The main goal of biologists has been the molecular study of viruses and their interaction with the host cell. The study of bacteriophage replication in bacteria discovered the existence of messenger RNA, carrying the genetic code of DNA needed for protein synthesis. Studies with these viruses have also been instrumental in defining the biochemical factors that start and end the use of genetic information. Knowledge of the mechanisms of control of viral replication is critical to understanding the biochemical events in higher organisms.

The viruses are useful as model systems for studying the mechanisms that control genetic information, because in essence are small pieces of information. This allows scientists to study replication systems simpler and more manageable, but that function on the same principles as those of the host cell. Much of the research on the virus replicative pretends to know the mechanism to find and how to control growth and eliminate viral diseases. Studies on viral diseases have greatly contributed to understanding the body’s immune response against infectious agents. Studying this response have been thoroughly described serum antibodies and the secretions of mucous membranes, which help the body eliminate foreign elements such as viruses. Now, the scientific interest is focused on research designed to isolate certain viral genes. They could clone to produce large quantities of certain proteins, which would be used as vaccines.

9. Bacteriophage T4

This transmission electron micrograph shows a T4 bacteriophage, a virus that infects only bacteria (in some cases only Escherichia coli). Phages lack any reproductive mechanism and exploit the mechanisms of the bacterium to replicate. They do this by holding on to the cell walls with fibers, by way of legs, visible here. The tail is a sheath that contracts to inject the contents of the head, the genetic material (DNA) within the host. In 25 minutes, are capable of successfully using the reproductive mechanisms of bacteria and viral progeny fills the cell. Then, the packed bacteria burst, releasing about 100 new copies of the bacteriophage.

10. Viral Structure

Some bacteriophages (viruses that parasitize bacteria), left, have a rather complicated structure and sophisticated. The phage T4, pictured here, consists of five proteins and the following parts: head, tail, a collar or necklace, a basal plate and fibers for legs. In contrast, a flu virus, right, is simpler. A lipid envelope surrounding the protein shell, or capsid, which, like the bacteriophage genetic material coiled locks. Since this involved projecting two types of protein in the form of spikes, which determine the properties of the virus infectivity. The human hosts must produce new immune defenses whenever they mutate, hence annual vaccinations are done.

11. Viral replication

Outside a host cell, a virus is an inert particle. But once inside the cell, the virus reproduces so many times and thousands of individuals who leave the cell to find others that parasitize. Pathogenic viruses act by destroying or damaging cells when they leave those in which have been reproduced.

12. Virus

Viruses are obligate intracellular parasites, particles composed of genetic material (DNA or RNA, but not both) surrounded by a protective protein coat. Are inert outside the host, inside, they enter a dynamic phase in which replicate using the host cell enzymes, nucleic acids, its amino acids and its mechanisms of reproduction. They carry out what they can not perform alone. Viral replication leads often damage to the host: diseases such as herpes, rabies, influenza, some cancers, polio and yellow fever are viral in origin. Between 1000 to 1500 known viruses, there are about 250 to cause disease in humans (about 100 of which cause the common cold), and 100 infect different animals.

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Viruses are spread from person to person, causing new cases of the disease. Many of them, as those responsible for influenza and measles, are transmitted by inhalation, through its dissemination in the infected droplets emitted by coughing and sneezing. Others, like those that cause diarrhea are spread by fecal-oral route. In other cases, the spread is through the bite of insects, such as yellow fever and arboviruses. Viral diseases may be endemic (specific to one area), affecting susceptible individuals, or epidemic, which appear in waves and attack much of the population. An example is the emergence of epidemic flu worldwide, almost always once a year.

6. Treatment

The treatments against viral infections are often not entirely satisfactory, since most of the drugs that kill viruses also affect cells in which they play. The alpha-adamantanamine is used in some countries to treat respiratory infections caused by influenza A and isatin-beta-thiosemicarbazone, effective against smallpox. Certain substances similar to precursors of nucleic acids may be useful against severe herpes infections.

One promising antiviral agent is interferon, which is a non-toxic protein produced by some animal cells infected with viruses and can protect other cells against such infections. Is currently studying the effectiveness of the drug to fight cancer. Until recently, these studies were limited by their limited availability, but new techniques of cloning the genetic material, produce large amounts of this protein. In a few years you may know whether interferon is really effective as an antiviral agent.

The only effective way to prevent viral infections is the use of vaccines. Vaccination against smallpox worldwide in the 1970s, eradicated the disease. It has developed many human virus vaccines and other animals. Among infections suffered by people include measles, rubella, polio and influenza. Immunization with an antiviral vaccine stimulates the body’s immune mechanism, which produces antibodies that protect you when you return to contact with the same virus. Vaccines contain viruses always altered so they can not cause disease.

7. Infections in plants

The virus originated wide variety of plant diseases and serious damage to crops. The most common are produced by the virus of turnip yellow mosaic, the potato virus X (potato) and snuff mosaic virus. Plants have rigid cell walls that viruses can not cross, so that the most important for propagation is provided by the animals that feed on them. Often, insects inoculated into healthy plants that carry the virus on its mouth parts, from other infected plants. Also nematodes, roundworms can transmit the infection when they feed on the roots.

Plant viruses can accumulate huge amounts within infected cells. For example, the snuff mosaic virus may represent up to 10% of the dry weight of the plant. Studies of the interaction between virus and host cells are limited because the infection is via an insect vector. Also, do not typically available in the laboratory of cell cultures susceptible to infection by plant viruses.

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The virus, lacking the enzymes and metabolic precursors necessary for its own replication, they must obtain them from the host cell they infect. Viral replication is a process that includes several separate synthesis and subsequent assembly of all components to give rise to new infectious particles. Replication is initiated when the virus enters the cell: cellular enzymes remove the cover and the DNA or RNA is contacted with the ribosome, directing the synthesis of proteins. The virus nucleic acid autoduplicates and, once synthesized protein subunits that form the capsid, the resulting components are assembled into new viruses. A single virus particle can cause a progeny of thousands. Some viruses are released by destroying the infected cell, and yet leave the cell without destroying it by a process of exocytosis that leverages own cell membranes. In some cases the infection is ‘silent’, ie the viruses replicate inside the cell without evident harm.

RNA-containing viruses are unique replicative systems, since the RNA autoduplicates without the involvement of DNA. In some cases, viral RNA functions as messenger RNA, and replicates indirectly using the ribosomal system and the metabolic precursors of the host cell. In others, the virus carried in the cover-dependent RNA enzyme that directs the synthesis process. Other RNA viruses, retroviruses, may produce an enzyme that synthesizes DNA from RNA. Formed DNA then acts as the viral genetic material.

During an infection, bacteriophages and animal viruses differ in their interaction with host cell surface. For example, in the cycle of bacteriophage T7, which infects the bacterium Escherichia coli, there are no stages or descapsidación adsorption. The virus binds first to the cell and then injects its DNA into it. However, once the nucleic acid enters the cell, the basic events of viral replication are the same.

4. Viruses in Medicine

Viruses represent a major challenge to medical science in combating infectious diseases. Many viruses cause major human diseases and diversity.

Among viral diseases include the common cold, which affects millions of people each year. Other diseases have serious consequences. Among them is rabies, hemorrhagic fevers, encephalitis, polio and yellow fever. However, most disease-causing viruses that cause severe discomfort only, provided that the patient will not be serious complications. Some of these are influenza, measles, mumps, fever with fever (herpes simplex), chickenpox, shingles (also known as herpes zoster), respiratory diseases, acute diarrhea, warts and hepatitis. Other viral agents as the cause of rubella (German measles) and cytomegalovirus, may cause serious anomalies or abortions. Acquired immunodeficiency syndrome (AIDS) is caused by a retrovirus. There are two known retrovirus associated with certain human cancers and is suspected of some forms of papillomavirus. There is evidence, increasingly, of viruses that might be involved in some types of cancer, chronic diseases such as multiple sclerosis and other degenerative diseases. Some viruses take a long time to cause symptoms, and produce so-called slow virus diseases such as Creutzfeldt-Jacob disease and kuru, which gradually destroys the brain.

Even today you find viruses responsible for important human diseases. Most can be isolated and identified with current methods of laboratory, although the process normally takes several days. One of them is rotavirus gastroenteritis caused by the child.

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(Latin for ‘poison’) organizational entities composed only of genetic material surrounded by a protective envelope. The term virus was used in the last decade of last century to describe the disease-causing agents smaller than bacteria. Lack of independent living but can replicate inside living cells, often damaging to his guest in this process. The hundreds of known viruses are the cause of many different diseases in humans, animals, bacteria and plants.

The existence of viruses was established in 1892, when Russian scientist Dmitry I. Ivanovsky, found microscopic particles, known later as the snuff mosaic virus. In 1898 the Dutchman botanist Martinus W. Beijerinck called these particles infectious virus. A few years later, viruses were found growing on bacteria, which are called bacteriophages. In 1935, the American biochemist Wendell Meredith Stanley crystallized the snuff mosaic virus, showing that consisted only of genetic material called ribonucleic acid (RNA) and an envelope protein. In the 1940s the development of electron microscopy enabled the visualization of the virus for the first time. Years later, the development of high-speed centrifuges able to concentrate and purify. The study of animal virus reached its peak in the 1950s with the development of cell culture methods, support of viral replication in the laboratory. Then they discovered many viruses, most of which were sampled in the 1960s and 1970s, in order to determine their physical and chemical characteristics.

2. Features

Viruses are submicroscopic intracellular parasites, consisting of RNA or deoxyribonucleic acid (DNA)-never both-and a protective layer of protein or lipid components combined with protein or carbohydrate. In general, the nucleic acid molecule is a unique single or double helix, however, certain viruses have genetic material segmented into two or more parties. The outer covering is called a capsid protein and the subunits that compose it, capsomeres. It’s called nucleocapsid, the set of all elements. Some viruses have an additional envelope usually acquired when the nucleocapsid exits the host cell. The complete viral particle is called a virion. Viruses are obligate intracellular parasites, ie only replicate in cells with active metabolism, and outside them are reduced to inert macromolecules.

The size and shape of the viruses are highly variable. There are two basic structural groups: isometric, rod-shaped or elongated, and complex virus with head and tail (as some bacteriophages). The smallest viruses are icosahedral (20-sided polygons) that measure between 18 and 20 nanometers wide (1 nanometer = 1 millionth of 1 mm). The larger ones are elongated, some measuring several micrometers in length, but rarely measure more than 100 nanometers wide. Hence the virus longer have a width that is below the limits of resolution of light microscopy used to study bacteria and other microorganisms.

Many viruses with helical internal structure have outer shell (also called decks) composed of lipoproteins, glycoproteins, or both. These viruses are like spheres, but may have different shapes and size ranges from 60 to more than 300 nanometers in diameter. Complex viruses, such as some bacteriophages, have heads and a tubular tail that binds to the host bacterium. The brick-shaped poxvirus and a complex protein composition. However, these latter types of virus are exceptions and most have a simple way.

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