A virus is a biological entity that requires a host cell, which he uses to multiply constituents. The viruses exist in one form or extracellular intracellular. In the form intracellular (inside the host cell), viruses are genetic elements that can replicate an independent report on the chromosome, but not independently of the host cell. Under the extracellular form, viruses are objects particulate infectious, made at least a nucleic acid and protein.

The virology is the science which studies the virus. It is studied by virologists or virologists.

The word virus comes from latin virus, i (neutral), which means "poison". It ended with an s, it takes no particular brand in the plural in french.

A virus is characterized by its inability to grow only by division. He needs to do is use a cell host: a virus is a parasite intracellular mandatory. It is composed of a molecule nucleic acid (DNA or RNA, single or double strand) surrounded by a shell protein called capsid and sometimes an envelope. It does not usually enzyme that can produce energy. The viruses are most often very small (compared to a bacterium, for example), usually less than 250 nanometers, but the mimivirus has a size of 400 nm, which makes it bigger than most small bacteria. The latter also has the peculiarity of having both the DNA and RNA.

The free form of the virus (or viral particles) is called the virion.

There is a huge variety of viruses, estimated in 2007 to 1031 which is much more than the diversity of the three kingdoms (Bacteria, Archaea, Eukaryota) combined.

All living beings can be infected by viruses. There are bacteria, viruses (bacteriophages), viruses Archaea, viruses algae (Phycodnaviridae), plant viruses, viruses, fungi, viruses invertebrates, vertebrates virus in which one finds Many pathogens.

The viral diseases as rabies, yellow fever, smallpox, affect humans for centuries. The hieroglyphics highlight the Poliomyelitis in ancient Egypt, the writings of ancient Greco-Roman and the Far East describe certain viral diseases. However, the cause of these diseases has remained unknown for a long time. At the end of the nineteenth century, the design of infectious agents that were neither bacteria nor fungi or parasites was still difficult.

Between 1887 and 1892, the botanist Russian Dimitri Ivanovski studied a plant disease, tobacco mosaic, and showed that the sap plans patients contained an infectious agent that was not accepted by the filters Chamberland designed by the biologist with the same name. Ivanovski thought it was a toxin or a very small bacteria. It is the chemist Dutch Martinus Beijerinck which deepens the work and rejected the assumption bacterial and dénomma the phenomenon Contagium vivum fluidum. At the same time, the FMD virus is the first virus identified by Friedrich Loeffler and Paul Frosch. The yellow fever virus is the first pathogenic virus man identified between 1900 and 1902.

It was during the First World War than English Frederick Twort and microbiologist Franco-Canadian Felix d'Hérelle highlight the phenomenon of "lysis transmissible" observable by lysis of bacteria grown in solid. This phenomenon is caused by a virus of bacteria that Felix d'Hérelle called bacteriophage. The virus of plants, animals, humans and bacteria were found and thus their lists did not cease to grow during the twentieth century. The appearance of electron microscopy in 1930 allowed the observation of viruses, but there was still not at that time what they really were.

The American biochemist Wendell Stanley crystallized the Tobacco Mosaic Virus in the form of a protein crystal in 1935. The following year additional studies showed that the crystal also contained the NRA. The later studies showed that depending on the virus studied, they were based on either proteins and RNA, proteins and DNA. In 1957, Andre Lwoff offered a clear definition and modern viruses.

Starting 1960, the development of cell culture, electron microscopy, and molecular biology enabled the scientific progress in understanding the mechanisms of virus replication, in achieving reliable diagnoses and in developing vaccines.

There are several hypotheses on the origin and evolution of the virus. It is likely that all viruses do not derive a single common ancestor and various viruses may have different origins.

* The virus and the cells were able to appear in the primordial soup at the same time and evolve in parallel. In this scenario, early in the emergence of life, the oldest genetic systems of self-replication (probably from the NRA) have become more complex and were wrapped in a bag lipid leading to progénote 's origin of cells. Another form replicative could keep its simplicity to form viral particles.
* The virus may derive cells that have undergone a regression. According to this hypothesis, the ancestors of the virus have been living free or micro-organisms become predators or parasites dependent on their host. Relations parasitism lead to the loss of many genes (including genes for metabolism provided by the host). This body would co-evolved with the host cell and would retain its ability to replicate its nucleic acid and the mechanism of transfer of cell to cell. This assumption is based on the existence rickettsia, small bacteria had fallen to such an extent that they can not survive in a host cell, and recalling the virus.
* The virus may have originated pieces of nucleic acids that have "escaped" Genome cell to become independent. This phenomenon may have occurred during mistakes during replication of genetic material. The virus could also have the origin of plasmids (circular DNA molecules) or transposons (DNA sequence that can move and multiply in a genome).

A complete viral particles, called virion, is composed of a filament nucleic acid, generally stabilized by nucleoproteins basic, locked in a protective shell protein called capsid. The shape of the capsid is the basis of different morphologies of the virus. The size of the virus is between 10 and 400 nm. The virus genomes contain only a few genes at 1 200 genes. The smallest known virus is the virus which delta parasite itself as Hepatitis B. It does not contain a single gene. The largest known virus is the mimivirus with a diameter reaching 400 nanometers and a genome which contains 1 200 genes.

Nucleic acid
The filament nucleic acid can be DNA or RNA. It represents the viral genome. It may be circular or linear bicaténaire (double strand) or single-(single strand). The genome in the form of DNA is generally bicaténaire. The genome in the form of RNA is usually single-and can be a positive polarity (in the same sense that a messenger RNA) or negative polarity (complementary messenger RNA). The platoon central nucleic acid is called nucleoide.

The capsid is a shell that surrounds and protects the viral nucleic acid. It is made by the assembly of protein structures. The capsid is made up of protein subunits called protomères. All capsid and nucléoïde is appointed nucléocapside. The structure of the capsid results in the form of the virus, which can be divided into two main groups of virus: the virus cubic symmetry and viruses to spiral symmetry.

Many viruses are surrounded by an envelope (or peplos), which shall arise during the crossing cell membranes. Its constitution is complex and presents a mix of cellular components and elements of viral origin. There are proteins, carbohydrates and lipids. The virus has an envelope are enveloped viruses. The virus has no envelope viruses are naked.

Viruses are classified according to the nature of the nucleic acid of their genome (DNA or RNA), the structure of nucleic acid (or single-bicaténaire), the form of nucleic acid (linear, circular, segmented or not) . The morphological data can also be taken into account (presence or absence envelope, symmetry of the capsid). Often, the sérogroupage is still used to refine the definition of the differences between viruses very similar.

Multiplication viral
The virus can not multiply within living cells to replicate 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 multiplying the virus can be summed up in different stages. However, after penetration of the virus in the cell, these steps may vary depending on the nature of the virus in question and in particular as it is a DNA virus or an RNA virus.

* The attachment or adsorption: during this stage, there is a link viral protein to a receptor on the cell surface. The receivers eukaryotes can be glycoproteins, or glycosphingolipids. The receivers of bacteriophages are glycoproteins or lipopolysaccharides. The plant cells do not have specific receptors the virus.
* The penetration: according to the virus, there are several mechanisms penetration of the virus inside the cell. Among bacteriophages, only the viral genome enters the bacterial cell. Among animal viruses the virus can enter through several mechanisms. The virus can enter through pinocytosis: nucleocapside viral surrounded by the plasma membrane enters the cell. It is often the case with naked virus. In the case of enveloped viruses, the virus may enter either by merger (there is fusion of the viral envelope and the plasma cell membrane) or by endocytosis (there is accumulation of viral particles in cytoplasmic vesicles).
* The decapsidation after penetration (or at the same time), there is liberation of the nucleic acid. According to the virus, decapsidation may take place in the cytoplasm or the nucleus.
* The viral replication or multiplication: in this phase, there is replication of the genome, the genome expression in the form of messenger RNA (transcript) and translation of mRNA protein by cellular machinery. According to the types of virus and the nature of their genome, the mechanism of viral multiplication can be very different.
* The assembly (maturation phase): there are assembly and maturation of the virus in infected cells. There is encapsidation of the genome. The virus wrapped acquire their envelope by budding, to the detriment of the plasma membrane or the nuclear membrane of the host cell.
* The liberation: the reconstructed virus is released outside the cell.

Culture viruses
To learn more about their biology, their proliferation, their reproductive cycle and possibly to prepare vaccines, it is necessary to cultivate the virus. They can multiply only within living cells. The virus infecting eukaryotic cells are grown on cell cultures obtained from animal tissues or plants. The cells are grown in a container made of glass or plastic, then are infected with the virus studied. The animal viruses can also be grown on embryonated eggs and sometimes in animals, when in vitro culture is impossible. The bacterial virus can also be cultivated by inoculation of a bacterial culture sensitive. The virus of plants can also be grown on monolayers plant tissue, cellular suspensions or whole plants.

The virus can then be quantified in different ways. They can be counted directly through electron microscopy. In the case of bacterial virus, the technique plates (or beaches) is widely used to assess the amount of virus in a suspension. A dilution of virus suspension is added to a bacterial suspension, then all is spread in a petri dish. After culture, bright areas (beaches) to the surface of the agar are a consequence of the destruction of bacteria and bacteria adjacent by a virion.

The virus can be purified through various methods of biochemistry (centrifuge differential precipitation, distorted, enzymatic digestion).

Prevention and treatment
Given that the virus uses the cellular machinery of the host to reproduce itself inside of the cell, it is difficult to eliminate without killing the host cell. The medical approach is the most effective vaccination, which allows to resist infection. Other drugs used to treat symptoms associated with infection. Patients often ask their doctors to prescribe their antibiotics, but they have no effect on viruses. Antibiotics indeed interfere with constituents or metabolism of bacteria and can therefore only treat bacterial diseases.
If viruses are considered as particles non-living outside the cellular context, they can not be "killed" but are inactivated. Various methods of disinfection in vitro can inactivate the virus (sodium hypochlorite 1%, 70% ethanol, 2% glutaraldehyde, formaldehyde).

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