1. Characterizing and Classifying Viruses, Viroids, and Prions13
Characterizing and Classifying Viruses, Viroids, and Prions 1

2. Characteristics of Viruses
Minuscule, acellular, infectious agent having either DNA or RNA
Cause infections of humans, animals, plants, and bacteria
Cause most of the diseases that plague the industrialized world
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
Have extracellular and intracellular state

3. 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

4. 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

6. 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

7. Characteristics of Viruses - Capsid Morphology
Capsids
Provide protection for viral nucleic acid
Means of attachment to host's cells
Composed of proteinaceous subunits called capsomeres
Capsomere may be made of single or multiple types of proteins

8. Figure 13.6 The complex shape of bacteriophage T4.
Head
Tail fibers
Tail
Base plate

9. 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 proteins and glycoproteins often play role in host recognition

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

14. Viral Replication: Overview
PLAY
Viral Replication: Overview

15. Figure 13.8 The lytic replication cycle in bacteriophages.

16. Viral Replication: Virulent Bacteriophages
PLAY
Viral Replication: Virulent Bacteriophages

17. 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

19. Viral Replication: Temperate Bacteriophages
PLAY
Viral Replication: Temperate Bacteriophages 19

20. 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

21. Viral Replication Replication of Animal Viruses
Attachment of animal viruses
Chemical attraction between viral protein and cell receptor
Animal viruses do not have tails or tail fibers
Have glycoprotein spikes or other attachment molecules that mediate attachment

22. Viral Replication Replication of Animal Viruses
Synthesis of DNA viruses of animals
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

23. Viral Replication Replication of Animal Viruses dsDNA viruses
Synthesis of DNA viruses of animals
dsDNA viruses
Similar to replication of cellular DNA
Viral genome replicated in the nucleus
Viral proteins are made in the cytoplasm
Some exceptions
Poxvirus replication occurs in the cytoplasm
Hepatitis B viruses replicate DNA from an RNA intermediary

24. Viral Replication Replication of Animal Viruses ssDNA viruses
Synthesis of DNA viruses of animals
ssDNA viruses
Cells do not use ssDNA
Parvoviruses have ssDNA genomes
Host enzymes produce DNA strand complementary to viral genome to form dsDNA molecule
dsDNA used for viral replication and transcription

25. Figure 13.13 Synthesis of proteins and genomes in animal RNA viruses.
Receptors on
cytoplasmic
membrane of host
–ssRNA
virus
dsRNA
virus
+ssRNA
virus
+ssRNA
–ssRNA
dsRNA
Transcription
by viral RNA
polymerase
Transcription by
RNA-dependent
RNA transcriptase
Unwinding
–ssRNA
+ssRNA
acts as
mRNA
Translation
of viral
proteins,
genome acts
as mRNA
Complementary
+ssRNA to act
as template
and as mRNA
Transcription
by viral RNA
polymerase
to make
complementary
RNA strands
Complementary –ssRNA
to act as template
Further
transcription
Further
transcription
Translation
of viral
proteins
Copies of
–ssRNA
Translation
of viral
proteins
Copies of
+ssRNA
Assembly
Assembly
Assembly
Positive-sense ssRNA virus
Negative-sense ssRNA virus
Double-stranded RNA virus

26. Viral Replication Replication of Animal Viruses
Synthesis of RNA viruses of animals
Retroviruses
Do not use their genomes as mRNA
Use DNA intermediary transcribed by viral reverse transcriptase as template to produce viral genomes

27. 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

29. Viral Replication: Animal Viruses
PLAY
Viral Replication: Animal Viruses

30. Viral Replication Replication of Animal Viruses
Latency of animal viruses
When animal viruses remain dormant in host cells
Viruses are called latent viruses or proviruses
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

32. The Role of Viruses in Cancer
Cell division is under strict genetic control
Genes dictate that some cells can no longer divide at all
Cells 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
Malignant tumors also called cancers
Metastasis occurs when tumors spread

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

34. 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
Specific viruses are known to cause ~15% of human cancers
Burkitt's lymphoma
Hodgkin's disease
Kaposi's sarcoma
Cervical cancer

35. Culturing Viruses in the Laboratory
Viruses cannot grow in standard microbiological media
Cultured inside host cells
Three types of media for culturing viruses
Media consisting of mature organisms
Embryonated eggs
Cell cultures

36. Culturing Viruses in the Laboratory
Culturing Viruses in Mature Organisms
Culturing viruses in bacteria
Phages grown in bacteria in liquid cultures or on agar plates
Lysis of bacteria produces plaques
Allows estimation of phage numbers by plaque assay

37. Culturing Viruses in the Laboratory
Culturing Viruses in Mature Organisms
Culturing viruses in plants and animals
Numerous plants and animals have been used to culture viruses
Laboratory animals can be difficult and expensive to maintain
Ethical concerns

38. Culturing Viruses in the Laboratory
Culturing Viruses in Embryonated Chicken Eggs
Inexpensive
Among the largest of cells
Free of contaminating microbes
Contain a nourishing yolk
Fertilized chicken eggs are often used
Embryonic tissues provide ideal site for growing viruses
Some vaccines prepared in chicken cultures

39. Culturing Viruses in the Laboratory
Culturing Viruses in Cell (Tissue) Culture
Cells isolated from an organism and grown on a medium or in a broth
Cell cultures sometimes inaccurately called tissue cultures
Two types of cell cultures
Diploid cell cultures
Continuous cell cultures

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

41. 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 hydrogen bonding
Figure One effect of viroids on plants.

42. Other Parasitic Particles: Viroids and Prions
Characteristics of Prions
Proteinaceous infectious agents
Cellular PrP
Made by all mammals
Normal, functional structure has -helices
Prion PrP
Disease-causing form has -sheets
Prion PrP causes cellular PrP to refold into prion PrP

43. Prions: Overview
PLAY
Prions: Overview

44. -helices -pleated sheet
Figure The two stable, three-dimensional forms of prion protein (PrP).
-helices
-pleated sheet
Cellular PrP
Prion PrP

45. Prions: Characteristics
PLAY
Prions: Characteristics

46. Other Parasitic Particles: Viroids and Prions
Characteristics of Prions
Prion diseases
Spongiform encephalopathies
Large vacuoles form in brain
Characteristic spongy appearance
BSE, vCJD, kuru
Transmitted by ingestion, transplantation, or contact of mucous membranes with infected tissues
Prions destroyed by incineration or autoclaving in concentrated sodium hydroxide
Figure A brain showing the large vacuoles and spongy appearance typical in prion-induced diseases.

47. Prions: Diseases
PLAY
Prions: Diseases