virology

• Virions


  • Very small and simple in structure
  • Most range in size from 10 to 300 nm in diameter
  • Some like Ebola virus can be up to 1 μm in length
• Scientists were unable to see viruses until electron microscopes were invented in the 1930s
• Organisms that viruses can infect


  • Humans
  • Animals
  • Plants
  • Fungi
  • Protozoa
  • Algae
  • Bacterial cells
• Oncogenic viruses or oncoviruses


  Viruses that cause specific types of cancer, including human cancers such as lymphomas and some types of leukemia
• Properties that distinguish viruses from living cells


  • Possess either DNA or RNA, unlike living cells which possess both
  • Unable to replicate (multiply) on their own
  • Replication is directed by the viral nucleic acid once it has been introduced into a host cell
  • Do not divide by binary fission, mitosis, or meiosis
  • Lack the genes and enzymes necessary for energy production
  • Depend on the ribosomes, enzymes, and metabolites of the host cell for protein and nucleic acid production
• Virion


  Consists of a genome of either DNA or RNA, surrounded by a capsid (protein coat), which is composed of many small protein units called capsomeres
• Nucleocapsid


  The nucleic acid and the capsid together
• Enveloped viruses


  • Have an outer envelope composed of lipids and polysaccharides
• Characteristics used to classify viruses


  • Type of genetic material (either DNA or RNA)
  • Shape of the capsid
  • Number of capsomeres
  • Size of the capsid
  • Presence or absence of an envelope
  • Type of host that it infects
  • Type of disease it produces
  • Target cell
  • Immunologic or antigenic properties
• Types of viral genomes


  • Double-stranded DNA
  • Single-stranded RNA
  • Single-stranded DNA
  • Double-stranded RNA
• Viral genomes


  • Usually circular molecules, but some are linear (having two ends)
• Shapes and symmetry of viral capsids


  • Polyhedral (many sided)
  • Helical (coiled tubes)
  • Bullet shaped
  • Spherical
  • Complex combination of these shapes
• Most scientists agree that viruses lack most of the basic features of cells and consider them to be nonliving entities
• Bacteriophages


  Viruses that infect bacteria
• Types of animal viruses


  • DNA viruses
  • RNA viruses
• Animal viruses


  • May consist solely of nucleic acid surrounded by a protein coat (capsid)
  • May be more complex, e.g. enveloped or containing enzymes that play a role in viral multiplication within host cells
• Viruses can only attach to and invade cells that bear a receptor that they can recognize
• Latent virus infections


  Viruses that remain dormant in the body for years and then resurface, e.g. shingles caused by the chickenpox virus
• Antibiotics do not work on viruses because they function by inhibiting certain metabolic activities within cellular pathogens, and viruses are not cells
• Antiviral agents


  Chemicals that interfere with virus-specific enzymes and virus production by either disrupting critical phases in viral cycles or inhibiting the synthesis of viral DNA, RNA, or proteins
• Human immunodeficiency virus (HIV)


  • Enveloped, single-stranded RNA virus that is the cause of AIDS
  • Member of the lentivirus genus in the Retroviridae family
  • Able to attach to and invade cells bearing the CD4 receptor, especially helper T cells
  • Destroys these important cells of the immune system
• Viroids


  Short, naked fragments of single-stranded RNA that can interfere with the metabolism of plant cells and stunt or kill plant growth
• Prions


  • Infectious protein molecules that cause fatal neurological diseases in animals and humans, e.g. "mad cow disease" and variant Creutzfeldt-Jakob disease (vCJD)
  • Most resistant to disinfectants of all pathogens