1. Viruses, Viroids, and Prions
Chapter 13
Viruses, Viroids, and Prions

2. Differentiate between a virus and a bacterium.
Viruses may be regarded as exceptionally complex aggregations of nonliving chemicals OR exceptionally simple living microbes.

3. Viruses Viruses contain DNA or RNA And a protein coat
Some are enclosed by an envelope (lipids, proteins, and carbohydrates)
Some viruses have spikes
Most viruses infect only specific types of cells in one host
Host range is determined by specific host attachment sites and cellular factors
Obligatory intracellular parasites, causing synthesis of specialized elements that transfer viral nucleic acid to other cells.

4. Viruses (range from 20 to 1000 nm)
nm = 10-9 m
Figure 13.1

5. Nonenveloped Polyhedral Viruses
Describe the chemical composition and physical structure of an enveloped and a nonenveloped virus.
Virion = complete, fully developed viral particle of nucleic acid surrounded by a coat
Figure 13.2a, b

6. Enveloped Helical Virus
Viruses contain either DNA or RNA, but never both. Nucleic acid may be single or double stranded, linear or circular, or divided into several separate molecules.

7. Helical Viruses Helical viruses look like long or coiled threads.
Their capsids are hollow cylinders surrounding the DNA/RNA.
Figure 13.4a, b

8. Complex Viruses Capsid – protein coat surrounding nucleic acid
Composed of capsomeres, single or multiple proteins
Figure 13.5a

9. Viral Taxonomy
Define viral species.
Classification based on type of nucleic acid, replication, and morphology.
Family names end in -viridae
Genus names end in -virus
Viral species: A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for species
Subspecies are designated by a number

10. Viral Taxonomy Herpesviridae Retroviridae Herpesvirus Lentivirus
Give an example of a family, genus, and common name for a virus.
Herpesviridae
Herpesvirus
Human herpes virus 1, HHV 2, HHV 3
Retroviridae
Lentivirus
Human Immunodeficiency Virus 1, HIV 2

15. Growing Viruses Viruses must be grown in living cells.
Describe how bacteriophages are cultured.
Viruses must be grown in living cells.
Bacteriophages form plaques (clearings) on a lawn of bacteria.
Easiest to grow
Figure 13.6

16. Growing Viruses Viruses must be grown in living host cells.
Describe how animal viruses are cultured.
Viruses must be grown in living host cells.
Animal viruses may be grown in living animals or in embryonated eggs.
Figure 13.7

17. Growing Viruses
Animal and plants viruses may be grown in cell culture.
Continuous cell lines may be maintained indefinitely.
Cytopathic effects due to viral growth
Figure 13.8

18. Cytopathic effect of viruses
Uninfected mouse cells form monolayer (left). Infected cells 24 hours later pile up and round up (right).
Figure 13.9

19. Virus Identification Serological tests
List three techniques that are used to identify viruses.
Serological tests
Detect antibodies against viruses in a patient
Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot
Nucleic acids
RFLPs – restriction fragment length polymorphisms
PCR – polymerase chain reaction (used to identify West Nile virus in U.S. in 1999)

20. Multiplication of Bacteriophages (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 phage particles
Release Phage lysozyme breaks cell wall

21. Lytic cycle of T-even bacteriophage
Bacterial cell wall
Bacterial chromosome
Capsid
DNA
Capsid
Sheath
Tail fiber 1
Attachment: Phage attaches to host cell.
Tail
Base plate
Pin
Cell wall
Plasma membrane 2
Penetration: Phage penetrates host cell and injects its DNA.
Sheath contracted
Tail core 3
Merozoites released into bloodstream from liver may infect new red blood cells
Describe the lytic cycle of T-even bacteriophages.
Figure

22. Lytic cycle of T-even bacteriophage
Burst time is generally about 20 – 40 minutes after phage absorption. Burst size ranges from 50 to 200 new phage cells.
Tail
DNA 4
Maturation: Viral components are assembled into virions.
Capsid 5
Release: Host cell lyses and new virions are released.
Tail fibers
Figure

23. One-step Growth Curve for bacteriophage
During biosynthesis and maturation, separate components of DNA and protein may be detected in the host cell.
Figure 13.11

24. Lytic cycle Phage causes lysis and death of host cell
Lysogenic cycle Prophage DNA incorporated in host DNA

25. The Lysogenic Cycle – bacteriophage lambda in E.coli
Describe the lysogenic cycle of bacteriophage lambda.
Figure 13.12

26. Specialized Transduction
gal gene
Prophage
Bacterial DNA 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
Figure 13.13

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, Penetration, and Uncoating
Entry of herpes simplex virus into an animal cell.
Figure 13.14

30. Compare and contrast the multiplication cycle of DNA- and RNA-containing animal viruses.

31. Multiplication of Papovarius, a DNA-containing Virus
Papovavirus 1
Virion attaches to host cell 7
Virions are released
Host cell
DNA
Capsid
DNA 2
Virion penetrates cell and its DNA is uncoated 6
Virions mature
Cytoplasm
Capsid proteins
mRNA 5
Late translation; capsid proteins are synthesized 3
Early transcription and translation; enzymes are synthesized 4
Late transcription; DNA is replicated
Figure 13.15

32. DNA-containing animal viruses: individual capsomeres visible

33. DNA-containing animal viruses: envelop around this herpes simplex virus broken (fried egg appearance)

34. Pathways of Multiplication for RNA-Containing Viruses
Figure 13.17

35. RNA-containing animal viruses: rubella (left), mouse mammary tumor virus (right).

36. Multiplication & Inheritance in a Retrovirus
Reverse transcriptase
Capsid
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
Viral RNA
Virion penetrates cell and its DNA is uncoated
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 transported into the host cell’s nucleus and integrated as a provirus. The provirus may divide indefinitely with the host cell DNA.
Provirus
Figure 13.19

37. Release of an enveloped virus by budding
Most enveloped viruses take part of host’s plasma membrane for their envelope.
Figure 13.20

38. Cancer
Define oncogene and transformed cell.
Activated oncogenes transform normal cells into cancerous cells. (malignant transformation)
Transformed cells have increased growth, loss of contact inhibition, tumor specific transplant and T antigens, chromosome abnormalities, can produce tumors when injected into susceptible animals.
Several DNA viruses and retroviruses are oncogenic.
The genetic material of oncogenic viruses becomes integrated into the host cell's DNA.

39. Oncogenic Viruses Oncogenic DNA Viruses Adenoviridae Heresviridae
Discuss the relationship of DNA- and RNA-containing viruses to cancer.
Oncogenic DNA Viruses
Adenoviridae
Heresviridae
Poxviridae
Papovaviridae
Hepadnaviridae
Oncogenic RNA viruses
Retroviridae
Viral RNA is transcribed to DNA which can integrate into host DNA
HTLV 1
HTLV 2
Retroviruses carry reverse transcriptase which allows RNA to DNA, permitting oncogenic properties

40. Latent Viral Infections
Provide an example of a latent viral infection.
Latent Viral Infections
Virus remains in asymptomatic host cell for long periods
Cold sores, shingles
Persistent Viral Infections
Disease processes occurs over a long period, generally fatal
Subacute sclerosing panencephalitis (measles virus)

41. Differentiate between persistant viral infections and latent viral infections.
Persistent viral infections are caused by conventional viruses, occur over a long period, generally fatal.

42. Prions Infectious proteins first discovered in 1980’s
Discuss how a protein can be infectious.
Infectious proteins first discovered in 1980’s
Inherited and transmissible by ingestion, transplant, & surgical instruments
Spongiform encephalopathies: Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease
PrPC, normal cellular prion protein, on cell surface
PrPSc, scrapie protein, accumulate in brain cells forming plaques

43. Prions
How a protein can be infectious: if an abnormal prion protein enters cell, it changes a normal prion to PrPSc, which changes another normal PrP (accumulation of abnormal PrPSc)
PrPSc
PrPc 1 2 3 4
Lysosome
Endosome 5 6 7 8
Figure 13.21

44. Some Plant Viruses Name a virus that causes a plant disease.
Table 13.6

45. Linear and circular potato spindle tuber viroid
Differentiate between virus, viroid, and prion.
Plant Viruses
Plant viruses enter through wounds or via insects
Viroids
Viroids are infectious RNA; potato spindle tuber disease
Prion = infectious protein
Figure 13.22

46. Virus Families Single-stranded DNA, nonenveloped viruses Parvoviridae
Human parvovirus
Fifth disease
Anemia in immunocompromised patients

47. Double-stranded DNA, nonenveloped viruses
Mastadenovirus
Respiratory infections in humans
Tumors in animals

48. Double-stranded DNA, nonenveloped viruses
Papillomavirus (human wart virus)
Polyomavirus
Cause tumors, some cause cancer

49. Double-stranded DNA, nonenveloped viruses
Orthopoxvirus (vaccinia and smallpox viruses)
Molluscipoxvirus
Smallpox, molluscum contagiosum, cowpox

50. Double-stranded DNA, nonenveloped viruses
Simplexvirus (HHV1 and HHV 2)
Varicellavirus (HHV 3)
Lymphocryptovirus (HHV 4)
Cytomegalovirus (HHV 5)
Roseolovirus (HHV 6)
HHV 7
Kaposi's sarcoma (HHV 8)
Some herpesviruses can remain latent in host cells

51. Double-stranded DNA, nonenveloped viruses
Hepadnavirus (Hepatitis B virus)
Use reverse transcriptase to produce DNA from mRNA

52. Single-stranded RNA, + strand, nonenveloped
Enterovirus
Enteroviruses include poliovirus and coxsackievirus
Rhinovirus
Hepatitis A virus

53. Single-stranded RNA, + strand, nonenveloped
Hepatitis E virus
Norovirus (Norwalk agent) causes gastroenteritis

54. Single-stranded RNA, + strand, nonenveloped
Alphavirus
Alphaviruses are transmitted by arthropods; include EEE, WEE
Rubivirus (rubella virus)

55. Single-stranded RNA, + strand, nonenveloped
Arboviruses can replicate in arthropods; include yellow fever, dengue, SLE, and West Nile viruses
Hepatitis C virus

56. Single-stranded RNA, + strand, nonenveloped
Coronavirus
Upper respiratory infections

57. Single-stranded RNA, – strand, one RNA strand
Vesiculovirus
Lyssavirus (rabies virus)
Cause numerous animal diseases

58. Single-stranded RNA, – strand, one RNA strand
Filovirus
Enveloped, helical viruses
Ebola and Marburg viruses

59. Single-stranded RNA, – strand, one RNA strand
Paramyxovirus
Morbillivirus
Paramyxovirus causes parainfluenza, mumps and Newcastle disease

60. Single-stranded RNA, – strand, one RNA strand
Hepatitis D virus
Depends on coinfection with Hepadnavirus

61. Single-stranded RNA, – strand, multiple RNA strands
Influenzavirus (Influenza viruses A and B)
Influenza C virus
Envelope spikes can agglutinate RBCs

62. Single-stranded RNA, – strand, multiple RNA strands
Bunyavirus (CE virus)
Hantavirus

63. Single-stranded RNA, – strand, multiple RNA strands
Arenavirus
Helical capsids contain RNA-containing granules
Lymphocytic choriomeningitis
VEE and Lassa Fever

64. Single-stranded RNA, two RNA strands, produce DNA
Lentivirus (HIV)
Oncogenic viruses
Use reverse transcriptase to produce DNA from viral genome
Includes all RNA tumor viruses

65. Double-stranded RNA, nonenveloped
Reovirus (Respiratory Enteric Orphan)
Rotavirus
Mild respiratory infections and gastroenteritis
Colorado tick fever