8 - Virus Structure Multiplication


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8 - Virus Structure Multiplication

  1. 1. Viruses: Their Structure and Replication
  2. 2. What are Viruses • Small, Filterable, infectious agents – Cannot be seen by light microscope – Electron Microscope • Obligatory Intracellular Parasites – Not enough ATP by itself
  3. 3. Characteristics Of Viruses • Genetic Material: DNA or RNA – never both! – Single stranded (ss) or double stranded (ds) – Linear or circular • Capsid – Protein subunits (shell) – Protect the genetic material – May be involved in cell entry • Envelope (required for entry if present) – Lipid, protein and carbohydrate – Protein: viral origin – Lipid, carbohydrate: host origin – Involved in cell entry (located outside the capsid) • Subunit Replication only – Assembled after parts are made (quick log phase) • NEVER contain enzymes for protein synthesis or ATP metabolism!! – Always appropriate the host cell machinery
  4. 4. Size of Viruses
  5. 5. Virus Classification • Based on Host Range (determined by specific receptors) – Bacterial Viruses (Bacteriophages) – Animal Viruses – Plant Viruses – Others – (amoeba, insects) • Based on genome structure – DNA or RNA (never both) • ssDNA viruses • dsDNA viruses • ssRNA viruses – Plus Strand (+RNA viruses) » Same – Negative strand (- RNA viruses) » Complementary – Retroviruses » Converted into complementary DNA, then into the cell for replication • dsRNA viruses – Linear or circular – Size • 2000 to 200,000 nucleotides
  6. 6. Virus Classification (cont.d) • Based on Shape – Polyhedral viruses (icosahedral -20 faces, 12 vertices) – Helical Viruses – Complex viruses • Based on Envelope – Naked viruses – Enveloped viruses • Based on Disease caused – Respiratory viruses – Gastrointestinal viruses – Sexually transmitted viruses
  7. 7. Viral Structure: Capsid Capsid = protein coat that encloses and protects the nucleic acid of a virus • Accounts for most of the viral mass • Composed of single or multiple proteins • Each subunit = capsomeres
  8. 8. Viral Structure: Envelope Sometimes, Capsid covered with envelope •spikes = carbohydrate-protein complexes (glycoproteins) that project from the envelope • Can be used to attach to host cell • Influenza virus causes hemagglutination – clumping of red blood cells by use of spikes Non-enveloped viruses/ Naked Viruses = viruses whose capsids are not covered by an envelope
  9. 9. An Enveloped Virus
  10. 10. Virus mutations and immunity When a virus infects a host cell: •Host immune system produces antibodies •Antibodies = proteins that inactivate the virus by reacting with virus surface proteins  Stops further infection Why you can get some viruses more than once: • Genes that code for viral surface proteins are susceptible to mutation • Antibodies can’t react with the altered surface proteins  infection • Ex) influenza; frequent mutations in its spikes Non-neutralizing antibodies do not inhibit function Neutralizing inhibits function
  11. 11. Viral Structure: General Morphology Capsid Structure determines shape: Helical Viruses = nucleic acid is inside a hollow cylindrical capsid with a helical structure • Rabies, Ebola viruses, Tobacco Mosaic Virus Polyhedral viruses = many sided; icosahedron is common with 20 equilateral triangles as sides and 12 vertices • Poliovirus, Adenovirus, herpes, others??
  12. 12. Viral Structure: General Morphology Enveloped Viruses = can be helical or polyhedral, but the capsid is surrounded by an envelope • Helical: influenza virus • Polyhedral (icosahedral): Herpes simplex virus Complex viruses = Complex structures; additional structures attached to capsids, combos of helical and polyhedral, may have several coats around nucleic acid • Bacteriophage, poxviruses
  13. 13. Virus Structure
  14. 14. Virus Taxonomy International Committee on Taxonomy of Viruses (ICTV) groups viruses based on: • Nucleic acid type • Mode of replication • Morphology Viral species = defined as a group of viruses sharing the same genetic information and host range • Viral species are given common names • Ex) human herpesvirus • The suffix –virus is used for genus names • Ex) Simplexvirus • Family of viruses is given the suffix –viridae • Ex) herpesviridae
  15. 15. Some Viral Families & Genera affecting humans
  16. 16. Some Viral Families & Genera affecting humans: Continued
  17. 17. Isolation & cultivation of viruses Our understanding of viruses comes mostly from bacteriophages, as they are easily grown on bacterial cultures • Liquid suspensions or solid media Plaque method for detecting & counting viruses: 1) bacteriophages are mixed with host bacteria and melted agar, poured onto petri plate with existing layer of solid growth medium 2) Top layer solidifies ~ one cell thick 3) After several rounds of infection, multiplication and lysis, bacteria surrounding the virus are destroyed  plaque
  18. 18. Bacteriophage lambda on a lawn of E. coli • Each plaque is from a single virus (theoretically) • Number of plaques can be used to calculate plaque forming units (PFU) in initial suspension
  19. 19. Bacterial Viruses • Bacteriophages • DNA and RNA viruses – ds and ss – Linear and circular • Important tool – Alternative to antibiotic therapy • Basis to study viral replication in host cells – Lytic cycle (lyses cell) – Lysogenic cycle http://www.youtube.com/watch?v=gU8XeqI7yts It is very easy to grow viruses in bacteria, so it another reason it’s an important tool Never marketed phage therapy because it doesn’t work too well
  20. 20. Lytic Cycle of a T-Even Bacteriophage 1 2 3 Figure 13.11 HAS TO BE A DNA VIRUS Only infects cells with specific receptors “syringes in”
  21. 21. 4 Figure 13.11 Lytic Cycle of a T-Even Bacteriophage Integrates genome into host genome
  22. 22. Steps for Replication • Adsorption • Penetration • Biosynthesis – Uncoating – Replication • Assembly • Release Memorize
  23. 23. Lysogenic cycle: bacteriophage lambda Prophage: DNA incorporated and now it will make bacteriophages DNA randomly pops off in lysogenic cycle then goes through lytic
  24. 24. Lysogeny: importance • Lysogenic cells are immune to infection by the same phage (but not to other phages) • Phage conversion = tendency of host cell to exhibit new properties when carrying lysogenic phage • Ex) Cornyebacterium diptheriae produces toxin only when carrying lysogenic phage  diptheria • Same is true for shiga toxin by pathogenic E. coli • Specialized transduction = since bacterial DNA is incorporated with phage DNA, adjacent genes on host DNA may remain attached when phage DNA is excised for initiation of the lytic cycle • Introduce foreign genes into a new cell’s genome Specialized transduction – horizontal gene transfer
  25. 25. Multiplication of Animal Viruses • Entry – Adsorption (not misspelled) • Sticks to the surface – Entry/ Penetration – Uncoating (if capsid goes in) • Replication/ Synthesis • Assembly/Maturation • Release – Affect on host cell DNA matures in the nucleus, RNA matures in the cytoplasm
  26. 26. Multiplication of Animal Viruses: Attachment • Attachment – Virus attachment sites • Spikes or capsid proteins – Receptor Sites • Proteins, glycoproteins on host cell membrane
  27. 27. Multiplication of Animal Viruses: Entry • Endocytosis – Pinocytosis (unseen) • Plasma membrane folds inward into vesicles – Receptor mediated Endocytosis (creates own vesicles) • Influenza virus • Fusion (membranes are the same) – Viral envelope fuses with cell membrane
  28. 28. Multiplication of Animal Viruses: Entry Above: Togavirus entering a cell through pinocytosis
  29. 29. Multiplication of Animal Viruses: Entry Above: Herpesvirus entering a cell through fusion
  30. 30. Multiplication of Animal Viruses: Uncoating • Separation of nucleic acid from protein capsid – Capsid digested by host enzymes – Viral proteins synthesized to uncoat • poxviruses
  31. 31. Replication/Biosynthesis/Maturation/Release: DNA viruses • Gene expression in most viruses – Stage specific – Temporal Cascade • DNA viruses – DNA shuttled to host nucleus – EARLY Gene transcription • Uses host RNA polymerase • mRNA shuttled to protein translation sites in cytoplasm • Viral Enzymes (proteins) shuttled back to nucleus – Viral DNA replication • Viral DNA polymerase replicates DNA – LATE gene expression • Uses host RNA polymerase • mRNA shuttled to protein translation sites in cytoplasm • Viral structural proteins Proteins shuttled back to nucleus – VIRIONS assembled, shuttled back to cytoplasm – Trafficked via ER, Golgi, to Cell membrane for release “Too much detail, I just need you to remember that some genes are expressed before replication, some are expressed after. It just depends on when it needs that expression.”
  32. 32. It’s acidity is what causes the endosome to fuse with the envelope and release the capsid budding
  33. 33. biosynthesis of DNA viruses example Back into nucleus
  34. 34. RNA viruses • Multiplication is same as that of DNA viruses except mechanisms of how mRNA is generated • Four nucleic acid types of RNA viruses • Single (+) strand of RNA • Ex) picornaviridae, togaviridae • Single (–) strand of RNA • Ex) rhaboviridae • Double stranded RNA • Ex) reoviridae • “Reverse transcriptase” RNA (retroviruses) • Ex) retroviridae +  transcription  - translation  + - strand makes message, so - stays - - Strand will make one part (like capsid) + will make other part (like the RNA) Prophage – bacteriophage DNA incorporated??? Provirus – any other virus DNA incorporated??? RNA polymerase is more likely to make errors than DNA polymerase
  35. 35. Avian Influenza Clinical Focus, p. 371 Antigenic drift – mutations in how it looks Antigenic shift – so many mutations, very virulent, no antibodies to fight it
  36. 36. Biosynthesis of DNA vs RNA viruses
  37. 37. Multiplication of Animal Viruses: Release Non-enveloped viruses = released from host through ruptures in plasma membrane  host cell death Enveloped viruses = the envelope develops around the capsid by budding: virus takes portion of plasma membrane as it pushes through it to extracellular space • Doesn’t immediately kill host cell
  38. 38. Budding: formation of the envelope
  39. 39. Viruses and Cancer Oncogenic viruses (cancer generating) = viruses capable of inducing tumors in animals (aka oncoviruses) • ~10% of cancers induced by viruses Oncogenes = parts of the genome that cause cancer when mutated; expressed at high levels in tumor cells Transformation = viral genetic material integrates into host DNA and replicates with it (like bacterial lysogeny) • Used by all oncogenic viruses Oncolytic – viruses that grow in and lyse cancer cells
  40. 40. DNA oncogenic viruses Herpesviridae •Epstein Barr virus causes infectious mononucleosis - remains latent in some throat and blood cells throughout life  various lymphomas (Hodgkin’s, Burkitt’s) Papovaviridae •all uterine (cervical) cancers are caused by human papillomavirus Hepadnaviridae (Hep B) & Flaviviridae (Hep C) •hepatitis B and C can cause liver cancer
  41. 41. RNA oncogenic viruses • ONLY the retroviridae family of RNA viruses cause cancer Human T-cell leukemia viruses 1 & 2 = cause adult T-cell (white blood cell) leukemia and lymphoma Mechanism of tumor generation: Viral reverse transcriptase generates the double stranded viral DNA (provirus) that integrates into the host chromosome • Changes in genetic material always put the cell at risk for tumor formation Know oncogenic viruses and diseases with them
  42. 42. Latent viral infections Viruses may infect host cells but cause disease only after a long period of time = latent infections All human herpesviruses can remain in host cells for a person’s lifespan, until reactivation: • Immune suppression (ex: AIDS) • Fever, sunburn (cold sores from herpes simplex) •Reactivation may never occur  no symptoms Chronic can be latent or persistant
  43. 43. Persistent viral infections Persistent (chronic) viral infections occur gradually over a long period of time • Infectious virus builds up over time, rather than appearing suddenly (like latent infections) • Typically fatal Example: Subacute sclerosing panencephalitis (SSPE) = a progressive, debilitating, and deadly brain disorder • Caused by immune resistant measles • No cure; may be managed with medication Remember this one
  44. 44. Figure 13.21 Latent and Persistent Viral Infections Peaks, looks like it will go away, then spikes (can be lethal)
  45. 45. Prions Prions = proteinaceous infectious particles • No nucleic acid, just purely protein • Cause infections diseases - neurological • Bovine spongiform encephalophathy (mad cow) • Creutzfeldt-Jakob disease (CJD) • Gerstmann-Straussler-Sheinker syndrome Run in families, indicating genetic component  not purely genetic:  Eating infected meat transmits mad cow  CJD transmitted via transplanted nerve tissue Only killed by formaldehyde, very resistant KNOW ALL PRIONS
  46. 46. Prion Diseases • Crutzfedt-Jakob • Kuru • Bovine Spongiform encephalopathy (mad cow) • vCJD http://www.beatricebiologist.com/2010/08/watch-out-for- prions.html
  47. 47. Plant viruses and viroids Plant viruses = similar in morphology and nucleic acid types to animal viruses Common crop viruses: - Bean mosaic virus - Wound tumor virus  corn and sugarcane - Potato yellow dwarf virus Must penetrate cell wall by: - Wounds - Parasites  Ex) aphids that eat sap Result = color change, deformed/stunted growth, wilting Only destructive
  48. 48. Plant viruses and viroids Infected plant spreads virus via pollen and seeds viroids = short pieces of RNA with no protein coat • Known to cause some plant diseases • Pathogens of plants only • Potato spindle tuber viroid Prions are only protein Viroids are only RNA
  49. 49. Some major plant viruses
  50. 50. DNA virus families Adenoviridae = cause acute respiratory disease (common cold) Poxviridae = cause skin lesion diseases • Pox = pus-filled lesions • Smallpox • Cowpox Herpesviridae = named after herpetic (spreading) appearance of cold sores • Genus simplexvirus (cold sores) • Genus varicellovirus (chickenpox) • Genus lymphocryptovirus (mononucleosis)
  51. 51. Figure 13.5b Poxviridae • Double-stranded DNA, enveloped viruses – Orthopoxvirus (vaccinia and smallpox viruses) – Molluscipoxvirus – Smallpox – Molluscum contagiosum – Cowpox
  52. 52. Mastadenovirus (adenoviridae) Herpes simplex virus (herpesviridae) DNA virus families
  53. 53. DNA virus families Papovaviridae = named for the papillomas (warts) polyomas (tumors) and vacuolation (development of cytoplasmic vacuoles) • Genus papillomavirus causes warts - HPV: cervical cancer and cauliflower-like growths in cervix - Vaccine: Gardasil • Polyomavirus diseases primarily affect the immunocompromised  tumors
  54. 54. DNA virus families Hepadnaviridae = named for their role in causing hepatitis and containing DNA • Only one genus  causes hepatitis B • The other hepatitis viruses (A,C,D,E,F,G) are RNA viruses Hepatitis = inflammation of liver • Hep B is similar to Hep C (an RNA virus) • Both transmitted through blood - Associated with intravenous drug use • Cirrhosis, liver failure, liver cancer • Vaccine for Hep B, no vaccine for Hep C!
  55. 55. RNA viruses Picornaviridae = small (-pico) and contain RNA • Single stranded RNA viruses Important genera: Rhinovirus = responsible for >50% of common colds Enterovirus = fecal oral transmission poliovirus, coxsackie virus (aseptic meningitis) Hepatovirus = only species in the genus causes Hep A • Fecal-oral transmission • Contaminated food or water • Primarily affects less developed countries • Replication: mucosa  intestine  liver • Symptoms: fever, nausea, diarrhea, jaundice • Prevention: vaccine
  56. 56. Togaviridae are enveloped (toga = covering) • Like picornaviruses, have a single strand of RNA Important genera: Rubivirus = only member is rubella virus • Part of MMR vaccination series • Rubella = (latin: little red) aka german measles - Itchy red rash - Swollen glands, fever • Transmission: respiratory droplets • Treatment: none, usually subsides in days - Less severe than measles (rubeola virus) RNA viruses
  57. 57. RNA viruses Paramyxoviridae = enveloped viruses with spikes • single stranded RNA viruses Important genera: Rubulavirus = contains the species Mumps virus • Transmitted by respiratory droplets • Was common before MMR vaccine (1960s) • Symptoms: fever, headache, muscle aches, tiredness - swelling of parotid (salivary) glands! - Orchitis = swelling of testicles (~30% of males)
  58. 58. RNA viruses Rhabdoviridae = bullet-shaped viruses with a single strand of RNA • ~150 viruses of vertebrates, invertebrates and plants Lyssavirus = genus that contains the species rabies virus • Transmission: animal bite • Salivary glands highly concentrated with virus • Spreads from muscle cells into CNS • Fatal if not treated prior to severe symptoms
  59. 59. RNA viruses Orthomyxoviridae = enveloped helical viruses with a single strand of RNA Influenza virus = three genera (A,B,C) that cause influenza, a contagious respiratory illness Symptoms: cough, sore throat, aches, fatigue and serious complications: - Pneumonia - Bronchitis - Worsening of chronic health problems TEM of H1N1 Influenza
  60. 60. Avian Influenza Clinical Focus, p. 371
  61. 61. RNA viruses Reoviridae = respiratory, enteric, orphan • Affect gastrointestinal system, respiratory tract • Double-stranded RNA viruses Rotavirus = genus in family reoviridae • Most common cause of severe diarrhea among infants and children  Fecal-oral transmission • 2009: included into U.S. recommended vaccination program by W.H.O. Stylized SEM: rotavirus
  62. 62. RNA viruses Retroviridae = reverse transcriptase viruses • Reverse transcriptase = uses viral RNA as template to produce double-stranded DNA • integrated into host chromosome  provirus - protected from host immune system & antivirals - Replicates with host DNA - Can be expressed to produce new virions and infect adjacent cells Human Immunodeficiency virus (HIV) • Infects immune cells, progresses to AIDS • No cure: hard to target latent infected cells
  63. 63. Retroviruses