Overview of Viruses and Virology Chapter 10 Lectures by Buchan & LeCleir
I. Virus Structure and Growth <ul><li>10.1 General Properties of Viruses </li></ul><ul><li>10.2 Nature of the Virion </li>...
10.1 General Properties of Viruses <ul><li>Virus : genetic element that cannot replicate independently of a living (host) ...
10.1 General Properties of Viruses <ul><li>Viral Genomes </li></ul><ul><ul><li>Either DNA or RNA genomes </li></ul></ul><u...
Viral Genomes Figure 10.1
10.1 General Properties of Viruses <ul><li>Viral Hosts and Taxonomy </li></ul><ul><ul><li>Viruses can be classified on the...
10.2 Nature of the Virion <ul><li>Most viruses are smaller than prokaryotic cells; range from 0.02 to 0.3 µm </li></ul><ul...
10.2 Nature of the Virion <ul><li>Viral Structure </li></ul><ul><ul><li>Capsid : the protein shell that surrounds the geno...
10.2 Nature of the Virion <ul><li>Viral Structure  (cont’d) </li></ul><ul><ul><li>Nucleocapsid : complete complex of nucle...
Comparison of Naked and Enveloped Virus Particles Figure 10.3
10.2 Nature of the Virion <ul><li>N ucleocapsids of viruses constructed in highly symmetric ways </li></ul><ul><ul><li>Hel...
Icosahedral Symmetry Figure 10.4
Icosahedral Symmetry Figure 10.4
10.2 Nature of the Virion <ul><li>Enveloped Viruses </li></ul><ul><ul><li>Have membrane surrounding nucleocapsid; lipid bi...
Electron Micrographs of Animal and Bacterial Viruses Figure 10.5
10.2 Nature of the Virion <ul><li>Complex Viruses   </li></ul><ul><ul><li>Virions composed of several parts, each with sep...
10.2 Nature of the Virion <ul><li>Some virions contain enzymes critical to infection </li></ul><ul><ul><li>Lysozyme </li><...
10.3 The Virus Host <ul><li>Viruses replicate only in certain types of cells or in whole organisms </li></ul><ul><li>Bacte...
10.4 Quantification of Viruses <ul><li>Titer : number of infectious units per volume of fluid </li></ul><ul><li>Plaque ass...
Quantification of Bacterial Virus by Plaque Assay Figure 10.6
Quantification of Bacterial Virus by Plaque Assay Figure 10.6
10.4 Quantification of Viruses <ul><li>The number of plaque-forming units is almost always lower than direct counts by mic...
II. Viral Replication <ul><li>10.5 General Features of Virus Replication </li></ul><ul><li>10.6 Viral Attachment and Penet...
10.5 General Features of Virus Replication <ul><li>The Phases of Viral Replication </li></ul><ul><ul><li>Attachment (adsor...
The Replication Cycle of a Bacterial Virus Figure 10.8
10.5 General Features of Virus Replication <ul><li>Virus replication typically characterized by a one-step growth curve </...
The One-Step Growth Curve of Virus Replication Figure 10.9
10.6 Viral Attachment and Penetration <ul><li>Attachment of virion to host cell is highly specific </li></ul><ul><ul><li>R...
10.6 Viral Attachment and Penetration <ul><li>The attachment of a virus to its host cell results in changes to both virus ...
10.6 Viral Attachment and Penetration <ul><li>Bacteriophage T4 : virus of  E. coli ; example of one of the most complex pe...
Attachment of Bacteriophage T4 to the Cell Wall of  E. coli Figure 10.10
10.6 Viral Attachment and Penetration <ul><li>Many eukaryotes possess mechanisms to diminish viral infections  </li></ul><...
10.6 Viral Attachment and Penetration <ul><li>Many bacteria employ  restriction-modification systems  to evade viral infec...
10.6 Viral Attachment and Penetration <ul><li>Viral mechanisms to evade bacterial restriction systems </li></ul><ul><ul><l...
10.7 Production of Viral Nucleic Acid and Protein <ul><li>David Baltimore, Howard Temin, and Renato Dulbecco discovered re...
The Baltimore Classification System of Viruses
10.7 Production of Viral Nucleic Acid and Protein <ul><li>Once a host has been infected, new copies of the viral genome mu...
10.7 Production of Viral Nucleic Acid and Protein <ul><li>Retroviruses : animal viruses responsible for causing certain ty...
10.7 Production of Viral Nucleic Acid and Protein <ul><li>Viral Proteins </li></ul><ul><ul><li>Production follows synthesi...
10.7 Production of Viral Nucleic Acid and Protein <ul><li>Production of Viral Proteins (cont’d) </li></ul><ul><ul><li>Late...
III. Viral Diversity <ul><li>10.8   Overview of Bacterial Viruses </li></ul><ul><li>10.9   Virulent Bacteriophages and T4 ...
10.8 Overview of Bacterial Viruses <ul><li>Bacteriophages are very diverse </li></ul><ul><li>Best-studied bacteriophages i...
10.8 Overview of Bacterial Viruses <ul><li>Viral Life Cycles </li></ul><ul><ul><li>Virulent mode : viruses lyse host cells...
10.10 Temperate Bacteriophages, Lambda, and P1 <ul><li>Temperate viruses : can undergo a different life cycle resulting in...
The Consequences of Infection by a Temperate Phage Figure 10.16
10.10 Temperate Bacteriophages, Lambda, and P1 <ul><li>Bacteriophage Lambda </li></ul><ul><ul><li>Linear, dsDNA genome </l...
Bacteriophage Lambda Figure 10.17
Integration of Lambda DNA into the Host Figure 10.18
10.11 Overview of Animal Viruses <ul><li>Unlike   prokaryotes, entire virion enters the animal cell </li></ul><ul><li>Euka...
Diversity of Animal Viruses: DNA Viruses Figure 10.21a
Diversity of Animal Viruses: RNA Viruses Figure 10.21b
10.11 Overview of Animal Viruses <ul><li>Consequences of Virus Infection in Animal Cells </li></ul><ul><ul><li>Persistent ...
10.12 Retroviruses <ul><li>Retroviruses : RNA viruses that replicate through a DNA intermediate </li></ul><ul><ul><li>Enve...
Retrovirus Structure and Function Figure 10.23a
10.12 Retroviruses <ul><li>Retroviruses have a unique genome </li></ul><ul><ul><li>Two identical ssRNA molecules of the pl...
10.12 Retroviruses <ul><li>Process of Replication of a Retrovirus </li></ul><ul><ul><li>Entrance into the cell  </li></ul>...
Replication Process of a Retrovirus Figure 10.24
IV. Subviral Entities <ul><li>10.13 Defective Viruses </li></ul><ul><li>10.14 Viroids </li></ul><ul><li>10.15 Prions </li>...
10.13 Defective Viruses <ul><li>Helper viruses (defective viruses) : viruses that are parasitic on other viruses </li></ul...
10.14 Viroids <ul><li>Viroids : infectious RNA molecules that lack a protein coat </li></ul><ul><ul><li>Small, circular, s...
Viroids and Plant Disease: Healthy vs. PSTV-Infected Figure 10.25
10.15 Prions <ul><li>Prions : infectious proteins whose extracellular form contains no nucleic acid </li></ul><ul><ul><li>...
Mechanisms of Prion Misfolding Figure 10.28
10.15 Prions <ul><li>Prion disease occurs by three distinct mechanisms </li></ul><ul><ul><li>Infectious prion disease : pa...
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  1. 1. Overview of Viruses and Virology Chapter 10 Lectures by Buchan & LeCleir
  2. 2. I. Virus Structure and Growth <ul><li>10.1 General Properties of Viruses </li></ul><ul><li>10.2 Nature of the Virion </li></ul><ul><li>10.3 The Virus Host </li></ul><ul><li>10.4 Quantification of Viruses </li></ul>
  3. 3. 10.1 General Properties of Viruses <ul><li>Virus : genetic element that cannot replicate independently of a living (host) cell </li></ul><ul><li>Virology : the study of viruses </li></ul><ul><li>Virus particle : extracellular form of a virus; allows virus to exist outside host and facilitates transmission from one host cell to another </li></ul><ul><li>Virion : the infectious virus particle; the nucleic acid genome surrounded by a protein coat and, in some cases, other layers of material </li></ul>
  4. 4. 10.1 General Properties of Viruses <ul><li>Viral Genomes </li></ul><ul><ul><li>Either DNA or RNA genomes </li></ul></ul><ul><ul><li>Some circular, but most linear </li></ul></ul>
  5. 5. Viral Genomes Figure 10.1
  6. 6. 10.1 General Properties of Viruses <ul><li>Viral Hosts and Taxonomy </li></ul><ul><ul><li>Viruses can be classified on the basis of the hosts they infect </li></ul></ul><ul><ul><ul><li>Bacterial viruses (bacteriophages) </li></ul></ul></ul><ul><ul><ul><li>Animal viruses </li></ul></ul></ul><ul><ul><ul><li>Plant viruses </li></ul></ul></ul>
  7. 7. 10.2 Nature of the Virion <ul><li>Most viruses are smaller than prokaryotic cells; range from 0.02 to 0.3 µm </li></ul><ul><li>Most viral genomes are smaller than those of biological cells </li></ul>
  8. 8. 10.2 Nature of the Virion <ul><li>Viral Structure </li></ul><ul><ul><li>Capsid : the protein shell that surrounds the genome of a virus particle </li></ul></ul><ul><ul><ul><li>Composed of a number of protein molecules arranged in a precise and highly repetitive pattern around the nucleic acid </li></ul></ul></ul><ul><ul><li>Capsomer : subunit of the capsid </li></ul></ul><ul><ul><ul><li>Smallest morphological unit visible with an electron microscope </li></ul></ul></ul>
  9. 9. 10.2 Nature of the Virion <ul><li>Viral Structure (cont’d) </li></ul><ul><ul><li>Nucleocapsid : complete complex of nucleic acid and protein packaged in the virion </li></ul></ul><ul><ul><li>Enveloped virus : virus that contains additional layers around the nucleocapsid </li></ul></ul>
  10. 10. Comparison of Naked and Enveloped Virus Particles Figure 10.3
  11. 11. 10.2 Nature of the Virion <ul><li>N ucleocapsids of viruses constructed in highly symmetric ways </li></ul><ul><ul><li>Helical symmetry : rod-shaped viruses (e.g., tobacco mosaic virus) </li></ul></ul><ul><ul><ul><li>Length of virus determined by length of nucleic acid </li></ul></ul></ul><ul><ul><ul><li>Width of virus determined by size and packaging of protein subunits </li></ul></ul></ul><ul><ul><li>Icosahedral symmetry : spherical viruses </li></ul></ul><ul><ul><ul><li>Most efficient arrangement of subunits in a closed shell </li></ul></ul></ul>
  12. 12. Icosahedral Symmetry Figure 10.4
  13. 13. Icosahedral Symmetry Figure 10.4
  14. 14. 10.2 Nature of the Virion <ul><li>Enveloped Viruses </li></ul><ul><ul><li>Have membrane surrounding nucleocapsid; lipid bilayer with embedded proteins </li></ul></ul><ul><ul><li>Make initial contact with host cell </li></ul></ul>
  15. 15. Electron Micrographs of Animal and Bacterial Viruses Figure 10.5
  16. 16. 10.2 Nature of the Virion <ul><li>Complex Viruses </li></ul><ul><ul><li>Virions composed of several parts, each with separate shapes and symmetries </li></ul></ul><ul><ul><li>Examples of most complex viruses in terms of structure can be found among bacterial viruses, which contain icosahedral heads and helical tails </li></ul></ul>
  17. 17. 10.2 Nature of the Virion <ul><li>Some virions contain enzymes critical to infection </li></ul><ul><ul><li>Lysozyme </li></ul></ul><ul><ul><li>Nucleic acid polymerases </li></ul></ul><ul><ul><li>Neuraminadases : enzymes that cleave gycosidic bonds; allows liberation of viruses from host </li></ul></ul>
  18. 18. 10.3 The Virus Host <ul><li>Viruses replicate only in certain types of cells or in whole organisms </li></ul><ul><li>Bacterial viruses are typically easiest to grow; model systems </li></ul><ul><li>Animal viruses (and some plant viruses) can be cultivated in tissue or cell cultures </li></ul><ul><li>Plant viruses typically are most difficult because study often requires growth of whole plant </li></ul>
  19. 19. 10.4 Quantification of Viruses <ul><li>Titer : number of infectious units per volume of fluid </li></ul><ul><li>Plaque assay : analogous to the bacterial colony; one of the most accurate ways to measure virus infectivity </li></ul><ul><ul><li>Plaques are clear zones that develop on lawns of host cells </li></ul></ul><ul><ul><ul><li>Each plaque results from infection by a single virus particle </li></ul></ul></ul>
  20. 20. Quantification of Bacterial Virus by Plaque Assay Figure 10.6
  21. 21. Quantification of Bacterial Virus by Plaque Assay Figure 10.6
  22. 22. 10.4 Quantification of Viruses <ul><li>The number of plaque-forming units is almost always lower than direct counts by microscopy </li></ul><ul><ul><li>Inactive virions </li></ul></ul><ul><ul><li>Conditions not appropriate for infectivity </li></ul></ul>
  23. 23. II. Viral Replication <ul><li>10.5 General Features of Virus Replication </li></ul><ul><li>10.6 Viral Attachment and Penetration </li></ul><ul><li>10.7 Production of Viral Nucleic Acid and Protein </li></ul>
  24. 24. 10.5 General Features of Virus Replication <ul><li>The Phases of Viral Replication </li></ul><ul><ul><li>Attachment (adsorption) of the virus to a susceptible host cell </li></ul></ul><ul><ul><li>Entry (penetration) of the virion or its nucleic acid </li></ul></ul><ul><ul><li>Synthesis of virus nucleic acid and protein by cell metabolism as redirected by virus </li></ul></ul><ul><ul><li>Assembly of capsids and packaging of viral genomes into new virions (maturation) </li></ul></ul><ul><ul><li>Release of mature virions from host cell </li></ul></ul>
  25. 25. The Replication Cycle of a Bacterial Virus Figure 10.8
  26. 26. 10.5 General Features of Virus Replication <ul><li>Virus replication typically characterized by a one-step growth curve </li></ul><ul><li>Latent period : eclipse + maturation </li></ul><ul><li>Burst size : number of virions released </li></ul>
  27. 27. The One-Step Growth Curve of Virus Replication Figure 10.9
  28. 28. 10.6 Viral Attachment and Penetration <ul><li>Attachment of virion to host cell is highly specific </li></ul><ul><ul><li>Requires complementary receptors on the surface of a susceptible host and its infecting virus </li></ul></ul><ul><ul><li>Receptors on host cell carry out normal functions for cell (e.g., uptake proteins) </li></ul></ul><ul><ul><li>Receptors include proteins, carbohydrates, glycoproteins, lipids, lipoproteins, or complexes </li></ul></ul>
  29. 29. 10.6 Viral Attachment and Penetration <ul><li>The attachment of a virus to its host cell results in changes to both virus and cell surface that facilitate penetration </li></ul><ul><li>Permissive cell : host cell that allows the complete replication cycle of a virus to occur </li></ul>
  30. 30. 10.6 Viral Attachment and Penetration <ul><li>Bacteriophage T4 : virus of E. coli ; example of one of the most complex penetration mechanisms known </li></ul><ul><ul><li>Virions attach to cells via tail fibers that interact specifically with polysaccharides on E. coli cell envelope </li></ul></ul><ul><ul><li>Tail fibers retract and tail core makes contact with E. coli cell wall </li></ul></ul><ul><ul><li>Lysozyme-like enzyme forms small pore in peptidoglycan </li></ul></ul><ul><ul><li>Tail sheath contracts and viral DNA passes into cytoplasm </li></ul></ul>
  31. 31. Attachment of Bacteriophage T4 to the Cell Wall of E. coli Figure 10.10
  32. 32. 10.6 Viral Attachment and Penetration <ul><li>Many eukaryotes possess mechanisms to diminish viral infections </li></ul><ul><ul><li>E.g., immune defense mechanisms, RNA interference </li></ul></ul>
  33. 33. 10.6 Viral Attachment and Penetration <ul><li>Many bacteria employ restriction-modification systems to evade viral infection </li></ul><ul><ul><li>DNA destruction system; only effective against double-stranded DNA viruses </li></ul></ul><ul><ul><li>Restriction enzymes (restriction endonucleases) cleave DNA at specific sequences </li></ul></ul><ul><ul><li>Modification of host’s own DNA at restriction enzyme recognition sites prevent cleavage of own DNA </li></ul></ul>
  34. 34. 10.6 Viral Attachment and Penetration <ul><li>Viral mechanisms to evade bacterial restriction systems </li></ul><ul><ul><li>Chemical modification of viral DNA (glycosylation or methylation) </li></ul></ul><ul><ul><li>Production of proteins that inhibit host cell restriction system </li></ul></ul>
  35. 35. 10.7 Production of Viral Nucleic Acid and Protein <ul><li>David Baltimore, Howard Temin, and Renato Dulbecco discovered retroviruses and reverse transcriptase </li></ul><ul><ul><li>Shared 1975 Nobel Prize for Physiology or Medicine </li></ul></ul><ul><li>Baltimore developed classification scheme for viruses based on relationship of viral genome to its mRNA </li></ul>
  36. 36. The Baltimore Classification System of Viruses
  37. 37. 10.7 Production of Viral Nucleic Acid and Protein <ul><li>Once a host has been infected, new copies of the viral genome must be made and virus-specific proteins synthesized in order for the virus to replicate </li></ul><ul><li>Generation of messenger RNA (mRNA) occurs first </li></ul><ul><li>Typically viral genome serves as template for viral mRNA </li></ul><ul><li>In some RNA viruses, viral RNA itself is the mRNA </li></ul><ul><li>In some cases essential transcriptional enzymes are contained in the virion </li></ul>
  38. 38. 10.7 Production of Viral Nucleic Acid and Protein <ul><li>Retroviruses : animal viruses responsible for causing certain types of cancers and acquired immunodeficiency syndrome (AIDS) </li></ul><ul><ul><li>Class VI and VII viruses </li></ul></ul><ul><ul><li>Require reverse transcriptase </li></ul></ul>
  39. 39. 10.7 Production of Viral Nucleic Acid and Protein <ul><li>Viral Proteins </li></ul><ul><ul><li>Production follows synthesis of viral mRNA </li></ul></ul><ul><ul><ul><li>Early proteins </li></ul></ul></ul><ul><ul><ul><ul><li>synthesized soon after infection </li></ul></ul></ul></ul><ul><ul><ul><ul><li>necessary for replication of virus nucleic acid </li></ul></ul></ul></ul><ul><ul><ul><ul><li>typically act catalytically </li></ul></ul></ul></ul><ul><ul><ul><ul><li>synthesized in smaller amounts </li></ul></ul></ul></ul>
  40. 40. 10.7 Production of Viral Nucleic Acid and Protein <ul><li>Production of Viral Proteins (cont’d) </li></ul><ul><ul><li>Late proteins </li></ul></ul><ul><ul><ul><li>Synthesized later </li></ul></ul></ul><ul><ul><ul><li>Include proteins of virus coat </li></ul></ul></ul><ul><ul><ul><li>Typically structural components </li></ul></ul></ul><ul><ul><ul><li>Synthesized in larger amounts </li></ul></ul></ul>
  41. 41. III. Viral Diversity <ul><li>10.8 Overview of Bacterial Viruses </li></ul><ul><li>10.9 Virulent Bacteriophages and T4 </li></ul><ul><li>10.10 Temperate Bacteriophages, Lambda, and P1 </li></ul><ul><li>10.11 Overview of Animal Viruses </li></ul><ul><li>10.12 Retroviruses </li></ul>
  42. 42. 10.8 Overview of Bacterial Viruses <ul><li>Bacteriophages are very diverse </li></ul><ul><li>Best-studied bacteriophages infect enteric bacteria </li></ul><ul><ul><li>E.g., E. coli , Salmonella enterica </li></ul></ul><ul><li>Most contain dsDNA genomes </li></ul><ul><li>Most are naked, but some possess lipid envelopes </li></ul><ul><li>They are structurally complex, containing heads, tails and other components </li></ul>
  43. 43. 10.8 Overview of Bacterial Viruses <ul><li>Viral Life Cycles </li></ul><ul><ul><li>Virulent mode : viruses lyse host cells after infection </li></ul></ul><ul><ul><li>Temperate mode : viruses replicate their genomes in tandem with host genome and without killing host </li></ul></ul>
  44. 44. 10.10 Temperate Bacteriophages, Lambda, and P1 <ul><li>Temperate viruses : can undergo a different life cycle resulting in a stable genetic relationship within the host </li></ul><ul><ul><li>But can also kill cells through lytic cycle </li></ul></ul><ul><li>Lysogeny : state where most virus genes not expressed and virus genome ( prophage ) is replicated in synchrony with host chromosome </li></ul><ul><li>Lysogen : a bacterium containing a prophage </li></ul><ul><li>Under certain conditions lysogenic viruses may revert to the lytic pathway and begin to produce virions </li></ul>
  45. 45. The Consequences of Infection by a Temperate Phage Figure 10.16
  46. 46. 10.10 Temperate Bacteriophages, Lambda, and P1 <ul><li>Bacteriophage Lambda </li></ul><ul><ul><li>Linear, dsDNA genome </li></ul></ul><ul><ul><li>Complementary, single-stranded regions 12 nucleotides long at the 5 ′ -terminus of each strand </li></ul></ul><ul><ul><li>Upon penetration, DNA ends base-pair forming the cos site, DNA ligates and forms double-stranded circle </li></ul></ul><ul><ul><li>When lysogenic, integrates into E. coli chromosome at the lambda attachment site ( att  </li></ul></ul>
  47. 47. Bacteriophage Lambda Figure 10.17
  48. 48. Integration of Lambda DNA into the Host Figure 10.18
  49. 49. 10.11 Overview of Animal Viruses <ul><li>Unlike prokaryotes, entire virion enters the animal cell </li></ul><ul><li>Eukaryotic cells contain a nucleus, the site of replication for many animal viruses </li></ul><ul><li>Animal viruses contain all known modes of viral genome replication </li></ul><ul><li>Many more kinds of enveloped animal viruses than bacterial viruses exist </li></ul><ul><ul><li>As animal viruses leave host cell, they can remove part of host cell’s lipid bilayer for their envelope </li></ul></ul>
  50. 50. Diversity of Animal Viruses: DNA Viruses Figure 10.21a
  51. 51. Diversity of Animal Viruses: RNA Viruses Figure 10.21b
  52. 52. 10.11 Overview of Animal Viruses <ul><li>Consequences of Virus Infection in Animal Cells </li></ul><ul><ul><li>Persistent infections : release of virions from host cell does not result in cell lysis </li></ul></ul><ul><ul><ul><li>Infected cell remains alive and continues to produce virus indefinitely </li></ul></ul></ul><ul><ul><li>Latent infections : delay between infection by the virus and lytic events </li></ul></ul><ul><ul><li>Transformation : conversion of normal cell into tumor cell </li></ul></ul>
  53. 53. 10.12 Retroviruses <ul><li>Retroviruses : RNA viruses that replicate through a DNA intermediate </li></ul><ul><ul><li>Enveloped viruses </li></ul></ul><ul><ul><li>Contain a reverse transcriptase (copies information from its RNA genome into DNA), integrase, and protease </li></ul></ul><ul><ul><li>Virion contains specific tRNA molecules </li></ul></ul>
  54. 54. Retrovirus Structure and Function Figure 10.23a
  55. 55. 10.12 Retroviruses <ul><li>Retroviruses have a unique genome </li></ul><ul><ul><li>Two identical ssRNA molecules of the plus (+) orientation </li></ul></ul><ul><ul><li>Contain specific genes </li></ul></ul><ul><ul><ul><li>gag : encode structural proteins </li></ul></ul></ul><ul><ul><ul><li>pol : encode reverse transcriptase and integrase </li></ul></ul></ul><ul><ul><ul><li>env : encode envelope proteins </li></ul></ul></ul>
  56. 56. 10.12 Retroviruses <ul><li>Process of Replication of a Retrovirus </li></ul><ul><ul><li>Entrance into the cell </li></ul></ul><ul><ul><li>Removal of virion envelope at the membrane </li></ul></ul><ul><ul><li>Reverse transcription of one of the two RNA genomes </li></ul></ul><ul><ul><li>Integration of retroviral DNA into host genome </li></ul></ul><ul><ul><li>Transcription of retroviral DNA </li></ul></ul><ul><ul><li>Assembly and packaging of genomic RNA </li></ul></ul><ul><ul><li>Budding of enveloped virions; release from cell </li></ul></ul>
  57. 57. Replication Process of a Retrovirus Figure 10.24
  58. 58. IV. Subviral Entities <ul><li>10.13 Defective Viruses </li></ul><ul><li>10.14 Viroids </li></ul><ul><li>10.15 Prions </li></ul>
  59. 59. 10.13 Defective Viruses <ul><li>Helper viruses (defective viruses) : viruses that are parasitic on other viruses </li></ul><ul><ul><li>Satellite viruses : defective viruses for which no intact version exists; rely on unrelated viruses as helpers </li></ul></ul>
  60. 60. 10.14 Viroids <ul><li>Viroids : infectious RNA molecules that lack a protein coat </li></ul><ul><ul><li>Small, circular, ssRNA molecules </li></ul></ul><ul><ul><li>Smallest known pathogens (246–399 bp) </li></ul></ul><ul><ul><li>Cause a number of important plant diseases </li></ul></ul><ul><ul><li>Do not encode proteins; completely dependent on host-encoded enzymes </li></ul></ul>
  61. 61. Viroids and Plant Disease: Healthy vs. PSTV-Infected Figure 10.25
  62. 62. 10.15 Prions <ul><li>Prions : infectious proteins whose extracellular form contains no nucleic acid </li></ul><ul><ul><li>Known to cause disease in animals (transmissible spongiform encephalopathies) </li></ul></ul><ul><ul><li>Host cell contains gene ( PrnP ) that encodes native form of prion protein that is found in healthy animals </li></ul></ul><ul><ul><li>Prion misfolding results in neurological symptoms of disease (e.g., resistance to proteases, insolubilty, and aggregation) </li></ul></ul>
  63. 63. Mechanisms of Prion Misfolding Figure 10.28
  64. 64. 10.15 Prions <ul><li>Prion disease occurs by three distinct mechanisms </li></ul><ul><ul><li>Infectious prion disease : pathogenic form of prion protein is transmitted between animals or humans </li></ul></ul><ul><ul><li>Sporadic prion disease : random misfolding of a normal, healthy prion protein in an uninfected individual </li></ul></ul><ul><ul><li>Inherited prion disease : mutation in prion gene yields a protein that changes more often into disease-causing form </li></ul></ul>
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