Virus

Characteristics

A virus is characterized by its incapacity to multiply by division. It needs for that to use a host cell: a virus is a parasitic intracellular obligatory. It is composed of a molecule of nucleic acid (either of DNA or of ARN, simple or double bit) surrounded by a hull of Protéine S called the Capside and sometimes of an envelope. It does not have in general any Enzyme which can produce energy. The viruses are generally of very small size (compared with that of a Bactérie for example), in general lower than 250 Nanomètre S; however, the Mimivirus has a size of 400 Nm, which makes it larger than the smallest bacteria. This last has also the characteristic to have at the same time DNA and ARN.

The free form of the virus (or viral particle) is called the Virion.

There exists a very great diversity of virus, estimated in 2007 at 10³¹ which is much more than diversity of the three reigns (Bacteria, Archaea, Eukaryota) joined together.

All the living beings can be infected by viruses. There exist viruses of bacteria (the Bactériophage S), viruses of Archaea, viruses of algae ( Phycodnaviridae ), viruses of plants, fungic viruses, viruses of invertebrates, viruses the vertebrate ones at which one finds many disease-causing agents.

Discovered

The viral diseases like the Rage, the Yellow fever, the Variola, affect human since centuries. Hiéroglyphe S highlight the Poliomyélite in the ancient Egypt, the writings of Antiquity gréco - Roman and of the Far East describes certain viral diseases. However, the cause of these diseases remained unknown for a long time. At the end of the 19th century, the design of infectious agents which were neither of the Bactérie S, neither of the Champignon S, nor of the parasite S was still difficult.

Between 1887 and 1892, the Russian botanist Dimitri Ivanovski studied a vegetable disease, the mosaic of the tobacco, and showed that the Sève sick plans contained an infectious agent which was not retained by the Filtres Chamberland conceived by the biologist of the same name. Ivanovski thought that it was about a Toxine or of a very small bacterium. It is the chemist Dutch Martinus Beijerinck which looks further into this work and dismissed the bacterial assumption, and named the phenomenon Contagium vivum fluidum . At the same time, the virus of the Foot-and-mouth disease is the first virus identified by Friedrich Loeffler and Paul Frosch. The virus of the yellow fever is the first pathogenic virus of the Man identified between 1900 and 1902.

It is during the First World War that the English Frederick Twort and the microbiologist free - Canadian Felix d' Hérelle highlights the phenomenon of “transmissible lysis” observable by the lysis of the bacteria cultivated in solid medium. This phenomenon is due to a virus of bacteria that Felix d' Hérelle baptized Bactériophage. The viruses of the plants, the animals, the Man and the bacteria were thus discovered and their lists did not cease lengthening during the 20th century. Appearance of electronic microscopy in the allowed Years 1930 observation of the viruses, but one still did not know at that time what they were really.

The American biochemist Wendell Stanley crystallized the virus of the mosaic of the proteinic tobacco in the form of Cristal in 1935. The following year of the complementary studies showed that this crystal also contained ARN. The later studies showed that according to the studied viruses, those were made up either of proteins and ARN, or of proteins and DNA. It is in 1957 that André Lwoff proposed a clear and modern definition viruses.

From the Years 1960, the development of the cellular cultures, electronic microscopy, then of the Molecular biology made it possible to the scientists to progress in the comprehension of the mechanisms of replication of the viruses, in the realization of reliable diagnoses and the development of Vaccin.

Origin

There exist several assumptions concerning the origin and the evolution of the viruses. It is probable that all the viruses do not derive from the same common ancestor and the various viruses can have different origins.

the viruses and the cells could appear in paramount soup at the same time and evolve/move in parallel. In this scenario, at the beginning of the appearance of the life, the oldest genetic systems of car-réplication (probably of ARN) became more complex and were wrapped in a lipidic bag to lead to the progénote at the origin of the cells. Another replicative form could have kept its simplicity to form viral particles.
the viruses could derive from cells having undergone a regression. According to this assumption, the ancestors of the viruses would have been free living beings or micro-organisms become of predatory or parasite S dependant on their host. The relations of Parasitisme involve the loss of many Gène S (in particular genes for the Métabolisme brought by the host). This organization would have Co-advanced with the host cell and would have preserved only its capacity to retort its Nucleic acid and the mechanism of transfer of cell at cell.
the viruses can originate in of the nucleic acid pieces of which are “escaped” cellular Génome to become independent. This phenomenon could have taken place at the time of errors during the replication of the genetic material. The viruses could also have for origin of the Plasmide S (circular molecules of DNA) or of the Transposon S (sequence of DNA able to move and to multiply in a Génome).

Structure

A complete viral particle, called virion, is composed of a filament of Nucleic acid, generally stabilized by basic Nucléoprotéine S, locked up in a called protective proteinic hull Capside. The shape of the capsid is at the base of various morphologies of the viruses. The size of the viruses ranges between 10 and 400 Nm. The Génome S of the viruses comprise only few genes with: 1200 genes. The smallest known virus is the virus delta which parasitizes itself that of the Hépatite B. It comprises one Gène. The largest known virus is the Mimivirus with a diameter which reaches 400 nanometers and a genome which comprises: 1200 genes.

Nucleic acid

Capsid

Wrap

Many viruses are surrounded by an envelope (or Péplos ) which occurs during the crossing of the cellular membranes. Its constitution is complex and present a mixture of cellular elements and elements of viral origin. One finds there Protéine S, Glucide S and Lipides. The viruses having an envelope are the wrapped viruses . The viruses not having an envelope are the naked viruses .

Classification

Viral multiplication

The viruses can multiply only within alive cells, by replication of their nucleic acid. It is the interaction of the viral genome and the host cell which leads to the production of new viral particles. The infection of a cell by a virus, then the multiplication of the virus can be summarized in various stages. However, after penetration of the virus in the cell, these stages can differ according to nature from the virus in question and in particular according to whether it is a question from a virus with DNA or a virus with ARN.

attachment or adsorption : during this stage, there is connection of a viral Protéine with a receiver of cellular surface. The receivers eucaryotes can be either of the glycoprotéines, or of the glycosphingolipides. The receivers of the Bactériophage S are glycoprotéines or lipopolysaccharides. The vegetable cells do not have receivers specific to the viruses.
the penetration : according to the viruses, there exist several mechanisms of penetration of the virus inside the cell. At the Bacteriophage S, only the viral Génome penetrates in the bacterial cell. At the animal viruses the virus can penetrate by several mechanisms. The virus can penetrate by Pinocytose: the viral nucléocapside, surrounded of the plasmic Membrane penetrates in the cell. It is often the case of the naked viruses. In the case of the wrapped viruses, the virus can penetrate either by fusion (there is fusion of the viral envelope and the plasmic Membrane cellular) or by Endocytose (there is accumulation of viral particles in cytoplasmic blisters).
the decapsidation after the penetration (or at the same time), there is release of the Nucleic acid . According to the viruses, the decapsidation can take place in the Cytoplasme or the core.
the replication or viral multiplication : at the time of this phase, there are replication of the genome, expression of the genome in the form of ARNm (transcription) and Traduction of ARNm in Protéine S by the cellular machinery. According to the types of germ and the nature of their Genome, the mechanism of the viral multiplication can be very different.
the assembly (phase of maturation): there are assembly and maturation of the viruses in the infected cells. There is encapsidation of the Génome. The wrapped viruses acquire their envelope by budding, with the detriment of the plasmic Membrane or the nuclear membrane of the cell-host.
the release : the reconstituted virus is released outside the cell.

Culture of the viruses

In order to better know their biology, their multiplication, their cycle of reproduction and possibly in order to prepare Vaccine S, it is necessary to cultivate the viruses. Those can multiply only within alive cells. The viruses infecting the cells eucaryotes are cultivated on cultures of cells obtained starting from animal or vegetable fabrics. The cells are cultivated in a container out of glass or plastic, then are infected by the studied virus. The animal viruses can also be cultivated on embryonnés eggs and sometimes in the animal, when in vitro culture is impossible. The bacterial viruses can also be cultivated by inoculation of a significant bacterial culture. The viruses of plants can also be cultivated on the full-course ones of vegetable fabrics, cellular suspensions or on whole plants.

The viruses can then be quantified various manners. They can be counted directly thanks to electronic microscopy. In the case of the bacterial viruses, the technique of the plates (or beaches) are very much used to evaluate the number of viruses in a suspension. A dilution of viral suspension is added to a bacterial suspension, then the unit is distributed in boxes of $petri. After culture, clear zones (beaches) on the surface of the gélose are the consequence of the destruction of a bacterium and adjacent bacteria by a virion.

The viruses can be purified thanks to various methods of biochemistry (differential Centrifugation, precipitation, denaturation, enzymatic digestion).

Discusses on its statute of form of life

The viruses in common have components with the alive cells, like a nucleic acid (DNA or ARN) and Protéine S. Cependant, according to the definition of the biochemist Wendell Stanley, the viruses are “simple” biological associations of molecules. They are the fruit of an car-organization of organic molecules and are thus not alive. François Jacob also insists on this characteristic of the viruses: “placed in suspension in a culture medium, they can neither metabolize, neither to produce or use energy, neither to grow, nor to multiply, all functions common to the living beings”. The viruses can multiply only by using the enzymatic equipment of an alive cell. Moreover, the viruses contain well a nucleic acid, DNA or ARN but never both, unlike the alive cells.

Nevertheless, during the last years, the description of new viruses starts again the debate on the or not alive character living viruses. The Mimivirus, infecting a Ameba, has in its Génome: 1200 genes, that is to say more than some Bactérie S. Moreover some of these genes would take part in the proteinic synthesis and mechanisms of repair of the DNA. There usually exists at the mimivirus about thirty genes present at the organizations cellular but absent at the viruses. In addition, virus ATV of archeobacteries shows to him also astonishing characteristics. This virus in the lemon shape has the effect of changing apart from the cellular context by an active mechanism. It is able to lengthen at each end at a temperature of 80°C, temperature to which saw its host Acidianus near the hydrothermal sources.

The viruses have also a role in the evolution. Patrick Forterre proposes even the assumption that the viruses would be the “inventors” of the DNA. With the Origin of life, the ARN dominated (assumption of the world with ARN) and provided at the same time the functions of storage and transmission of genetic information and of catalysis of the chemical reactions. The DNA would have appeared then and selected because of its greater stability. According to Patrick Forterre the first organization with DNA would be a virus. The DNA would confer on the virus the capacity to resist Enzyme S degrading the Génome S ARN, arms with probable defense of the protocellules. One finds the same principle at current viruses, which deteriorate their DNA to resist enzymes produced by infected bacteria.

The debate between the alive or inert character of the viruses is open still today. To answer this question in pleasing another: what life? According to Ali Saïb, the notion of alive is a dynamic concept, evolving/moving according to our knowledge. Consequently, the border between the inert matter and living it is quite as unstable .

Virus of the procaryotes

There exist two categories of virus of Procaryote S according to the type of host whom they parasitize. The first category gathers those which infect the bacteria and are called Bactériophage S. the second category gathers those which infect the archaeobacteries. There exist four great morphological groups of virus of procaryotes.

viruses with binary symmetry. This group represents nearly 96% of the viruses of procaryotes and corresponds to the families of the Myoviridae , the Siphoviridae and the Podoviridae .
viruses with cubic symmetry with an icosahedral capsid but not of tail like the Microviridae .
the viruses with helicoid symmetry which have the shape of filaments like the Inoviridae like the phage M13.
pleomorphic viruses, without capsids true but having an envelope. This group gathers six families of virus of which five gather viruses infecting only the archeobacteries. Certain viruses of archeobacteries are pleomorphic, whereas others have the shapes of bottles, lemon, spindle.

The bacteriophages have a role in the ecosystems. For example, in the watery ecosystems, they take part in the control of abundance and bacterial diversity.

Virus of the plants

The structure of the viruses of the Plant S or Phytovirus, is similar to that of the bacterial and animal viruses. Many vegetable viruses are appeared as thin and long propellers. The majority have a genome made up of ARN. The viruses of plants can be disseminated by the wind or vectors like the Insecte S and the Nématode S, sometimes by seeds and the Pollen. The viruses can also contaminate the plant via a wound or of a Clerc's Office. Various types of symptoms can appear on the infected plant. The viruses can cause spots or fadings on the sheets and the flowers. Tumors can occur on the stems or the feuilles.
The virus of the mosaic of the tobacco (TMV or tobamovirus) is a very studied example of virus of plants.

Virus of the insects

The Baculovirus are very studied viruses of Insecte S. They infect mainly the Lépidoptère S. the larva of the insect is infected by introducing food. Starting from the digestive tract, the infection can be transmitted to other fabrics. The use of pathogenic viruses of invertebrates in the fight against the devastating insects of the cultures and the forests could be one of the means to limit or replace the insecticides chimiques.
The baculovirus are also used in Molecular biology to express a foreign gene (recombining protein) in cultures of cells of insecte.
In addition, certain viruses of plants are transmitted by invertebrates but do not multiply at these vectors.

Virus of mushrooms

The viruses of the Champignon S are particular because they are propagated during cellular fusion. There are no extracellular virions. At the Yeast S like Saccharomyces , the viruses are transmitted to the moment of cytoplasmic mixing during cellular fusion. The filamentous mushrooms like Penicillium or the cultivated mushroom Agaricus bisporus can also be infected by viruses. That can involve problems during production.

Virus and disease at the Man

The Cold, the Influenza, the Chicken pox, the Measles, the infectious Mononucléose are examples of viral human diseases relatively current. More severe diseases like the AIDS, the SARS, the Avian flu, the Variole are also caused by viruses. The virus Ebola involves hemorrhagic fevers. The capacity of a virus to involve a disease is described in term of Virulence.

The viruses have various strategies, various mechanisms to which they can produce diseases. The virus penetrates in a specific host cell and takes the control of its normal functions. At the cellular level, the cytopathogenic effects of the viruses can involve various harmful effects. The capacities of synthesis of the Protéine S of the infected cells can be inhibited, the Chromatine is split up by viral enzymes. Viral particles accumulate in the Cytoplasme or the core of the infected cells. The viruses can then cause the lysis and the death of the host cells. The lysis of the cells involves the release of the viral particles and allows the dissemination of the virus.

When the virus penetrates in a nonpermissive cell, it cannot multiply. Its genome can however remain in the form of a épisome free or integrated into the cellular genome. There is viral cellular transformation when the genome of the virus enters in interaction with the DNA of the cellular genome. One calls these viruses of the Virus oncogene S. Among those, the Rétrovirus, while being integrated in the cellular genome, can become Tumorigène and thus has the capacity to involve Cancer S.

The classification of the independent groups of virus and their correspondence in pathology, is in the medical encyclopedia Vulgaris. It is based on the type of molecule of nucleic acids (ARN or DNA) which constitutes the viral particle or virion.

Prevention and treatment

Since the viruses use the cellular machinery of the host to reproduce even cell inside, it is difficult to eliminate them without killing the host cell. The medical approach most effective is the Vaccination which makes it possible to resist the infection. Various drugs make it possible to treat the Symptôme S related to the infection. The patients often ask their doctors that they their prescribe Antibiotique S, but they are without effect on the viruses. The antibiotics interfere indeed with components or the metabolism of the Bactérie S and thus make it possible to treat only the diseases of origin bactérienne.
If the viruses are regarded as nonalive particles apart from the cellular context, they cannot be “killed” but are inactivated. Various methods of disinfection in vitro allow inactiver the viruses (Hypochlorite of sodium to 1%, ethanol to 70%, Glutaraldéhyde to 2%, Formaldéhyde).

Application

The viruses are important for the study of the Molecular biology and the cellular Biologie, because they provide simple systems which can be used for the handling and the comprehension of the cellular functions. For example, the viruses present a simplified genetic material in general and help with the comprehension of the molecular mechanisms of the Génétique like the replication of the DNA, the transcription, the modifications post-transcriptionnels of ARN, the translation, the transport of the Protéine S and the Immunologie.

The viruses can moreover be used like vector to introduce a gene into a cell. This tool is used for example to make it possible the cell to produce a recombining protein or to study the effect of the introduction of new gene into the Génome.
Certain viruses are used in genic Thérapie like vector, to look after various diseases. In certain genetic diseases, a defective gene causes the symptoms. The viruses vector would make it possible to target specific cells and to replace gene in question by a normal gene.
The viruses are also used in the fight against cancer. Certain viruses are able to destroy cancer cells specifically.

Pathogenic examples of virus for the Man

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