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

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. 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

40. 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

41. 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

42. 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

43. 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

44. HIV replication

45. Viruses associated with cancers

46. Viruses and tumors
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

47. 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

48. Influenza virus

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

50. Antigenic shift vs antigenic drift

51. Ways to study viruses
Since viruses grow in living cells….need a live cell to culture them
Cell culture/tissue culture
Embryonated chicken eggs

52. Cell Culture

53. Proteinaceous infectious particles: PRIONS
1982 Stanley Prusiner proposed that there were infectious proteins
Caused the disease “scrapie” in sheep
Caused the “mad-cow”disease in 1987
Human forms suggest a genetic component

54. Prions Contain no nucleic acid
Abnormal protein promotes conformational change to normal protein
Results in damage to neurons…transmissible spongiform encephalopahthies

55. Brain with spongiform encephalopathy

56. Infections caused by prions

57. Mechanism of prion replication

58. Rhabdoviridae (-)ssRNA, enveloped
Rabies virus
-enters the skin and multiplies in skeletal muscle and connective tissue
-virus travels along nerves to the CNS causing encephalitis

59. Pathology of rabies