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  • 1. Chapter 13 - Viruses General Characteristics of Viruses Viral Structure Taxonomy of Viruses
  • 2. GENERAL CHARACTERISTICS OF VIRUSES
    • Depending on one's viewpoint, Viruses may be regarded as exceptionally complex aggregations of nonliving chemicals or as exceptionally simple living microbes.
    • Viruses contain a single type of nucleic acid (DNA or RNA) and a protein coat, sometimes enclosed by an envelope composed of lipids, proteins, and carbohydrates.
    • Viruses are obligatory intracellular parasites. They multiply by using the host cell's synthesizing machinery to cause the synthesis of specialized elements that can transfer the viral acid to other cells.
  • 3. Host range
    • Host range refers to the spectrum of host cells in which a virus can multiply.
    • Host range is determined by the specific attachment on the host cell's surface and the availability of host cellular factors.
    Viral Size
    • Viral size is ascertained by electron microscopy
    • Viruses range from 20 to 1000 nm in length
    • Viral Structure
    • A virion is a complete, fully developed viral particle composed of nucleic acid surrounded by a coat .
  • 4. Nucleic acid
    • Viruses contain either DNA or RNA. Never both, and the nucleic acid may be single or double stranded, linear or circular, or divided into several separate molecules.
    • The proportion of nucleic acid in relation to protein in viruses ranges from about 1% to about 50%
    Capsid and Envelope
    • The protein coat surrounding the nucleic acid of a virus is called the capsid.
    • The capsid is composed of subunits, capsomeres, which can be a single type of protein or several types
    • The capid of some viruses is enclosed by an envelope consisting of lipids, proteins and carbohydrates
    • Some envelopes are covered with carbohydrate-protein complexes called spikes.
  • 5. Taxonomy of Viruses
    • Classification of viruses is based on type of nucleic acid, strategy for replication, and morphology.
    • Virus family names end in -viridae; genus names end in -virus.
    • A viral species is a group of viruses sharing the same genetic information and ecological niche.
    Isolation, Cultivation, and Identification of Viruses
    • Viruses must be grown in living cells.
    • The easiest viruses to grow are bateriophages.
  • 6. Growing Bacteriophages in the Laboratory ( Plaque assay)
    • The plaque method mixes bacteriophages with the host bacteria nutrient agar.
    • After several viral multiplication cycles, the bacteria in the area surrounding the original virus are destroyed and the area of lysis is called a plaque
    • Each plaque originates with a single viral particle.
    • The concentration of viral suspensions is measured by the number of plaques that are in the terms of plaque forming units (PFU).
  • 7. Viral Reproduction
    • Requires host cell
    • Bacteriophages (prokaryotes)
    • Animal viruses (eukaryotes)
    Bacteriophages Reproduction Cycle
    • Lytic Cycle
      • Mass production of phage
      • Ends in host cell death (lysis)
    • Lysongenic cycle
      • Virus reproduces through binary fission along with host cell
      • Can spontaneously convert to lytic cycle
  • 8. Bacteriophage Reproduction cycle
    • Attachment
      • Random encounter
      • Attachment – Receptor connection (lock & key)
    •  
    • Penetration
      • Break down of cell wall (phage lysozyme)
      • Injection of DNA into cytoplasm
  • 9. Bacteriophage Reproduction cycle (Lytic)
    • Biosynthesis
      • Viral DNA takes over
      • Host metabolic machinery synthesizes new viral parts
      • Spontaneous assembly
    • Release
      • Break down of host cell wall
      • New virion escape to infect more cells
  • 10. Lysogenic cycle
    • Viral DNA integrates into host’s genome
    • Bacteria continues normal metabolic processes (DNA replication, protein synthesis, binary fission)
    • Virus remains latent until triggered to enter lytic cycle  
  • 11. Multiplications of Animal Viruses
    • Animal viruses may differ from phages while entering the host cell.
    • The virus is different due to the prokaryotic and eukaryotic cells.
    • Animal viruses may not have all the enzymes found in the phage.
    • Multiplication of this virus is shared by the DNA and RNA which contains animal viruses.
  • 12. Bacteriophages and animal viral multiplication compared
    • Tail fibers attach to cell wall proteins
    • Viral DNA injected into host cell
    • Not required
    • In cytoplasm
    • Lysogeny
    • Host cell lysed
    • Attachment sites are plasma membrane proteins and glycoproteins.
    • Capsid enters by endocytosis or fusion
    • Enzymatic removal of capsid proteins
    • In nucleus DNA viruses or cytoplasm RNA viruses
    • Latency; slow viral infections; cancer
    • Enveloped viruses bud out; noneveloped viruses rupture plasma membrane.
  • 13. There are six phrases in a virus replication
    • Attachment
    • Entry
    • Uncoating
    • The Biosynthesis of DNA Viruses
    • The Biosynthesis of RNA Viruses
    • Maturation and Release
  • 14.  
  • 15. Chickenpox ( Varicellovirus )
    • Chickenpox is one of the classic childhood diseases, and one of the most contagious. The affected child or adult may develop hundreds of itchy, fluid-filled blisters that burst and form crusts. Chickenpox is caused by a virus.
  • 16. Latent Viral Infections
    • A latent viral infection is one in which the virus remains in the host cell for long periods without producing an infection.
    • Examples are cold sores and shingles.
    Persistent Viral Infections
    • Persistent viral infections are disease process that occur over a long period and are generally fatal.
    • Persistent viral infections are caused by conventional viruses; viruses accumulate over a long period .
    Persistent Viral Infections
  • 17.  
  • 18. Prions: Definition
    • An infectious agent consisting of a self-replicating protein, with no detectable nucleic acids.
    • Involve degeneration of the brain tissue
    • Cause some infectious diseases
    • Prions have a high resistance to all forms of sterilization
    • Listed as most resistant in the “Decreasing order of resistance of microorganisms to chemical biocides”
    • e.g.Mad Cow disease
      • Great Britain, 1987
  • 19. Viroids
    • Infectious RNA
      • Short pieces of naked RNA
      • 300 – 400 nucleotides long
      • No protein coat
    • Some plant diseases are caused by viroids
    • Conclusively identified as pathogens only of plants