Chapter 9
Viruses
By Boundless
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Viruses are infectious particles about 100 times smaller than bacteria and can only be observed by electron microscopy.
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A virion is a complete viral particle consisting of RNA or DNA surrounded by a protein shell, constituting the infective form of a virus.
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The viral genome is the complete genetic complement contained in a DNA or RNA molecule in a virus.
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A virus' host range is the range of cell types and host species a virus is able to infect.
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Most viruses range in size from 5 to 300 nanometers (nm), although some Paramyxoviruses can be up to 14,000 nm long.
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Viruses of all shapes and sizes consist of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope.
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Viruses have a variety of shapes and structures.
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Complex viruses are often asymetrical or symetrical in combination with other structures such as a tail.
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The International Committee on Taxonomy of Viruses (ICTV) establishes guidelines to maintain viral family uniformity.
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The Baltimore classification groups viruses into families depending on their type of genome.
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The evolution of viruses is speculative as they do not fossilize; biochemical and genetic information is used to create virus histories.
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Viruses are obligate intracellular parasites that hijack a host cell's machinery to replicate, thereby causing disease.
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Bacteriophage cultures require host cells in which the virus or phage multiply.
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Viruses cannot be grown in standard microbiological broths or on agar plates, instead they have be to cultured inside suitable host cells.
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Live animal inoculation is a method used to cultivate viruses.
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The genetic material within virus particles varies considerably between different types of viruses.
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Virologists describe the formation of viruses during the infection process in target host cells as viral replication.
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Viral infection involves the incorporation of viral DNA into a host cell, replication of that material, and the release of the new viruses.
Host tropism refers to the way in which viruses/pathogens determine which cells become infected by a given pathogen.
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Animal viruses have their genetic material copied by a host cell after which they are released into the environment to cause disease.
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Plant viruses are often spread from plant to plant by organisms known as vectors.
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Replication defective viruses (also known as satellites) are those that need the presence of other viruses to help them reproduce.
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Viroids are plant pathogens without the protein coat that is typical for viruses.
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Prions are infectious agents composed of protein in a misfolded form, unlike all other known infectious agents which contain nucleic acids.
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Bacteriophages are viruses that infect bacteria and are among the most common and diverse entities in the biosphere.
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Nineteen families of bacteriophages that infect bacteria and archaea are currently recognized; of these, only two families have RNA genomes.
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Of the viral families with DNA genomes, only two have single-stranded genomes, the Inoviridae and the Microviridae.
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The dsDNA tailed phages, or Caudovirales, account for 95% of all known phages and possibly make up the majority of phages on the planet.
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Bacteriophage Mu is a temperate bacteriophage that uses DNA-based transposition in its lysogenic cycle.
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T-4 bacteriophage is a virulent bacteriophage that infects E. coli bacteria; virulent bacteriophages have a lytic life cycle.
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In virology, temperate refers to the ability of some bacteriophages to display a lysogenic life cycle.
Most viruses infecting Archaea are double-stranded DNA viruses that are unrelated to any other form of virus.
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Positive strand RNA viruses are the single largest group of RNA viruses with 30 families.
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Attachment is a specific binding between viral capsid proteins and specific receptors on the host cellular surface.
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RNA viruses are classified into distinct groups depending on their genome and mode of replication.
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Viruses are released from the host cell following assembly.
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Negative-strand RNA viruses are single-stranded viruses that can infect several types of animals.
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For influenza viral propagation to begin, there first must be viron attachment and entry into a host cell.
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Influenza A follows the typical life cycle of most influenza virus: infection and replication are a multi-step process.
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Retroviruses are viruses that are able to reverse transcribe their RNA genome into DNA, which is then integrated into a host genome.
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The attachment and fusion of HIV virons to host cells are crucial to HIV infection.
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The retroviral genome contains the elements needed for reverse transcription and all other activities of a retrovirus.
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HIV replication depends on a complex, coordinated series of events where the virus integrates into the DNA of host cells.
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DNA viruses are relatively rare in plants, but are responsible for a significant amount of crop damage worldwide.
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Most double-stranded DNA viruses replicate within the host cell nucleus.
Herpes viruses cause a wide range of latent, recurring infections including oral and genital herpes, cytomegalovirus, and chicken pox.
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Herpes simplex virus attaches to a host's cells with viral envelope glycoproteins, which then allows entry of the viral capsid into the host cell.
Herpes replication entails three phases: gene transcription, viral assembly in the nucleus, and budding through the nuclear membrane.
Immunodeficiency occurs when the immune system cannot appropriately respond to infections.
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The poxviruses are a family of large, complex, enveloped DNA viruses that infect a variety of vertebrate and invertebrate hosts.
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Adenoviruses are non-enveloped, icosahedral DNA viruses which cause upper respiratory infections, primarily in children.
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Hepadnaviruses, retroviruses, use virally encoded reverse transcriptase to convert RNA into DNA.
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Interferons play pivotal roles in shaping the immune responses in mammals.