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

2. Characteristics of Viruses
Miniscule, acellular, infectious agent having either DNA or RNA
Causes many infections of humans, animals, plants, and bacteria
Causes most of the diseases that plague the industrialized world

3. Characteristics of Viruses
Cannot carry out any metabolic pathway
Neither grow nor respond to the environment
Cannot reproduce independently
Recruit the cell’s metabolic pathways to increase their numbers
No cytoplasmic membrane, cytosol, organelles (with one exception)
Have extracellular and intracellular state

4. Characteristics of Viruses
Extracellular State
Called virion
Protein coat (capsid) surrounding nucleic acid
Nucleic acid and capsid also called nucleocapsid
Some have phospholipid envelope
Outermost layer provides protection and recognition sites for host cells
Intracellular State
Capsid removed
Virus exists as nucleic acid

5. Virions, complete virus particles
Figure 13.1

6. Characteristics of Viruses
Genetic Material of Viruses
Show more variety in nature of their genomes than do cells
Primary way scientists categorize and classify viruses
May be DNA or RNA, but never both
Can be dsDNA, ssDNA, dsRNA, ssRNA
May be linear and segmented or single and circular
Much smaller than genomes of cells

7. The relative sizes of genomes
Figure 13.2

8. Characteristics of Viruses
Hosts of Viruses
Most viruses infect only particular host’s cells
Due to affinity of viral surface proteins for complementary proteins on host cell surface
May be so specific they only infect particular kind of cell in a particular host
Generalists – infect many kinds of cells in many different hosts

9. Some examples of hosts of viral infections
Figure 13.3

10. Sizes of selected virions
Figure 13.4

11. Characteristics of Viruses
Capsid Morphology
Capsids
Protein coats that provide protection for viral nucleic acid and means of attachment to host’s cells
Composed of proteinaceous subunits called capsomeres
Capsomere may be made of single or multiple types of proteins

12. The shapes of virions
Figure 13.5

13. The complex shape of bacteriophage T4
Figure 13.6

14. Enveloped viruses
Figure 13.7

15. Characteristics of Viruses
The Viral Envelope
Acquired from host cell during viral replication or release
Envelope is portion of membrane system of host
Composed of phospholipid bilayer and proteins
Some proteins are virally coded glycoproteins (spikes)
Envelope’s proteins and glycoproteins often play role in host recognition

16. Classification of Viruses
Table

17. Classification of Viruses
Table

18. Viral Replication
Dependent on hosts’ organelles and enzymes to produce new virions
Lytic replication
Replication cycle usually results in death and lysis of host cell
Stages of lytic replication cycle
Attachment
Entry
Synthesis
Assembly
Release

19. Animation: Viral Replication: Overview

20. The lytic replication cycle in bacteriophage
Figure 13.8

21. Pattern of virion abundance in lytic cycle
Figure 13.9

22. Animation: Viral Replication: Virulent Bacteriophages

23. Viral Replication Lysogeny Modified replication cycle
Infected host cells grow and reproduce normally for generations before they lyse
Temperate phages
Prophages – inactive phages
Lysogenic conversion results when phages carry genes that alter phenotype of a bacterium

24. Bacteriophage lambda
Figure 13.10

25. The lysogenic replication cycle in bacteriophages
Figure 13.11

26. Animation: Viral Replication: Temperate Bacteriophages

27. Viral Replication Replication of Animal Viruses
Same basic replication pathway as bacteriophages
Differences result from
Presence of envelope around some viruses
Eukaryotic nature of animal cells
Lack of cell wall in animal cells

28. Viral Replication Replication of Animal Viruses
Attachment of animal viruses
Chemical attraction
Animal viruses do not have tails or tail fibers
Have glycoprotein spikes or other attachment molecules that mediate attachment

29. Three mechanisms of entry of animal viruses
Figure 13.12

30. Viral Replication Replication of Animal Viruses
Synthesis of animal viruses
Each type of animal virus requires different strategy depending on its nucleic acid
DNA viruses often enter the nucleus
RNA viruses often replicate in the cytoplasm
Must consider
How mRNA is synthesized
What serves as template for nucleic acid replication

31. Synthesis of proteins and genomes in animal RNA viruses
Figure 13.13

32. Viral Replication Replication of Animal Viruses
Assembly and release of animal viruses
Most DNA viruses assemble in nucleus
Most RNA viruses develop solely in cytoplasm
Number of viruses produced depends on type of virus and size and initial health of host cell
Enveloped viruses cause persistent infections
Naked viruses are released by exocytosis or lysis

33. The process of budding in enveloped viruses
Figure 13.14

34. Pattern of virion abundance in persistent infections
Figure 13.15

35. Animation: Viral Replication: Animal Viruses

36. Viral Replication Replication of Animal Viruses
Latency of animal viruses
When animal viruses remain dormant in host cells
May be prolonged for years with no viral activity
Some latent viruses do not become incorporated into host chromosome
Incorporation of provirus into host DNA is permanent

37. The Role of Viruses in Cancer
Animal’s genes dictate that some cells can no longer divide and those that can divide are prevented from unlimited division
Genes for cell division “turned off” or genes inhibiting division “turned on”
Neoplasia
Uncontrolled cell division in multicellular animal; mass of neoplastic cells is tumor
Benign vs. malignant tumors
Metastasis
Cancers

38. The oncogene theory of the induction of cancer in humans
Figure 13.16

39. The Role of Viruses in Cancer
Environmental factors that contribute to the activation of oncogenes
Ultraviolet light
Radiation
Carcinogens
Viruses

40. The Role of Viruses in Cancer
Viruses cause 20–25% of human cancers
Some carry copies of oncogenes as part of their genomes
Some promote oncogenes already present in host
Some interfere with tumor repression when inserted into host’s repressor gene
Specific viruses are known to cause ~15% of human cancers
Burkitt’s lymphoma
Hodgkin’s disease
Kaposi’s sarcoma
Cervical cancer

41. Culturing Viruses in the Laboratory
Culturing Viruses in Mature Organisms
In bacteria
In plants and animals
Culturing Viruses in Embryonated Chicken Eggs
Inexpensive, among the largest of cells, free of contaminating microbes, and contain a nourishing yolk
Culturing Viruses in Cell (Tissue) Culture

42. Viral plaques in a lawn of bacterial growth on an agar plate
Figure 13.17

43. Inoculation sites for the culture of viruses in eggs
Figure 13.18

44. Culturing Viruses in the Laboratory
Culturing Viruses in Cell (Tissue) Culture
Consists of cells isolated from an organism and grown on a medium or in a broth
Two types of cell cultures
Diploid cell cultures
Continuous cell cultures

45. An example of cell culture
Figure 13.19

46. Are Viruses Alive?
Some consider them complex pathogenic chemicals that lack the characteristics of life
Others consider them to be the least complex living entities because they
Use sophisticated methods to invade cells
Have the ability to take control of their host cell
Are able to replicate themselves

47. Other Parasitic Particles: Viroids and Prions
Characteristics of Viroids
Extremely small, circular pieces of RNA that are infectious and pathogenic in plants
Similar to RNA viruses, but lack capsid
May appear linear due to H bonding

48. The RNA strand of the small potato spindle tuber viroid
Figure 13.20

49. One effect of viroids on plants
Figure 13.21

50. Other Parasitic Particles: Viroids and Prions
Characteristics of Prions
Proteinaceous infectious agents
Cellular PrP protein
Made by all mammals
Normal structure with -helices called cellular PrP
Prion PrP
Disease-causing form with -sheets called prion PrP
Prion PrP converts cellular PrP into prion PrP by inducing conformational change

51. Other Parasitic Particles: Viroids and Prions
Animation: Prions: Overview

52. The two stable forms of prion protein (PrP)
Figure 13.22

53. Other Parasitic Particles: Viroids and Prions
Characteristics of Prions
Normally, nearby proteins and polysaccharides force PrP into cellular shape
Excess PrP or PrP mutations result in formation of prion PrP
Cause newly synthesized cellular PrP to refold into prion PrP

54. Other Parasitic Particles: Viroids and Prions
Animation: Prions: Characteristics

55. Other Parasitic Particles: Viroids and Prions
Characteristics of Prions
Prion diseases
Fatal neurological degeneration, fibril deposits in brain, and loss of brain matter
Large vacuoles form in brain
Characteristic spongy appearance
Spongiform encephalopathies – BSE, vCJD, kuru
Prions only destroyed by incineration or autoclaving in 1 N NaOH

56. Brain with appearance typical in prion-induced diseases
Figure 13.23

57. Other Parasitic Particles: Viroids and Prions
Animation: Prions: Disease