Saturday, February 16, 2008

Anti virus comparison

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Anti virus comparison

virus


A virus (from the Latin virus meaning toxin or poison) is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell. Each viral particle, or virion, consists of genetic material, DNA or RNA, within a protective protein coat called a capsid. The capsid shape varies from simple helical and icosahedral (polyhedral or near-spherical) forms, to more complex structures with tails or an envelope. Viruses infect cellular life forms and are grouped into animal, plant and bacterial types, according to the type of host infected. In the late 19th century Charles Chamberland developed a porcelain filter with pores small enough to remove cultured bacteria from their culture medium.[10] Dimitri Ivanovski used this filter to study an infection of tobacco plants, now known as tobacco mosaic virus. He passed crushed leaf extracts of infected tobacco plants through the filter, then used the filtered extracts to infect other plants, thereby proving that the infectious agent was not a bacterium. Similar experiments were performed by several other researchers, with similar results. These experiments showed that viruses are orders of magnitudes smaller than bacteria. The term virus was coined by the Dutch microbiologist Martinus Beijerinck who showed, using methods based on the work of Ivanovski, that tobacco mosaic disease is caused by something smaller than a bacterium. He coined the Latin phrase "contagium vivum fluidum" (which means “soluble living germ”) as first the idea of the virus.[11] A major problem for early virologists was the inability to propagate viruses on sterile culture media, as is done with cellular microorganisms. This limitation required medical virologists to infect living animals with infectious material, which is dangerous. The first breakthrough came in 1931, when Ernest William Goodpasture demonstrated the growth of influenza and several other viruses in fertile chicken eggs.[17] However, some viruses would not grow in chicken eggs, and a more flexible technique was needed for propagation of viruses. The solution came in 1949 when John Franklin Enders, Thomas H. Weller and Frederick Chapman Robbins together developed a technique to grow polio virus in cultures of living animal cells.[18] Their methods have since been extended and applied to the growth of viruses and other infectious agents that do not grow on sterile culture media. Small viruses with only a few genes may be runaway stretches of nucleic acid originating from the genome of a living organism. Their genetic material could have been derived from transferable genetic elements such as plasmids or transposons, that are prone to moving within, leaving, and entering genomes. New viruses are emerging de novo and therefore, it is not always the case that viruses have "ancestors".[22] Viruses with larger genomes, such as poxviruses, may have once been small cells that parasitised larger host cells. Over time, genes not required by their parasitic lifestyle would have been lost in a streamlining process known as retrograde-evolution or reverse-evolution. The bacteria Rickettsia and Chlamydia are living cells that, like viruses, can only reproduce inside host cells. They lend credence to the streamlining hypothesis, as their parasitic lifestyle is likely to have caused the loss of genes that enabled them to survive outside a host cell. The International Committee on Taxonomy of Viruses (ICTV) developed the current classification system and put in place guidelines that put a greater weighting on certain virus properties to maintain family uniformity. A universal system for classifying viruses, and a unified taxonomy, has been established since 1966. In determining order, taxonomists should consider the type of nucleic acid present, whether the nucleic acid is single- or double-stranded, and the presence or absence of an envelope. After these three main properties, other characteristics can be considered: the type of host, the capsid shape, immunological properties and the type of disease it causes. The system makes use of a series of ranked taxons. The general structure is as follows: The Baltimore classification of viruses is based on the mechanism of mRNA production. Viruses must generate positive strand mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. This classification places viruses into seven groups: As an example of viral classification, the chicken pox virus, Varicella zoster (VZV), belongs to family Herpesviridae, subfamily Alphaherpesvirinae and genus Varicellovirus. It remains unranked in terms of order. VZV is in Group I of the Baltimore Classification because it is a dsDNA virus that does not use reverse transcriptase. The viral envelope can give a virion a few distinct advantages over other capsid-only virions, such as protection from enzymes and certain chemicals. The proteins in it can include glycoproteins functioning as receptor molecules, allowing host cells to recognise and bind these virions, resulting in the possible uptake of the virion into the cell. Most enveloped viruses are dependent on the envelope for infectivity. A medium sized virion next to a flea is roughly equivalent to a human next to a mountain twice the size of Mount Everest. Some filoviruses have a total length of up to 1400�nm, however their capsid diameters are only about 80�nm. Most viruses which have been studied have a capsid diameter between 10 and 300 nanometres. Most viruses are unable to be seen with a light microscope but some are as large or larger than the smallest bacteria and can be seen under high optical magnification. More commonly, both scanning and transmission electron microscopes are used to visualise virus particles. 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. A virus can still cause degenerative effects within a cell without causing its death; collectively these are termed cytopathic effects. Attachment is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. This specificity determines the host range of a virus. For example, the human immunodeficiency virus (HIV) infects only human T cells, because its surface protein, gp120, can interact with CD4 and receptors on the T cell's surface. This mechanism has evolved to favour those viruses that only infect cells that they are capable of replicating in. Attachment to the receptor can induce the viral-envelope protein to undergo changes that results in the fusion of viral and cellular membranes. Animal DNA viruses, such as herpesviruses, enter the host via endocytosis, the process by which cells take in material from the external environment. Frequently after a chance collision with an appropriate surface receptor on a cell, the virus penetrates the cell, the viral genome is released from the capsid and host polymerases begin transcribing viral mRNA. New virions are assembled and released either by cell lysis or by budding off the cell membrane. Reverse transcribing viruses replicate using reverse transcription, which is the formation of DNA from an RNA template. Viruses containing RNA genomes use a DNA intermediate to replicate, whereas those containing DNA genomes use an RNA intermediate during genome replication. Both types use the reverse transcriptase enzyme to carry out the nucleic acid conversion. both types are susceptible to antiviral drugs that inhibit the reverse transcriptase enzyme, e.g. zidovudine and lamivudine. Bacteriophages infect specific bacteria by binding to surface receptor molecules and then enter the cell. Within a short amount of time, in some cases, just minutes, bacterial polymerase starts translating viral mRNA into protein. These proteins go on to become either new virions within the cell, helper proteins which help assembly of new virions, or proteins involved in cell lysis. Viral enzymes aid in the breakdown of the cell membrane, and in the case of the T4 phage, in just over twenty minutes after injection over three hundred phages could be released. Viruses do not have a cell structure (regarded as the basic unit of life), although they do have genes. Additionally, although they reproduce, they do not self-metabolize and require a host cell to replicate and synthesise new products. However, bacterial species such as Rickettsia and Chlamydia, are considered living organisms, but are unable to reproduce outside a host cell. Examples of common human diseases caused by viruses include the common cold, the flu, chickenpox and cold sores. Serious diseases such as Ebola, AIDS, 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 as to whether they too have a virus as the causative agent, such as the possible connection between Human Herpesvirus Six (HHV6) and neurological diseases such as multiple sclerosis and chronic fatigue syndrome. There is current controversy over whether the borna virus, previously thought of as causing neurological disease in horses, could be responsible for psychiatric illness in humans.[56] Viruses have different mechanisms by which they produce disease in an organism, which largely depends on the species. Mechanisms at the cellular level primarily include cell lysis, the breaking open and subsequent death of the cell. In multicellular organisms, if enough cells die the whole organism will start to suffer the effects. Although viruses cause disruption of healthy homeostasis, resulting in disease, they may exist relatively harmlessly within an organism. An example would include the ability of the herpes simplex virus, which cause coldsores, to remain in a dormant state within the human body. This is called latency[57] and is a characteristic of the herpes viruses including Epstein-Barr virus which causes glandular fever and Varicella zoster virus which causes chicken pox. Latent chickenpox infections return in later life as the disease called shingles. Because viruses use the machinery of a host cell to reproduce and reside within them, they are difficult to eliminate without killing the host cell. The most effective medical approaches to viral diseases so far are vaccinations to provide resistance to infection, and antiviral drugs which treat the symptoms of viral infections. Not all virus infections produce a protective immune response in this way. HIV evades the immune system by constantly changing the amino acid sequence of the proteins on the surface of the virion. These persistent viruses evade immune control by sequestration, blockade of antigen presentation, cytokine resistance, evasion of natural killer cell activities, escape from apoptosis, and antigenic shift.[91] Other viruses, called "neurotropic viruses", are disseminated by neural spread where the immune system may be unable to reach them. Other antiviral drugs in use target different stages of the viral life cycle. HIV is dependent on a proteolytic enzyme called the HIV-1 protease for it to become fully infectious. There is a class of drugs called protease inhibitors which have been designed to inactivate the enzyme. Viruses are important to the study of molecular and cellular biology as they provide simple systems that can be used to manipulate and investigate the functions of cells. The study and use of viruses have provided valuable information about aspects of cell biology. For example, viruses been useful in the study of genetics and helped our understanding of the basic mechanisms of molecular genetics, such as DNA replication, transcription, RNA processing, translation, protein transport, and immunology. Geneticists often use viruses as vectors to introduce genes into cells that they are studying. This is useful for making the cell produce a foreign substance, or to study the effect of introducing a new gene into the genome. In similar fashion, virotherapy uses viruses as vectors to treat various diseases, as they can specifically target cells and DNA. It shows promising use in the treatment of cancer and in gene therapy. In April 2006 scientists at the Massachusetts Institute of Technology (MIT) created nanoscale metallic wires using a genetically-modified virus.[108] The MIT team was able to use the virus to create a working battery with an energy density up to three times more than current materials. The potential exists for this technology to be used in liquid crystals, solar cells, fuel cells, and other electronics in the future. The ability of viruses to cause devastating epidemics in human societies has led to the concern that viruses could be weaponized for biological warfare. Further concern was raised by the successful recreation of the infamous 1918 influenza virus in a laboratory.[109] The smallpox virus devastated numerous societies throughout history before its eradication. It currently exists in several secure laboratories in the world, and fears that it may be used as a weapon are not totally unfounded. The vaccine for smallpox is not safe and during the years before the eradication of smallpox disease more people became seriously ill as a result of vaccination than did people from smallpox.[110] and smallpox vaccination is no longer universally practiced.[111] Thus, the modern global human population has almost no established resistance to smallpox; if it were to be released, a massive loss of life could be sustained before the virus is brought under control.


Anti virus comparison

anti


His worship is quite ancient, dating from at least the 2nd dynasty, at which point he already had priests dedicated to his cult. Originally, Anti appears to have been the patron of the ancient area around Badari, which was the centre of the cult of Horus. Due to lack of surviving information, it is not very well known what the original function of Anti was, or whether he was more than just a title of Horus referring to some specific function.[1] Over time, he became considered simply as the god of ferrymen, and was consequently depicted as a falcon standing on a boat, a reference to Horus, who was originally considered as a falcon. As god of ferrymen, he gained the title Nemty, meaning (one who) travels. His later cult centre Antaeopolis was known as Per-Nemty (House of Nemty). Anti appears in the tale The Contendings of Horus and Seth which describes the settlement of the inheritance of Osiris, seen as a metaphor for the conquest of Lower Egypt (whose patron was Set) by Upper Egypt, at the beginning of the Old Kingdom. In this tale, one of Set's attempts to gain power consists of his gathering together the gods, and providing good arguments, convincing all of them (in later traditions, all except Thoth). Set fears magical intervention by Isis, Horus' wife (in early Egyptian mythology), and so holds the gathering on an island, instructing Anti not to allow anyone resembling Isis to be ferried there. However, Isis disguises herself as an old woman, and unknowingly Anti takes her across after being paid a gold ring, having rejected the first offer of gruel, resulting in the disruption of the council by her use of magic. Anti is punished for his error, by having his toes cut off, which is more severe than it appears, since as a falcon, he would no longer be able to perch, and thus would not be able to reside on the boat.[2]




Anti virus comparison

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Thursday, February 14, 2008

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