Micro Ch13 Edited

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Micro Ch13 Edited

  1. 1. Chapter 13 - Viruses General Characteristics of Viruses Viral Structure Taxonomy of Viruses
  2. 2. GENERAL CHARACTERISTICS OF VIRUSES <ul><li>Depending on one's viewpoint, Viruses may be regarded as exceptionally complex aggregations of nonliving chemicals or as exceptionally simple living microbes. </li></ul><ul><li>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. </li></ul><ul><li>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. </li></ul>
  3. 3. Host range <ul><li>Host range refers to the spectrum of host cells in which a virus can multiply. </li></ul><ul><li>Host range is determined by the specific attachment on the host cell's surface and the availability of host cellular factors. </li></ul>Viral Size <ul><li>Viral size is ascertained by electron microscopy </li></ul><ul><li>Viruses range from 20 to 1000 nm in length </li></ul><ul><li>Viral Structure </li></ul><ul><li>A virion is a complete, fully developed viral particle composed of nucleic acid surrounded by a coat . </li></ul>
  4. 4. Nucleic acid <ul><li>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. </li></ul><ul><li>The proportion of nucleic acid in relation to protein in viruses ranges from about 1% to about 50% </li></ul>Capsid and Envelope <ul><li>The protein coat surrounding the nucleic acid of a virus is called the capsid. </li></ul><ul><li>The capsid is composed of subunits, capsomeres, which can be a single type of protein or several types </li></ul><ul><li>The capid of some viruses is enclosed by an envelope consisting of lipids, proteins and carbohydrates </li></ul><ul><li>Some envelopes are covered with carbohydrate-protein complexes called spikes. </li></ul>
  5. 5. Taxonomy of Viruses <ul><li>Classification of viruses is based on type of nucleic acid, strategy for replication, and morphology. </li></ul><ul><li>Virus family names end in -viridae; genus names end in -virus. </li></ul><ul><li>A viral species is a group of viruses sharing the same genetic information and ecological niche. </li></ul>Isolation, Cultivation, and Identification of Viruses <ul><li>Viruses must be grown in living cells. </li></ul><ul><li>The easiest viruses to grow are bateriophages. </li></ul>
  6. 6. Growing Bacteriophages in the Laboratory ( Plaque assay) <ul><li>The plaque method mixes bacteriophages with the host bacteria nutrient agar. </li></ul><ul><li>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 </li></ul><ul><li>Each plaque originates with a single viral particle. </li></ul><ul><li>The concentration of viral suspensions is measured by the number of plaques that are in the terms of plaque forming units (PFU). </li></ul>
  7. 7. Viral Reproduction <ul><li>Requires host cell </li></ul><ul><li>Bacteriophages (prokaryotes) </li></ul><ul><li>Animal viruses (eukaryotes) </li></ul>Bacteriophages Reproduction Cycle <ul><li>Lytic Cycle </li></ul><ul><ul><li>Mass production of phage </li></ul></ul><ul><ul><li>Ends in host cell death (lysis) </li></ul></ul><ul><li>Lysongenic cycle </li></ul><ul><ul><li>Virus reproduces through binary fission along with host cell </li></ul></ul><ul><ul><li>Can spontaneously convert to lytic cycle </li></ul></ul>
  8. 8. Bacteriophage Reproduction cycle <ul><li>Attachment </li></ul><ul><ul><li>Random encounter </li></ul></ul><ul><ul><li>Attachment – Receptor connection (lock & key) </li></ul></ul><ul><li>  </li></ul><ul><li>Penetration </li></ul><ul><ul><li>Break down of cell wall (phage lysozyme) </li></ul></ul><ul><ul><li>Injection of DNA into cytoplasm </li></ul></ul>
  9. 9. Bacteriophage Reproduction cycle (Lytic) <ul><li>Biosynthesis </li></ul><ul><ul><li>Viral DNA takes over </li></ul></ul><ul><ul><li>Host metabolic machinery synthesizes new viral parts </li></ul></ul><ul><ul><li>Spontaneous assembly </li></ul></ul><ul><li>Release </li></ul><ul><ul><li>Break down of host cell wall </li></ul></ul><ul><ul><li>New virion escape to infect more cells </li></ul></ul>
  10. 10. Lysogenic cycle <ul><li>Viral DNA integrates into host’s genome </li></ul><ul><li>Bacteria continues normal metabolic processes (DNA replication, protein synthesis, binary fission) </li></ul><ul><li>Virus remains latent until triggered to enter lytic cycle   </li></ul>
  11. 11. Multiplications of Animal Viruses <ul><li>Animal viruses may differ from phages while entering the host cell. </li></ul><ul><li>The virus is different due to the prokaryotic and eukaryotic cells. </li></ul><ul><li>Animal viruses may not have all the enzymes found in the phage. </li></ul><ul><li>Multiplication of this virus is shared by the DNA and RNA which contains animal viruses. </li></ul>
  12. 12. Bacteriophages and animal viral multiplication compared <ul><li>Tail fibers attach to cell wall proteins </li></ul><ul><li>Viral DNA injected into host cell </li></ul><ul><li>Not required </li></ul><ul><li>In cytoplasm </li></ul><ul><li>Lysogeny </li></ul><ul><li>Host cell lysed </li></ul><ul><li>Attachment sites are plasma membrane proteins and glycoproteins. </li></ul><ul><li>Capsid enters by endocytosis or fusion </li></ul><ul><li>Enzymatic removal of capsid proteins </li></ul><ul><li>In nucleus DNA viruses or cytoplasm RNA viruses </li></ul><ul><li>Latency; slow viral infections; cancer </li></ul><ul><li>Enveloped viruses bud out; noneveloped viruses rupture plasma membrane. </li></ul>
  13. 13. There are six phrases in a virus replication <ul><li>Attachment </li></ul><ul><li>Entry </li></ul><ul><li>Uncoating </li></ul><ul><li>The Biosynthesis of DNA Viruses </li></ul><ul><li>The Biosynthesis of RNA Viruses </li></ul><ul><li>Maturation and Release </li></ul>
  14. 15. Chickenpox ( Varicellovirus ) <ul><li>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. </li></ul>
  15. 16. Latent Viral Infections <ul><li>A latent viral infection is one in which the virus remains in the host cell for long periods without producing an infection. </li></ul><ul><li>Examples are cold sores and shingles. </li></ul>Persistent Viral Infections <ul><li>Persistent viral infections are disease process that occur over a long period and are generally fatal. </li></ul><ul><li>Persistent viral infections are caused by conventional viruses; viruses accumulate over a long period . </li></ul>Persistent Viral Infections
  16. 18. Prions: Definition <ul><li>An infectious agent consisting of a self-replicating protein, with no detectable nucleic acids. </li></ul><ul><li>Involve degeneration of the brain tissue </li></ul><ul><li>Cause some infectious diseases </li></ul><ul><li>Prions have a high resistance to all forms of sterilization </li></ul><ul><li>Listed as most resistant in the “Decreasing order of resistance of microorganisms to chemical biocides” </li></ul><ul><li>e.g.Mad Cow disease </li></ul><ul><ul><li>Great Britain, 1987 </li></ul></ul>
  17. 19. Viroids <ul><li>Infectious RNA </li></ul><ul><ul><li>Short pieces of naked RNA </li></ul></ul><ul><ul><li>300 – 400 nucleotides long </li></ul></ul><ul><ul><li>No protein coat </li></ul></ul><ul><li>Some plant diseases are caused by viroids </li></ul><ul><li>Conclusively identified as pathogens only of plants </li></ul>

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