Chapter 13

1. Chapter 13 Viruses, Viroids and Prions

2. Adolf Mayer, 1886 • tobacco mosaic disease (TMD) transmissible • Dimitri Iwanowski, 1892 • Filtered sap still caused TMD • contagious fluid or filterable agent • Walter Reed,1901 • Yellow Fever

3. Felix d’Herelle, 1917 • Bacteriophage • Suggested phage therapy • 1930’s, term virus introduced and electron microscopy invented • Wendell Stanley, 1935 • Isolated tobacco mosaic virus

4. [INSERT FIGURE 13.4]

5. Viral Features • Obligate intracellular pathogen • Host range • highly specific or generalists • DNA or RNA • Protein coat • Surface proteins highly susceptible to mutations • Cause synthesis of specialized structures to transfer viral particles to other cells

6. Virion • complete, fully developed, infectious viral particle • Living or non-living entities?????

8. Viral Classification • Oldest system based on symptomology • International Committee on Taxonomy of Viruses (ICTV), 1966 • Describe viruses as elementary bio-systems • Classified into orders, families, genera and species • Over 1,500 officially recognized species

9. 3 primary means of classification: Nucleic acid Replication strategy Morphology of protein coat (capsid)

10. Helical Viruses Polyhedral Viruses

11. Viral Envelope • Acquired from host cell • Phospholipids and proteins • Some glycoproteins are virally coded spikes • Often play role in host recognition

12. Critical Swine Flu prevention tip: Don't DO this!

13. Complex Viruses

14. [INSERT FIGURE 13.5]

15. Viral Taxonomy • Order –virales • Family –viridae • Genus – virus • Species • Common names • Subspecies designated by a number

16. Viral Taxonomy • Retroviridae– family • Lentivirus – genus • Human Immunodeficiency Virus– species • Herpesviridae • Simplexvirus • Human herpesvirus 1, HHV 2, HHV 3

17. Isolation and Cultivation of Viruses • Viruses must be grown in living cells • Cytopathic effects

18. Animal viruses may be grown in living animals or in embryonated eggs

19. Animal & plants viruses may be grown in cell cultures • Primary cell lines • Continuous cell lines (transformed cells )

20. Virus Identification • Serological tests • Detect antibodies against viruses in a patient • Nucleic acids • RFLPs • PCR

21. Lytic cycle Phage causes lysis and death of host cell Lysogenic cycle Temperate phages incorporate DNA into host DNA (prophage) Multiplication of Bacteriophages

22. Lytic Cycle • Attachment Phage attaches by tail fibers to host cell • Penetration Phage lysozyme opens cell wall, tail sheath contracts to force tail core and DNA into cell • Biosynthesis Production of phage DNA and proteins • Maturation Assembly of newly synthesized phage particles • Release Phage lysozyme breaks cell wall

23. [INSERT FIGURE 13.8]

24. One-step Growth Curve

25. Lysogenic Cycle

26. 3 Important results of lysogeny • Immunity to re-infection • Phage conversion • Specialized transduction

27. Specialized Transduction gal gene Bacterial DNA Prophage 1 Prophage exists in galactose-using host (containing the gal gene). Galactose-positive donor cell gal gene 2 Phage genome excises, carrying with it the adjacent gal gene from the host. 3 Phage matures and cell lyses, releasing phage carrying gal gene. gal gene 4 Phage infects a cell that cannot utilize galactose (lacking gal gene). Galactose-negative recipient cell 5 Along with the prophage, the bacterial gal gene becomes integrated into the new host’s DNA. 6 Lysogenic cell can now metabolize galactose. Galactose-positive recombinant cell

28. Multiplication of Animal viruses • Attachment Viruses attaches to cell membrane • Penetration By endocytosis or fusion • Uncoating By viral or host enzymes • Biosynthesis Production of nucleic acid and proteins • Maturation Nucleic acid and capsid proteins assemble • Release By budding (enveloped viruses) or rupture

29. Attachment of animal viruses • Chemical attraction • No tails or tail fibers • Glycoprotein spikes or other attachment molecules

30. Replication of Animal Viruses • Biosynthesis • Each virus requires different strategy depending on its nucleic acid • DNA viruses often enter the nucleus • RNA viruses typically replicate in cytoplasm • Must consider: • What serves as template for replication and how mRNA is transcribed

31. Multiplication of DNA Virus Papovavirus 1 Virion attaches to host cell 7 Virions are released Host cell DNA Capsid 2 DNA Virion penetrates cell and its DNA is uncoated Cytoplasm 6 Virions mature Capsid proteins mRNA 5 Late translation; capsid proteins are synthesized 3 Viral DNA penetrates host nucleus Early transcription and translation; enzymes are synthesized 4 Late transcription; DNA is replicated

32. Multiplication of RNA Virus [INSERT FIGURE 13.13]

33. Multiplication of a Retrovirus Capsid Reverse transcriptase DNA Virus Two identical + stands of RNA 1 Retrovirus penetrates host cell. Host cell DNA of one of the host cell’s chromosomes 5 Mature retrovirus leaves host cell, acquiring an envelope as it buds out. Reverse transcriptase 2 Virion penetrates cell and its DNA is uncoated Viral RNA Identical strands of RNA 4 Transcription of the provirus may also occur, producing RNA for new retrovirus genomes and RNA that codes for the retrovirus capsid and envelope proteins. Viral proteins RNA 3 The new viral DNA is tranported into the host cell’s nucleus and integrated as a provirus. The provirus may divide indefinitely with the host cell DNA. Provirus

35. Assembly and release of animal viruses • Most DNA viruses assemble in and are released from nucleus into cytosol • Most RNA viruses develop solely in cytoplasm • Enveloped viruses cause persistent infections • Naked viruses are released by exocytosis or may cause lysis and death of host cell

38. Viruses and Cancer • Oncogenes transform normal cells into tumor cells • Activated by mutagenic chemicals, radiation, viruses • Causes increased growth, loss of contact inhibition • Cells tend to be misshapen and exhibit chromosomal abnormalities

39. Oncoviruses • Viral DNA integrated into host DNA • Induces tumors

40. Oncogenic Viruses • Oncogenic DNA Viruses • Adenoviridae • Herpesviridae • Poxviridae • Papovaviridae • Hepadnaviridae • Oncogenic RNA viruses • Retroviridae • DNA • HTLV 1 • HTLV 2

41. Latent Viral Infections • Virus remains dormant in asymptomatic host cell for long periods • Cold sores, shingles • Persistent Viral Infections • Disease progresses slowly over a long period, generally fatal • Subacute sclerosing panencephalitis (measles virus)

43. Plant Viruses • Plant viruses enter through wounds or via biting insects • May be transmitted in pollen • Viroids • infectious naked RNA Potato Spindle Tuber Viroid

45. Prions • Proteinaceous infectious particle • Inherited and transmissible diseases • Spongiform encephalopathies • Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease

46. PrPC, normal cellular prion proteinPrPSc, scrapie protein

47. [INSERT FIGURE 13.23] scrapie proteinsaccumulate in brain cells forming large vacuoles