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CHAPTER 4 Virus and Subvirus

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  1. CHAPTER 4Virus and Subvirus

  2. Outline • 4.1 General Characteristics of Virus • 4.2 Size and Shapes of Viruses • 4.3 Classification of Viruses • 4.4 Viroid • 4.5 Virusoid • 4.6 Prion • 4.7 The Life Cycle of Viruses • 4.8 Life Cycle of Bacteriophages

  3. 4.1 General Characteristics of Virus • Tobacco Mosaic Virus------The Beginning of Virology. Tobacco mosaic virus (TMV, RNA virus) ×49,500

  4. General Characteristics of Virus • Viruses are infectious agents with both living and nonliving characteristics. • They can infect animals, plants, and even other microorganisms. • Viruses that infect only bacteria are called bacteriophages and those that infect only fungi are termed mycophages.

  5. Influenza A virus (RNA virus, Orthomyxoviridae Family) ×31,710 Herpes simplex virus (HSV6, DNA virus) on a peripheral blood lymphocyte ×25,120 Rhabdovirus infecting a fish epithelial cell (RNA virus, Rhabdoviridae Family) ×6,315 Mature virus and budding release of HIV in human lymph tissue ×14,555 Animal virus

  6. Cowpea chlorotic mosaic virus (CCMV) ×42,900 Tobacco mosaic virus (TMV, RNA virus) ×27,300 Plant virus

  7. T4 bacteriophage (DNA virus) ×55,065 Microbial virus

  8. Living characteristics of viruses • They reproduce at a fantastic rate, but only in living host cells. • They can mutate.

  9. Nonliving characteristics of viruses • They are acellular, that is, they contain no cytoplasm or cellular organelles. • They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. In other words, viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell. • The vast majority of viruses possess either DNA or RNA but not both.

  10. Criteria used to define a virus • The vast majority of viruses contain only one type of nucleic acid: DNA or RNA, but not both. • They are totally dependent on a host cell for replication. (They are strict intracellular parasites.) • Viral components must assemble into complete viruses (virions) to go from one host cell to another.

  11. Laboratory cultivation of viruses • Since viruses lack metabolic machinery of their own and are totally dependent on their host cell for replication, they cannot be grown in synthetic culture media. • Animal viruses are normally grown in animals, embryonated eggs, or in cell cultures where in animal host cells are grown in a synthetic medium and the viruses are then grown in these cells.

  12. 4.2 Size and Shapes of Viruses • Size • Viruses are usually much smaller than bacteria and are submicroscopic. Most range in size from 5 to 300 nanometers (nm), although some Paramyxoviruses can be up to 14,000nm long. Can you see the virus?

  13. Sizes of Viruses (Animal RNA Viruses)

  14. Sizes of Viruses (Animal DNA Viruses)

  15. Sizes of Viruses (Bacteriophages)

  16. Shapes of Viruses • Helical viruses • Polyhedral viruses • Enveloped viruses • Complex (binal) viruses

  17. Helical viruses • consist of nucleic acid surrounded by a hollow protein cylinder or capsid and possessing a helical structure.

  18. Helical viruses

  19. Polyhedral viruses • consist of nucleic acid surrounded by a polyhedral (many-sided) shell or capsid, usually in the form of an icosahedron.

  20. Polyhedral viruses Transmission Electron Micrograph ofAdenovirus腺病毒 Transmission Electron Micrograph of Poliomyelitis Virus脊髓灰质炎病毒

  21. Viral Structure (Enveloped Polyhedral Virus) Viral Structure (Enveloped Helical Virus) Enveloped viruses • consist of nucleic acid surrounded by either a helical or polyhedral core and covered by an envelope.

  22. Influenza A Virus Hepatitis B Viruses Herpes Simplex Type 6 Virus Coronavirus HIV-1 Enveloped viruses

  23. A T-even bacteriophage consisting of a head, sheath, and tail Complex (binal ) viruses • have neither helical nor polyhedral forms, are pleomorphic (irregular shaped), or have complex structures.

  24. T4 bacteriophage (DNA virus) ×55,065 Complex (binal ) viruses

  25. 4.3 Classification of Viruses • Viruses can store their genetic information in six different types of nucleic acid which are named based on how that nucleic acid eventually becomes transcribed to the viral mRNA capable of binding to host cell ribosomes and being translated into viral proteins. • Only a (+) viral mRNA strand can be translated into viral protein.

  26. Transcription of Viral Nucleic Acid into Viral mRNA • A (+) RNA can be translated into viral protein. (+) and (-) strands are complementary.

  27. Six forms of viral nucleic acid • (+/-) double-stranded DNA • To replicate the viral genome, DNA-dependent DNA polymerase enzymes copy both the (+) and (-) DNA strands producing dsDNA viral genomes. To produce viral mRNA molecules. DNA-dependent RNA polymerase enzymes copy the (-) DNA strand into (+) viral mRNA. The (+) viral mRNA can then be transtated into viral proteins by host cell ribosomes. Examples include most bacteriophages, Papovaviruses, Adenoviruses, and Herpesviruses.

  28. Six forms of viral nucleic acid • (+) single-stranded DNA • To replicate the viral genome, DNA-dependent DNA polymerase enzymes copy the (+) DNA strand of the genome producing a dsDNA intermediate. DNA-dependent DNA polymerase enzymes then copy the (-) DNA strand into ss (+) DNA genomes. To produce viral mRNA molecules. DNA-dependent RNA polymerase enzymes copy the (-) DNA strand into (+) viral mRNA. The (+) viral mRNA can then be transtated into viral proteins by host cell ribosomes. Examples include Phage M13 and Parvoviruses.

  29. Six forms of viral nucleic acid • (+/-) double-stranded RNA • To replicate the viral genome, RNA-dependent RNA polymerase enzymes copy both the (+) RNA and (-) RNA strands of the genome producing a dsRNA genomes. To produce viral mRNA molecules. RNA-dependent RNA polymerase enzymes copy the (-) RNA strand into (+) viral mRNA. The (+) viral mRNA can then be transtated into viral proteins by host cell ribosomes. Reoviruses are an example.

  30. Six forms of viral nucleic acid • (-) RNA • To replicate the viral genome, RNA-dependent RNA polymerase enzymes copy the (-) RNA genome producing ss (+) RNA. RNA-dependent RNA polymerase enzymes then copy the (+) RNA strands producing ss (-) RNA viral genome. The (+) mRNA strands also function as viral mRNA and can then be transtated into viral proteins by host cell ribosomes. Examples include Orthomyxoviruses, Paramyxoviruses, Rhabdoviruses.

  31. Six forms of viral nucleic acid • (+) RNA • To replicate the viral genome, RNA-dependent RNA polymerase enzymes copy the (+) RNA genome producing ss (-) RNA. RNA-dependent RNA polymerase enzymes then copy the (-) RNA strands producing ss (+) RNA viral genome. To produce viral mRNA molecules. RNA-dependent RNA polymerase enzymes copy the (-) RNA strand into (+) viral mRNA. The (+) viral mRNA can then be transtated into viral proteins by host cell ribosomes. Examples include Picornaviruses, Togaviruses, and Coronaviruses.

  32. Six forms of viral nucleic acid • (+) RNA Retroviruses • To replicate the viral genome, reverse transcriptase enzymes (RNA-dependent DNA polymerases) copy the (+) RNA genome producing ss (-) DNA strands. DNA-dependent DNA polymerase enzymes then copy the (-) DNA strands to produce a dsDNA intermediate. DNA-dependent RNA polymerase enzymes then copy the (-) DNA strands to produce ss (+) RNA genomes. To produce viral mRNA molecules. DNA-dependent RNA polymerase enzymes copy the (-) DNA strand into (+) viral mRNA. The (+) viral mRNA can then be transtated into viral proteins by host cell ribosomes. Retroviruses, such as HIV-1, HIV-2, and HTLV-1 are examples.

  33. 4.4 Viroid • Viroids are small, circular, single-stranded molecules of infectious RNA lacking even a protein coat, even more simple than viruses. • They are the cause of a few plant diseases such as, • Potato spindle-tuber disease, • Cucumber pale fruit disease, • Citrus exocortis disease, • Cadang-cadang (coconuts).

  34. Potato spindle-tuber disease • Potato spindle tuber viroid gets its name because of the oblong tubers produced from infected plants. • Potato spindle tuber viroid causes a stiff and upright growth habit on infected potatoes.

  35. Potato tuber spindle viroid Potato Spindle Tuber Viroid (PSTV) Magnified 350000×

  36. 4.5 Virusoid • Virusoids are circular single-stranded RNAs dependent on plant viruses for replication and encapsidation. • The genome of virusoids consist of several hundred nucleotides and only encodes structural proteins. • Virusoids are similar to viroids in size, structure and means of replication (rolling-circle replication) • Virusoids, while being studied in virology, are not considered as viruses but as subviral particles. Since they depend on helper viruses, they are classified as satellites. In the virological taxonomy they appear as Satellites/Satellite nucleic acids/Subgroup 3: Circular satellite RNAs. • The term virusoid is also sometimes used more generally to refer to all satellites.

  37. Hepatitis D virus • HDV is a defective single-stranded RNA virus that requires the helper function of HBV (Hepatitis B virus) to replicate. • HDV requires HBV for synthesis of envelope protein composed of HBsAg, which is used to encapsulate the HDV genome.

  38. 4.6 Prion • Prions are infectious protein particles thought to be responsible for a group of transmissible and/or inherited neurodegenerative diseases, including Creutzfeldt-Jakob disease, kuru, and Gerstmann-Straussler-syndrome in humans as well as scrapie in sheep and goats.

  39. Scrapie • Scrapie is a chronic disease of sheep which is transmitted by a filterable particle that is resistant to heat and formalin fixation.

  40. spongy appearance congestion of blood vessels spikeball Kuru

  41. Creutzfeldt-Jakob Disease spongy appearance

  42. Prion Creutzfeldt-Jakob Disease brain showing immunohistochemical staining of prion plaque at 1:200 dilution in formalin-fixed, paraffin-embedded section of cerebral cortex.

  43. Stabilities of the scrapie agent and viriods (PSTV) “+” - inactivated; “-”- no change in infectivity

  44. Proposed three-dimensional structure PrPsc PrPc 43% α-helix 30%α-helix, 43%β-sheet

  45. Stanley B. Prusiner The Nobel Assembly at the Karolinska Institute in Stockholm, Sweden, has awarded the Nobel Prize in Physiology or Medicine for 1997 to Stanley B. Prusiner, for his discovery of "prions - a new biological principle of infection".

  46. 4.7 The Productive Life Cycle of Animal Viruses • For many animal viruses, the details of each step in their life cycle have not yet been fully characterized, and among the viruses that have been well studied there is great deal of variation. What follows is a generalized productive life cycle for animal viruses consisting of the following steps: adsorption, viral entry, viral movement to the site of replication and release of the viral genome from the remainder of the virus, viral replication, viral assembly, and viral release.

  47. Adsorption of a Naked Virus to a Susceptible Host Cell • Attachment sites on the viral envelope bind to corresponding host cell receptors.

  48. Penetration of a Naked Virus by Endocytosis

  49. Uncoating of a Naked Virus Entering by Endocytosis

  50. Viral Replication