal stimulation of
15600-420: The host cell varies among polyomaviruses; some express proteins that facilitate cell exit, such as the agnoprotein or VP4 . In some cases high levels of encapsidated virus result in cell lysis , releasing the virions. The large tumor antigen plays a key role in regulating the viral life cycle by binding to the viral origin of DNA replication where it promotes DNA synthesis. Also as the polyomavirus relies on
15750-406: 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
15900-432: The hypothetical MX. This occurs because of the much greater lattice energy afforded by the more highly charged Ca cation compared to the hypothetical Ca cation. Calcium, strontium, barium, and radium are always considered to be alkaline earth metals ; the lighter beryllium and magnesium , also in group 2 of the periodic table, are often included as well. Nevertheless, beryllium and magnesium differ significantly from
16050-517: 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
16200-417: The information for children and adolescents was not sufficient to determine ULs. Calcium is an essential element needed in large quantities. The Ca ion acts as an electrolyte and is vital to the health of the muscular, circulatory, and digestive systems; is indispensable to the building of bone in the form of hydroxyapatite ; and supports synthesis and function of blood cells. For example, it regulates
16350-525: The kidneys of monkeys without causing disease, but can cause cancer in rodents under laboratory conditions. In the 1950s and early 1960s, well over 100 million people may have been exposed to SV40 due to previously undetected SV40 contamination of polio vaccine , prompting concern about the possibility that the virus might cause disease in humans. Although it has been reported as present in some human cancers, including brain tumors , bone tumors , mesotheliomas , and non-Hodgkin's lymphomas , accurate detection
16500-405: The late region of the genome. The agnoprotein is a small multifunctional phospho-protein found in the late coding part of the genome of some polyomaviruses, most notably BK virus , JC virus , and SV40 . It is essential for proliferation in the viruses that express it and is thought to be involved in regulating the viral life cycle, particularly replication and viral exit from the host cell, but
16650-428: The low calcium concentration in the cytoplasm. Both expression of viral genes and replication of the viral genome occur in the nucleus using host cell machinery. The early genes - comprising at minimum the small tumor antigen (ST) and large tumor antigen (LT) - are expressed first, from a single alternatively spliced messenger RNA strand. These proteins serve to manipulate the host's cell cycle - dysregulating
16800-416: 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
16950-501: The most common isotope of calcium in nature is Ca, which makes up 96.941% of all natural calcium. It is produced in the silicon-burning process from fusion of alpha particles and is the heaviest stable nuclide with equal proton and neutron numbers; its occurrence is also supplemented slowly by the decay of primordial K . Adding another alpha particle leads to unstable Ti, which decays via two successive electron captures to stable Ca; this makes up 2.806% of all natural calcium and
17100-416: 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,
17250-434: The neighbouring group 2 metals. It crystallises in the face-centered cubic arrangement like strontium and barium; above 443 °C (716 K), it changes to a body-centered cubic . Its density of 1.526 g/cm (at 20 °C) is the lowest in its group. Calcium is harder than lead but can be cut with a knife with effort. While calcium is a poorer conductor of electricity than copper or aluminium by volume, it
17400-525: The only element with two primordial doubly magic isotopes. The experimental lower limits for the half-lives of Ca and Ca are 5.9 × 10 years and 2.8 × 10 years respectively. Apart from the practically stable Ca, the longest lived radioisotope of calcium is Ca. It decays by electron capture to stable K with a half-life of about 10 years. Its existence in the early Solar System as an extinct radionuclide has been inferred from excesses of K: traces of Ca also still exist today, as it
17550-461: 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
17700-419: The other hand increases the compound's solubility, volatility, and kinetic stability. Natural calcium is a mixture of five stable isotopes ( Ca, Ca, Ca, Ca, and Ca) and one isotope with a half-life so long that it is for all practical purposes stable ( Ca , with a half-life of about 4.3 × 10 years). Calcium is the first (lightest) element to have six naturally occurring isotopes. By far
17850-400: The other hand, the metal in pure form has few applications due to its high reactivity; still, in small quantities it is often used as an alloying component in steelmaking, and sometimes, as a calcium–lead alloy, in making automotive batteries. Calcium is the most abundant metal and the fifth-most abundant element in the human body . As electrolytes , calcium ions (Ca ) play a vital role in
18000-428: The other members of the group in their physical and chemical behavior: they behave more like aluminium and zinc respectively and have some of the weaker metallic character of the post-transition metals , which is why the traditional definition of the term "alkaline earth metal" excludes them. Calcium metal melts at 842 °C and boils at 1494 °C; these values are higher than those for magnesium and strontium,
18150-436: The other two capsid proteins, VP2 or VP3 . The genome of a typical polyomavirus codes for between 5 and 9 proteins , divided into two transcriptional regions called the early and late regions due to the time during infection in which they are transcribed. Each region is transcribed by the host cell's RNA polymerase II as a single pre-messenger RNA containing multiple genes. The early region usually codes for two proteins,
18300-425: The plaster in the tomb of Tutankhamun . The ancient Romans instead used lime mortars made by heating limestone (CaCO 3 ). The name "calcium" itself derives from the Latin word calx "lime". Vitruvius noted that the lime that resulted was lighter than the original limestone, attributing this to the boiling of the water. In 1755, Joseph Black proved that this was due to the loss of carbon dioxide , which as
18450-416: 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
18600-578: The polyomaviruses are highly common childhood and young adult infections. Most of these infections appear to cause little or no symptoms. These viruses are probably lifelong persistent among almost all adults. Diseases caused by human polyomavirus infections are most common among immunocompromised people; disease associations include BK virus with nephropathy in renal transplant and non-renal solid organ transplant patients, JC virus with progressive multifocal leukoencephalopathy , and Merkel cell virus (MCV) with Merkel cell cancer . SV40 replicates in
18750-960: 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
18900-408: The rate of removal of Ca by mineral precipitation exceeds the input of new calcium into the ocean. In 1997, Skulan and DePaolo presented the first evidence of change in seawater Ca/ Ca over geologic time, along with a theoretical explanation of these changes. More recent papers have confirmed this observation, demonstrating that seawater Ca concentration is not constant, and that the ocean is never in
19050-422: 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
19200-479: 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
19350-407: The salt involved: calcium citrate , malate , and lactate are highly bioavailable, while the oxalate is less. Other calcium preparations include calcium carbonate , calcium citrate malate , and calcium gluconate . The intestine absorbs about one-third of calcium eaten as the free ion , and plasma calcium level is then regulated by the kidneys . Parathyroid hormone and vitamin D promote
19500-451: 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
19650-421: The similar ionic radii of Yb (102 pm) and Ca (100 pm). Most of these compounds can only be prepared at low temperatures; bulky ligands tend to favor stability. For example, calcium di cyclopentadienyl , Ca(C 5 H 5 ) 2 , must be made by directly reacting calcium metal with mercurocene or cyclopentadiene itself; replacing the C 5 H 5 ligand with the bulkier C 5 (CH 3 ) 5 ligand on
19800-500: The simplest terms, mountain-building exposes calcium-bearing rocks such as basalt and granodiorite to chemical weathering and releases Ca into surface water. These ions are transported to the ocean where they react with dissolved CO 2 to form limestone ( CaCO 3 ), which in turn settles to the sea floor where it is incorporated into new rocks. Dissolved CO 2 , along with carbonate and bicarbonate ions, are termed " dissolved inorganic carbon " (DIC). The actual reaction
19950-399: The small and large tumor antigens, produced by alternative splicing . The late region contains the three capsid structural proteins VP1, VP2, and VP3, produced by alternative translational start sites. Additional genes and other variations on this theme are present in some viruses: for example, rodent polyomaviruses have a third protein called middle tumor antigen in the early region, which
20100-522: The steel and become small and spherical, improving castability, cleanliness and general mechanical properties. Calcium is also used in maintenance-free automotive batteries , in which the use of 0.1% calcium– lead alloys instead of the usual antimony –lead alloys leads to lower water loss and lower self-discharging. Due to the risk of expansion and cracking, aluminium is sometimes also incorporated into these alloys. These lead–calcium alloys are also used in casting, replacing lead–antimony alloys. Calcium
20250-399: 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
20400-457: 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
20550-587: The surficial system (atmosphere, ocean, soils and living organisms), storing it in carbonate rocks where it is likely to stay for hundreds of millions of years. The weathering of calcium from rocks thus scrubs CO 2 from the ocean and atmosphere, exerting a strong long-term effect on climate. The largest use of metallic calcium is in steelmaking , due to its strong chemical affinity for oxygen and sulfur . Its oxides and sulfides, once formed, give liquid lime aluminate and sulfide inclusions in steel which float out; on treatment, these inclusions disperse throughout
20700-473: The surrounding solution enriched in heavier isotopes at a magnitude of roughly 0.025% per atomic mass unit (amu) at room temperature. Mass-dependent differences in calcium isotope composition are conventionally expressed by the ratio of two isotopes (usually Ca/ Ca) in a sample compared to the same ratio in a standard reference material. Ca/ Ca varies by about 1- 2‰ among organisms on Earth. Calcium compounds were known for millennia, though their chemical makeup
20850-611: The tissue where the tumor develops is determined by the MMTV promoter. The polyomavirus capsid consists of one major component, major capsid protein VP1 , and one or two minor components, minor capsid proteins VP2 and VP3 . VP1 pentamers form the closed icosahedral viral capsid , and in the interior of the capsid each pentamer is associated with one molecule of either VP2 or VP3. Some polyomaviruses, such as Merkel cell polyomavirus , do not encode or express VP3. The capsid proteins are expressed from
21000-446: The transition from G1 phase to S phase , when the host cell's genome is replicated - because host cell DNA replication machinery is needed for viral genome replication. The precise mechanism of this dysregulation depends on the virus; for example, SV40 LT can directly bind host cell p53 , but murine polyomavirus LT does not. LT induces DNA replication from the viral genome's non-coding control region (NCCR), after which expression of
21150-429: The tree shrew. Most polyomaviruses do not infect humans. Of the polyomaviruses cataloged as of 2017, a total of 14 were known with human hosts. However, some polyomaviruses are associated with human disease, particularly in immunocompromised individuals. MCV is highly divergent from the other human polyomaviruses and is most closely related to murine polyomavirus. Trichodysplasia spinulosa-associated polyomavirus (TSV)
21300-446: 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
21450-824: 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
21600-418: The viral capsid structure is likely to be disrupted by action of host cell disulfide isomerase enzymes. The details of transit to the nucleus are not clear and may vary among individual polyomaviruses. It has been frequently reported that an intact, albeit distorted, virion particle is released from the endoplasmic reticulum into the cell cytoplasm, where the genome is released from the capsid, possibly due to
21750-441: The viral load can be done through PCR. This is also true for the blood. Renal biopsy can also be used if the two methods just described are inconclusive or if the specific viral load for the renal tissue is desired. Similarly to the urine cytology, the renal cells are examined under light microscopy for polyomavirus inclusion of the nucleus, as well as cell lysis and viral partials in the extra cellular fluid. The viral load as before
21900-446: The viral load in both urine and blood, and a renal biopsy . The reactivation of polyomavirus in the kidneys and urinary tract causes the shedding of infected cells, virions, and/or viral proteins in the urine. This allows urine cytology to examine these cells, which if there is polyomavirus inclusion of the nucleus, is diagnostic of infection. Also as the urine of an infected individual will contain virions and/or viral DNA, quantitation of
22050-503: 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
22200-460: 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
22350-716: Was identified as a virus by Sarah Stewart and Bernice Eddy , after whom it was once called "SE polyoma". The term "polyoma" refers to the viruses' ability to produce multiple (poly-) tumors (-oma) under certain conditions. The name has been criticized as a "meatless linguistic sandwich" ("meatless" because both morphemes in "polyoma" are affixes) giving little insight into the viruses' biology; in fact, subsequent research has found that most polyomaviruses rarely cause clinically significant disease in their host organisms under natural conditions. Dozens of polyomaviruses have been identified and sequenced as of 2017, infecting mainly birds and mammals. Two polyomaviruses are known to infect fish,
22500-504: Was not understood until the 17th century. Lime as a building material and as plaster for statues was used as far back as around 7000 BC. The first dated lime kiln dates back to 2500 BC and was found in Khafajah , Mesopotamia . About the same time, dehydrated gypsum (CaSO 4 ·2H 2 O) was being used in the Great Pyramid of Giza . This material would later be used for
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