| Virology, often considered a part of
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| | a giant virus that infects amoebae and
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| microbiology or of pathology, is the
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| | carries much of the molecular machinery
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| study of biological viruses and virus
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| | traditionally associated with bacteria.
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| like agents: their structure and
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| | Is it a simplified version of a parasitic
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| classification, their ways to infect and
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| | prokaryote, or did it originate as a
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| exploit cells for virus reproduction, the
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| | simpler virus that acquired genes from
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| diseases they cause, the techniques to
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| | its host?
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| isolate and culture them, and their
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| | While viruses reproduce and evolve, they
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| potential uses in research and therapy. A
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| | don't engage in metabolism and depend on
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| researcher in virology is a virologist.
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| | a host cell for reproduction. The
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| Virus structure and classification
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| | often-debated question of whether they
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| A major branch of virology is virus
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| | are alive or not is a matter of
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| classification. Viruses can be classified
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| | definition that does not affect the
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| according to the host cell they infect:
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| | biological reality of viruses.
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| animal viruses, plant viruses, fungal
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| | Viral diseases and host defenses
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| viruses, and bacteriophages (viruses
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| | One main motivation for the study of
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| infecting bacteria, which include the
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| | viruses is the fact that they cause many
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| most complex viruses). Another
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| | important infectious diseases, among them
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| classification uses the geometrical shape
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| | the common cold, influenza, rabies,
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| of their capsid (often a helix or an
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| | measles, many forms of diarrhea,
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| icosahedron) or the virus's structure
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| | hepatitis, yellow fever, polio, smallpox
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| (e.g. presence or absence of a lipid
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| | and AIDS. Some viruses, known as
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| envelope). Viruses range in size from
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| | oncoviruses, contribute to certain forms
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| about 30 nm to about 450 nm, which means
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| | of cancer; the best studied example is
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| that most of them cannot be seen with
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| | the association between Human
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| light microscopes. The shape and
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| | papillomavirus and cervical cancer. Some
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| structure of viruses can be studied with
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| | subviral particles also cause disease:
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| electron microscopy, with NMR
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| | Kuru and Creutzfeldt-Jakob disease are
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| spectroscopy, and most importantly with
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| | caused by prions, and hepatitis D is due
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| X-ray crystallography.
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| | to a satellite virus.
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| The most useful and most widely used
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| | The study of the manner in which viruses
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| classification system distinguishes
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| | cause disease is viral pathogenesis. The
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| viruses according to the type of nucleic
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| | degree to which a virus causes disease is
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| acid they use as genetic material and the
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| | its virulence.
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| viral replication method they employ to
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| | When the immune system of a vertebrate
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| coax host cells into producing more
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| | encounters a virus, it produces specific
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| viruses:
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| | antibodies which bind to the virus and
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| DNA viruses (divided into double-stranded
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| | mark it for destruction. The presence of
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| DNA viruses and the much less common
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| | these antibodies is often used to
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| single-stranded DNA viruses),
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| | determine whether a person has been
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| RNA viruses (divided into positive-sense
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| | exposed to a given virus in the past,
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| single-stranded RNA viruses,
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| | with tests such as ELISA. Vaccinations
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| negative-sense single-stranded RNA
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| | protect against viral diseases, in part,
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| viruses and the much less common
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| | by eliciting the production of
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| double-stranded RNA viruses),
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| | antibodies. Specifically constructed
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| reverse transcribing viruses
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| | monoclonal antibodies can also be used to
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| (double-stranded reverse-transcribing DNA
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| | detect the presence of viruses, with a
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| viruses and single-stranded
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| | technique called fluorescence microscopy.
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| reverse-transcribing RNA viruses
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| | A second defense of vertebrates against
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| including retroviruses).
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| | viruses, cell-mediated immunity, involves
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| In addition virologists also study
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| | immune cells known as T cells: the body's
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| subviral particles, infectious entities
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| | cells constantly display short fragments
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| even smaller than viruses: viroids (naked
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| | of their proteins on the cell's surface,
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| circular RNA molecules infecting plants),
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| | and if a T cell recognizes a suspicious
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| satellites (nucleic acid molecules with
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| | viral fragment there, the host cell is
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| or without a capsid that require a helper
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| | destroyed and the virus-specific T-cells
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| virus for infection and reproduction),
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| | proliferate. This mechanism is
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| and prions (proteins that can exist in a
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| | jump-started by certain vaccinations.
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| conformation which induces other protein
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| | RNA interference, an important cellular
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| molecules to assume that same
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| | mechanism found in plants, animals and
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| conformation).
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| | many other eukaryotes, most likely
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| The latest report by the International
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| | evolved as a defense against viruses. An
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| Committee on Taxonomy of Viruses (2005)
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| | elaborate machinery of interacting
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| lists 5450 viruses, organized in over
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| | enzymes detects double-stranded RNA
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| 2,000 species, 287 genera, 73 families
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| | molecules (which occur as part of the
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| and 3 orders.
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| | life cycle of many viruses) and then
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| The taxa in virology are not necessarily
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| | proceeds to destroy all single-stranded
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| monophyletic. In fact, the evolutionary
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| | versions of those detected RNA molecules.
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| relationships of the various virus groups
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| | Every lethal viral disease presents a
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| remain unclear, and three hypotheses
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| | paradox: killing its host is obviously of
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| regarding their origin exist:
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| | no benefit to the virus, so how and why
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| 1. Viruses arose from non-living matter,
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| | did it evolve? Today it is believed that
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| separately from and in parallel to other
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| | most viruses are relatively benign in
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| life forms, possibly in the form of
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| | their natural host; the lethal viral
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| self-reproducing RNA ribozymes similar to
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| | diseases are explained as resulting from
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| viroids.
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| | an "accidental" jump of the virus from a
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| 2. Viruses arose from earlier, more
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| | species in which it is benign to a new
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| competent cellular life forms that became
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| | one that is not accustomed to it (see
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| parasites to host cells and subsequently
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| | zoonosis). For example, serious influenza
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| lost most of their functionality;
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| | viruses probably have pigs or birds as
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| examples of such tiny parasitic
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| | their natural host, and HIV is thought to
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| prokaryotes are Mycoplasma and
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| | derive from the benign monkey virus SIV.
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| Nanoarchaea.
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| | While it has been possible to prevent
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| 3. Viruses arose as parts of the genome
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| | (certain) viral diseases by vaccination
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| of cells, most likely transposons or
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| | for a long time, the development of
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| plasmids, that acquired the ability to
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| | antiviral drugs to treat viral diseases
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| "break free" from the host cell and
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| | is a comparatively recent development.
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| infect other cells.
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| | The first such drug was interferon, a
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| It is of course possible that different
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| | substance that is naturally produced by
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| alternatives apply to different virus
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| | certain immune cells when an infection is
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| groups.
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| | detected, thus stimulating other parts of
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| Of particular interest here is mimivirus,
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| | the immune system.
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