Viruses

1. Viruses Not Composed of Cells

2. Characteristics • Obligate intracellular parasites • Single type of nucleic acid • Protein coat • Envelope

3. Two Forms • Extracellular form • Intracellular form

4. Host Range • Can infect many hosts • Determined by attachment to host cell • cell wall, flagella/fimbriae • Receptor sites, or plasma membrane in animal cells

5. Structure • Nucleic acid-core • Either DNA or RNA, not both • Follows central dogma of molecular biology • Genetic info flows from NA to protein

6. Structure • Protein coat-capsid • Envelope in some virions • May or may not have spikes-glycoproteins

7. Envelope • Acquire membrane when budding or pass through membranes • Advantages of lipid membrane • Lose infectivity when envelope destroyed

8. Envelope • Host’s phospholipids & viral proteins • Disadvantage -damaged easily • Without envelope- naked viruses • More resistant to chemicals/ disinfectants

9. Enzymes • Required early in infection process • Bacterial virus or bacteriophage • lysozyme • Lysis of cell and release of virions

10. Enzymes • Some have own NA polymerase • RNA polymerase in some RNA virions • Reverse transcriptase in retro viruses • RNA dependent DNA polymerase • Neuramidases-release of virions

11. Morphology • Helical • Polyhedral- shape of icosohedron-20 sides • Enveloped- usually spherical

12. Morphology • Complex viruses • combination of helical and icosohedral • bacterial viruses-head and tail • poxviruses- several coats of protein

13. Growth of Bacteriophage • Grow in suspensions of bacteria or in bacteria cultures on plate • Plaque method for counting

14. Growth of Animal Viruses • Living animals • Embryonated eggs-influenza • Cell cultures-continuous lines

15. CPE Cytopathic Effect • Visible effect of viruses on cells • Stop multiplication of cells • Lysosomes release enzymes • Inclusion bodies • Syncytium- • Interferons-

16. CPE • Mark infected cells for destruction by immune system • Transformation-abnormal cells

17. Multiplication of Bacteriophage • Lytic cycle: produces virions • T-even phages, virulent phages, on E. coli • ds DNA for over 100 genes-head •   tail sheath-retracts • DNA moves from head into host

18. Lytic Cycle • Attachment stage (adsorption) • attachment site on virus with complementary receptor on bacteria cell wall • use fibers at end of tail as attachment sites • may attach to flagella or fimbriae

19. Penetration • Injects DNA • Tail releases enzyme lysozyme • Tail core driven through cell wall • Tail reaches cell membrane • Capsid remains outside: uncoating

20. Biosynthesis • In cytoplasm • Host protein synthesis is stopped • Uses host nucleotides and enzymes to synthesis copies of phage DNA

21. Biosynthesis • Synthesis of phage capsid proteins • Uses host ribosomes and amino acids for translation

22. Maturation • Assemble into mature phages • Head assembled and packed with DNA • Phage tails assembled from plates, sheaths, • Each head attached to tail • Then fibers are attached

23. Release • Lysis of PM, cell breaks open • Virulent (lytic) phages

24. One Step Growth Curve • One step growth curve • Always present are mutant bacteria with altered receptors

25. Lysogenic Cycle • Temperate phages do not always under go lytic cycle: • Lysogeny- • Phage NA incorporated into the host NA • Lambda phage in E. coli • Integrates into bacterial chromosome • Prophage

26. Lysogenic Cycle • Prophage replicated along with host DNA • On rare event- can lead to popping out of phage DNA

27. Lysogenic Conversion • Alteration of characteristics of bacteria • Cells are immune to reinfection by same phage •  Not immune to infection by different phage

28. Phage Conversion • Medical significance of conversion • C. diphtheriae and C. botulinum • Without prophage do not cause disease • Strep with prophage can cause scarlet fever

29. Specialized Transduction • ·  Mediated by lysogenic phage ( only temperate virions) • · DNA on either side of prophage can be picked up • ·  Phage lambda picks up gene for galactose fermentation-gal from host • · Carry this gene to new host which is gal negative

30. Multiplication of Animal Viruses • Attachment • receptor sites on animal cells - proteins and glycoproteins of PM • sites are distributed all over surface of virus • Spikes or capsid

31. Penetration • Trigger endocytosis-folding inward of PM-vesicle • Enveloped viruses also can fuse with PM • Fusion protein facilitates this • Releases capsid via endocytosis

32. Uncoating-Removal of Capsid • Varies with virus • Separation of NA and protein coat • lysosomal enzymes inside vesicles • Some enzymes in host cytoplasm

33. Biosynthesis of DNA viruses • DNA viruses replicate DNA in nucleus of host • Synthesize proteins in cytoplasm • Early transcription- for enzymes & proteins needed for viral DNA replication • Late transcription-capsid & structural proteins

34. Maturation • Assembly of virus • Takes place in nucleus • Proteins transported via ER into nucleus • Released from host cell • Budding • Lysis

35. Biosynthesis of RNA Viruses • Multiply in host cell’s cytoplasm • Picornavirus-polio, ss RNA, • RNA is a sense strand or positive since it acts as mRNA • Early translation-2 proteins • Inhibits host cell synthesis of RNA & protein • Produces RNA-dependent RNA polymerase

36. Biosynthesis of SS RNA Virus • · Synthesizes another strand of RNA-antisense strand or negative strand • Serves as template for all + strands • Late transcription and translation -proteins for capsids

37. Rhabdoviruses • Rabies, bullet shaped • Contains a single minus strand and RNA dependent RNA polymerase • makes + strands from minus strand • + strand serves as mRNA for new viral RNA and for proteins

38. Maturation and Release • Assemble capsids spontaneously • Enveloped viruses • proteins in envelope encoded by viral genes • envelope wraps around capsid -budding • Lipids and CH2O encoded by host cell • Noneveloped viruses released via rupture

39. Retrovirus • HIV • Positive strand RNA virus • Own RNA polymerase • RNA dependent DNA polymerase • Reverse transcriptase • RNA to DNA

40. Provirus • Viral DNA incorporated into host DNA –provirus • Never comes out of host chromosome • Protected from host’s immune system and antiviral drugs

41. Retrovirus Replication • Provirus may remain in latent state replicating with host DNA OR • Provirus may be expressed and produces new viruses

42. Consequences of Virus Infection • Lytic infection: destruction of host cell • Acute infection-influenza • Persistent infection : slow release of virions • Budding without lyzing cell

43. Consequences of Virus Infection • Latent infections: delay between infection and lytic events • Transformation: change in cell

44. Herpes Viruses • Large, enveloped, latent • Herpes type 1 and 2 • Cold sores, genital and neonatal herpes • Varicella zoster- chickenpox, and shingles • EBV-mononucleosis

45. Herpes Viruses • CMV • Salivary gland virus-acute febrile illness, birth defects • Roseolovirus (6) • Infants with rash and fever • HHV-7 rashes in infants • HHV-8 Kaposi’s sarcoma

46. Transformation • Normal cells become tumor cells • Benign and malignant • Oncogene-cancer gene

47. Oncogenes • Genes always turned on • Continuous cell division • Activated to abnormal functioning by chemicals, radiation and viruses • Loss of control of cell cycle • Result in formation of tumors

48. Oncogenic viruses • 10- 20% of cancers known to be virus induced • Oncogenic viruses incorporate into host DNA • Cells lack contact inhibition

49. DNA Oncogenic Viruses • EBV- herpes virus, causes 2 human cancers • Burkitt’s lymphoma ( rare affecting children in Africa) • Nasopharyngeal cancer is worldwide • 90% of population carry latent stage of EBV in lymphocytes • Hepatitis B virus has casual role in liver cancer • Papilloma virus- can cause cervical & penile cancer-vaccine

50. RNA Viruses • Human T cell leukemia viruses