10 Viruses and Virology

• Viruses


  Unique group of biological entities known to infect every type of cell, including bacteria, algae, fungi, protozoa, plants, and animals
• Viruses are much smaller than bacteria, fungi, and protozoa, and had to be indirectly studied until the 20th century, when they were finally seen with an electron microscope
• Viruses


  Obligate intracellular parasites
• Virology


  Field of microbiology concerned with the study of viruses and other infectious particles such as prions
• Virion


  Complete virus particle
• Oncogenic viruses or oncoviruses


  Viruses that cause specific types of cancer
• Virion


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


  Viruses with an outer envelope composed of lipids and polysaccharides
• Viruses


  • Ultramicroscopic size, ranging from 20 nm up to 450 nm (diameter)
  • Not cellular in nature; structure is very compact and economical
  • Do not independently fulfill the characteristics of life
  • Inactive macromolecules outside the host cell and active only inside host cells
  • Lack organelles and locomotion of any kind
  • Contain large, complex molecules and can be crystalline in form
• Nucleocapsid


  Combination of a nucleic acid core (DNA or RNA, not both) and a capsid (a geometric protein shell surrounding the nucleic acid)
• Naked viruses


  Viruses without an envelope
• Enveloped viruses


  Viruses with a membranous envelope containing viral protein spikes
• Viruses


  • Come in many shapes and forms (icosahedral, helical, spherical, or cylindrical)
  • Smallest infectious forms range from the largest poxvirus (0.45 mm or 450 nm) to the smallest viruses (0.02 mm or 20 nm)
  • Lack enzymes for processing food or generating energy; are tied entirely to the host cell for all needs
• Viruses are known to parasitize all types of cells, including bacteria, algae, fungi, protozoa, animals, and plants
• Each viral type is limited in its host range to a single species or group, mostly due to specificity of adsorption of virus to specific host receptors
• Major types of viruses

  • DNA viruses
  • RNA viruses
• Viral classification


  Based on shape and size of capsid, presence or absence of an envelope, whether double- or single-stranded nucleic acid, antigenic similarities, and host cell
• Categories of viruses based on nucleic acid type

  • Double-stranded DNA viruses
  • Single-stranded RNA viruses
  • Single-stranded DNA viruses
  • Double-stranded RNA viruses
• Most viral genomes are circular molecules, but some are linear
• Multiplication cycle of an animal virus

  Adsorption, penetration, synthesis and assembly, and release from the host cell
• Some animal viruses cause chronic and persistent infections
• Viruses that alter host genetic material may cause oncogenic effects
• Bacteriophages


  Viruses that attack bacteria, penetrating by injecting their nucleic acid and being released as virulent phages upon lysis of the cell
• Lysogenic cycle


  Some viruses go into a latent phase in which they integrate into the DNA of the host cell and later may be active and produce a lytic infection
• Techniques for cultivating and identifying animal viruses

  • Growing viruses in living cells, either in isolated cultures of host cells (cell culture), in bird embryos, or in the intact host animal
  • Identification by cytopathic effects (CPE) in host cells, direct examination of viruses or their components in samples, genetic analysis to detect virus nucleic acid, and growing viruses in culture
• Cell culture


  Cells from animal tissue are separated and suspended in a solution that provides the osmotic pressure, nutrients, and growth factors needed for the cells to grow
• Cultivating animal viruses in a developing bird embryo

  The shell is perforated and a virus preparation is injected into a site selected to grow the viruses, such as the allantoic cavity, amniotic cavity, chorioallantoic membrane, yolk sac, or embryo itself
• Growing viruses in intact host animals

  Used to study the immune system's response to viral infections and to distinguish between viruses that produce similar lesions
• Latent virus infections


  Viral infections in which the virus is able to hide from a host's immune system by entering cells and remaining dormant
• Antiviral agents


  Drugs that interfere with virus-specific enzymes and virus production by disrupting critical phases in viral multiplication or inhibiting synthesis of viral DNA, RNA, or proteins
• Oncogenic viruses or oncoviruses


  Viruses that cause cancer, such as Epstein–Barr virus, human papillomaviruses, and human T-lymphotrophic virus type 1 (HTLV-1)
• Human immunodeficiency virus (HIV)


  Enveloped, single-stranded RNA virus that causes acquired immunodeficiency syndrome (AIDS) by targeting CD4+ cells
• Viruses have become an invaluable tool for studying basic genetic principles
• Current research is focused on the connection of viruses to afflictions of unknown causes, such as type I diabetes and multiple sclerosis
• Prions


  Infectious agents composed of a protein that can alter the structure of nerve cells, causing spongiform encephalopathies such as "mad cow disease" and Creutzfeldt-Jakob disease
• Viroids


  Short, naked fragments of single-stranded RNA that can interfere with the metabolism of plant cells, causing diseases like potato spindle tuber and citrus exocortis
• Satellites and satellite viruses (virophage)

  Subviral agents that depend on co-infection of a host cell with a helper virus for productive multiplication, with the satellite virus encoding the coat protein in which it is encapsulated
• Satellite viral particles should not be confused with satellite DNA