Viruses

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Viruses
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Characteristics of Viruses

• Not normally classified into any of the kingdoms
  of organisms
• Do not have the characteristics of life except
  when infecting a living cell (do not
  independently reproduce or carry out metabolism)
• Extremely small usually must be viewed with an
  electron microscope
• Parasitic
• Acellular
• Contain DNA or RNA, but not both

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Structure of a Virus

• Consist of a core of nucleic acid (DNA or RNA)
  surrounded by a protein coat
• Some viruses have additional outer coverings for
  protection
• The DNA or RNA can be passed from one generation
  of viruses to another

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viral RNA
DNA
protein coat
protein subunits of coat
sheath
base plate
18 nm diameter, 250 nm length
tail fiber
65-nm diameter head, 225-nm total length
viral coat (proteins)
reverse transcriptase
100-120 nm diameter
viral RNA
lipid envelope proteins span the envelope, line
its inner surface, spike out above it
80-nm diameter
Fig. 20.11, p. 317
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Methods of Viral Infection

• A virus targets specific cell types, based upon
  its ability to attach to cell receptor sites
• Once a virus does successfully attach to a host
  cell, it can infect it by injecting its nucleic
  acid material into the cell

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Methods of Viral Infection (continued)

• The viral genetic material takes over the cells
  activities, and viral nucleic acids are
  replicated (instead of the cells DNA)
• New virus particles are created
• An enzyme produced by the virus destroys the
  plasma membrane and/or cell wall of the host
  cell, causing the release of the new viruses

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Types of Viruses

• Bacteriophagea virus that infects bacteria
  usually a DNA virus
• Human immunodeficiency virusthe virus that
  causes AIDS, or acquired immune deficiency
  syndrome an RNA virus

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Human Diseases Caused by Viruses

• AIDS Common Cold
• Hepatitis B Influenza
• Smallpox Herpes
• Chickenpox Rabies
• Measles Mononucleosis
• Mumps Ebola
• Polio

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Virus Facts

• Viruses can infect all types of organisms
  bacteria, plants, animals
• Scientists believe that some cancers are caused
  by viruses
• Some viruses can only cause infection in an
  organism once others can re-occur

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Virus Facts (continued)

• Antibiotics are used to kill bacteria, not
  viruses a viral infection has to run its
  course the body must eliminate it on its own
• Medications can be used to treat viral symptoms
  (such as a cough or fever)
• Some anti-viral medications help to slow or
  prevent viral reproduction