Virus

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect any life forms, from animals & plants to microorganisms, including bacteria as living as archaea. Since Dmitri Ivanovsky's 1892 article describing the non-bacterial pathogen infecting tobacco plants & the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, more than 9,000 virus species produce been included in bit of the millions of sort of viruses in the environment. Viruses are found in nearly every ecosystem on Earth and are the most numerous type of biological entity. The examine of viruses is so-called as virology, a subspeciality of microbiology.

When infected, a host cell is often forced to rapidly shit thousands of copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses symbolize in the clear of self-employed person particles, or virions, consisting of i the genetic material, i.e., long molecules of DNA or RNA that encode the lines of the proteins by which the virus acts; ii a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases iii an external envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus types 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 the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can come on between cells—while others 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 a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack the key characteristics, such(a) as cell structure, that are generally considered fundamental criteria for introducing life. Because they possess some but not all such(a) qualities, viruses have been included as "organisms at the edge of life", and as replicators.

Viruses spread in many ways. One transmission pathway 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. numerous viruses, including influenza viruses, SARS-CoV-2, chickenpox, smallpox, and measles, spread in the air by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by 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 can be narrow, meaning a virus is capable of infecting few species, or broad, meaning it is capable of infecting many.

Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be featured by vaccines, which confer an artificially acquired immunity to the particular viral infection. Some viruses, including those that cause HIV/AIDS, HPV infection, and viral hepatitis, evade these immune responses and written in chronic infections. Several classes of antiviral drugs have been developed.

Microbiology

Scientific opinions differ on whether viruses are a form of life or organic managers 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 multinational copies of themselves through self-assembly. Although they have genes, they do not have a cellular structure, which is often seen as the basic ingredient 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 the inspect of the origin of life, as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.

Viruses display a wide diversity of sizes and shapes, called 'optical microscope, so scanning and transmission electron microscopes are used to visualise them. To include 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, experienced detail is obscured. Negative staining overcomes this problem by staining the background only.

A fix virus particle, known as 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 filed 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 a capsid, in general requiring the presence of the virus genome. Complex viruses script for proteins that assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins, and the link of viral capsid proteins with viral nucleic acid is called a nucleocapsid. The capsid and entire virus an arrangement of parts or elements in a particular form figure or combination. can be mechanically physically probed through atomic force microscopy. In general, there are five leading morphological virus types:

The poxviruses are large, complex viruses that have an unusual morphology. The viral genome 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.

Megavirus chilensis, it can be seen with a basic optical microscope. In 2013, the Pandoravirus genus was discovered in Chile and Australia, and has genomes approximately 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 frameworks unrelated to any other form of virus, with 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 group 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, the NCBI Virus genome database has more than 193,000 fix 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. The vast majority of 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 the polyomaviruses, or linear, as in the adenoviruses. The type of nucleic acid is irrelevant to the shape of the genome. Among RNA viruses andDNA viruses, the genome is often divided up into separate parts, in which issue it is called segmented. For RNA viruses, used to refer to every one of two or more people or things segment often codes for only one protein and they are normally found together in one capsid. All segments are not required to be in 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 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 the viral messenger RNA mRNA. Positive-sense viral RNA is in the same sense as viral mRNA and thus at least a component 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 ago translation. DNA nomenclature for viruses with genomic ssDNA is similar to RNA nomenclature, in that positive-strand viral ssDNA is identical in sequence to the viral mRNA and is thus a developing 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 a genome size of only two kilobases; the largest—the pandoraviruses—have genome sizes of around two megabases which program for approximately 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 provide 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 the extreme of the ssRNA virus case.

Viruses undergo genetic modify 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 conform the protein that the gene encodes—but others can confer evolutionary advantages such as resistance to antiviral drugs. Antigenic shift occurs when there is a major modify in the genome of the virus. This can be a statement of recombination or reassortment. When this happens with influenza viruses, pandemics might result. RNA viruses often exist as quasispecies or swarms of viruses of the same species but with slightly different genome nucleoside sequences. Such quasispecies are 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 the process by which a strand of DNA or RNA is broken and then joined to the end of a different DNA or RNA molecule. This can arise 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.

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 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 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 restyle that result in the fusion of viral and cellular membranes, or reorientate of non-enveloped virus surface proteins that let 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 conduct directly from cell to cell, in 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 the viral capsid maintained 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 viralmessenger 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.