Chapter 13-Viruses. Viroids, and Prions

1. Chapter 13-Viruses. Viroids, and Prions

2. General Characteristics of all viruses • Contain a single type of nucleic acid • Contain a protein coat • Obligate intracellular parasites • Are viruses the only known obligate intracellular parasites?

3. History began with the Tobacco Mosaic Virus (TMV) • 1886 Aldolf Mayer showed that a virus was transmissable between plants • 1892 Iwanowski tried to isolate it by filtering with porcelain filter

4. Sizes of viruses

5. Common Shapes • Capsid coat made of capsomeres • Nucleic acid inside

6. Two different types of Viruses

7. Relationship of virus with host cell

8. Bacterial viruses • Known as bacteriophages or phages • Two different life cycles • Lytic cycle (lytic or virulent phage)-results in lysis of the cell • Lysogenic cycle (temperate or lysogenic phage)-may result in lysis of the cell or becomes a permanent part of the chromosome by integrating

9. T4 phage replication

10. Lambda Phage replication

12. Lambda integrates into the chromosome

13. Properties conferred by prophage

14. Some phage are filamentous

15. Replication of filamentous phage

16. M13 is ssDNA…how does it replicate the ssDNA?

17. How do bacteria protect themselves against phage? • Prevent phage attachment • Attacking foreign DNA with restriction enzymes, protecting native DNA with methylation • CRISPR system degrades incoming viral nucleic acid

18. CRISPR defense system against phage

19. Methods to study bacteriophage • Plaque Assay used to quantitate phage

20. How do animal viruses differ from bacterial viruses? • Attachment or entry into the cell • Replication of viral nucleic acid (remember eukaryotic cells have a nucleus) • Uncoating step is required by animal viruses • Exit the host cell by budding or shedding

21. Effects of animal virus on cells

22. Entry of animal virus

23. Replication strategies • Watch the type of nucleic acid • What enzymes are needed for the process?

24. Release of enveloped viruses

25. Acute viral infections • Usually short in duration • Host develops long lasting immunity • Infection of the virus results in a productive infection…host cells die as a result of infection

26. General Steps of Acute Viral infection • Attachment • Entry into host cell • Targeting where it will reproduce • Uncoating of the capsid • Synthesis of proteins, replication of nucleic acid • Maturation • Cell lysis

27. Can you identify some examples of viruses that produce an acute viral infection?

28. Persistent infections • Virus is continually present in the body, released by budding • Three categories • Latent infections • Chronic infections • Slow infections

29. Persistent: Latent Infections • Persistent infection with symptomless period followed by reactivation of virus and symptoms • Example of latent viruses are found in the family Herpesviridae • Herpes simplex virus -1 • Herpes simplex virus -2

30. All of these viruses are in the Herpesviridae family Latent Viral infections

31. Herpesviridae Family • Double stranded DNA (dsDNA), enveloped viruses -herpes simplex virus type 1(cold sores) -herpes simplex virus type 2 (genital herpes) -Varicella-zoster virus (chicken pox, shingles) -Epstein-Barr (infectious mono and Burkitt’s lymphoma)

32. Herpes Simplex virus-1

33. HSV-1 reactivation

34. Herpes simplex-1 • HSV-1 causes fever blisters, HSV-2 genital herpes • Symptoms: fluid filled skin lesions • Treatment: Acyclovir

35. Varicella (chickenpox) and Herpes Zoster (Shingles) • HSV-3 causes chicken pox and latent activation known as shingles • Acquired by respiratory route, 2 weeks later see vesicles on skin • Vaccine established in 1995 for chickenpox

36. Epstein Barr • Causes infectious mononucleosis • Acquire by saliva, incubation period is 4-7 weeks • Identify by -lobed lymphocytes -heterophile antibodies -fluorescent antibody tests

37. Chronic infections • Infectious virus present at all times • Disease may be present or absent • Examples are Hepatitis Type B and Type C viruses

38. Type Hepadnaviridae family: Hepatitis B • dsDNA virus, enveloped • Hepatitis B -passes through intermediate stage (RNA) for replication -three particles found in blood sample 1. Dane 2. filamentous 3. sphericle -exposure through blood/body fluids

39. Replication of Hepatitis B

40. Hepatitis Type B • Incubation period is ~12 weeks • 10% of cases become chronic, mortality rate is less than 1% • About 40% of the chronic cases die of liver cirrhosis

41. Flaviviridae Family: Hepatitis Type C • Hepatitis C virus • (+) ssRNA virus, enveloped • Obtain from blood/body fluids • Incubation period averages 6 weeks • Hard to screen blood for the virus • 85% of all cases become chronic

42. What other types of Hepatitis viruses are known to infect humans? • Hepatitis Type A • Found in the Picornaviridae family (+) ssRNA -obtain through fecal-oral route, enters GI tract and multiplies -incubation period is ~4 weeks -symptoms include: anorexia, malaise, nausea, diarrhea, abdominal discomfort, fever, and chills lasting 2-21 days

43. Slow Infections • Infectious agent increases in amount over a long time during which there are no symptoms • Examples are HIV found in the Retroviridae family • Retroviruses use reverse transcriptase to replicate ssRNA

44. Retroviridae-multiple strands of (-)RNA • HIV -infects Helper T cells -requires the enzyme reverse transcriptase -integrates as a provirus -is released by budding, or lyses the cell

45. HIV replication

46. Viruses associated with cancers

47. dsDNA viruses are most common to cause viral-induced tumors Cancer is result of integration of viral genes into the host chromosome Transforming genes are called oncogenes Examples: papillomavirus, herpesvirus Viruses and tumors

48. Orthomyxoviridae-multiple strands of (-)RNA • Influenza virus • Consists of 8 segments of RNA • Envelope has H spikes (hemagglutinin) and N spikes (neuraminidase) • Incubation is 1-3 days • Symptoms include: chills, fever, headache, muscle aches, may lead to cold-like symptoms

49. Influenza virus

50. If multiple forms infect one cell…reassortment can occur