1. Chapter 6 An Introduction to Viruses

2. The Search for the Elusive Virus
Louis Pasteur postulated that rabies was caused by a virus (1884)
Ivanovski and Beijerinck showed a disease in tobacco was caused by a virus (1890s)
1950s virology was a multifaceted discipline
Viruses: noncellular particles with a definite size, shape, and chemical composition

3. The Position of Viruses in the Biological Spectrum
There is no universal agreement on how and when viruses originated
Viruses are considered the most abundant microbes on earth
Viruses played a role in the evolution of Bacteria, Archaea, and Eukarya
Viruses are obligate intracellular parasites

4. General Size of Viruses
Size range – most <0.2 μm; requires electron microscope

5. Viral Structure Viruses bear no resemblance to cells
Lack protein-synthesizing machinery
Viruses contain only the parts needed to invade and control a host cell

6. General Structure of Viruses
Capsids
All viruses have capsids (protein coats that enclose and protect their nucleic acid)
The capsid together with the nucleic acid is the nucleocapsid
Some viruses have an external covering called an envelope; those lacking an envelope are naked
Each capsid is made of identical protein subunits called capsomers

7. General Structure of Viruses
Two structural capsid types:
Helical - continuous helix of capsomers forming a cylindrical nucleocapsid
Icosahedral

8. General Structure of Viruses
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Two structural capsid types:
Helical -
Icosahedral sided with 12 corners
(a) Capsomers
Facet
Capsomers
Vertex
Nucleic
acid
(b)
Capsomers
Vertex
Fiber
(c) 8
(d)
© Dr. Linda Stannard, UCT/Photo Researchers, Inc. 8

9. General Structure of Viruses
Viral envelope
Mostly animal viruses
Acquired when the virus leaves the host cell
Exposed proteins on the outside of the envelope, called spikes, are essential for attachment of the virus to the host cell

10. Functions of Capsid/Envelope
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Protects the nucleic acid when the virus is outside of the host cell
Helps the virus bind to a cell surface and assists the penetration of the viral DNA or RNA into a suitable host cell
Capsomers
© Dr. Linda Stannard, UCT/Photo Researchers, Inc.
Fred P. Williams, Jr./EPA
(a)
Envelope
Capsid
DNA core
© Eye of Science/Photo Researchers, Inc.
(b)

11. General Structure of Viruses
Complex viruses: atypical viruses
Poxviruses lack a typical capsid and are covered by a dense layer of lipoproteins
Some bacteriophages have a polyhedral nucleocapsid along with a helical tail and attachment fibers

12. Types of Viruses

13. Concept Check: How would you describe this virus?
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How would you describe this virus?
A. Icosahedral and Naked
Helical and Naked
Complex and Naked
Icosahedral and Enveloped
Helical and Enveloped
Complex and Enveloped

14. Concept Check: How would you describe this virus?
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
How would you describe this virus?
A. Icosahedral and Naked
Helical and Naked
Complex and Naked
Icosahedral and Enveloped
Helical and Enveloped
Complex and Enveloped
© Dennis Kunkel/CNRI/Phototake

15. Nucleic Acids Viral genome – either DNA or RNA but never both
Carries genes necessary to invade host cell and redirect cell’s activity to make new viruses
Number of genes varies for each type of virus – few to hundreds

16. Nucleic Acids DNA viruses RNA viruses
Usually double stranded (ds) but may be single stranded (ss)
Circular or linear
RNA viruses
Usually single stranded, may be double stranded, may be segmented into separate RNA pieces
ssRNA genomes ready for immediate translation are positive-sense RNA
ssRNA genomes that must be converted into proper form are negative-sense RNA

17. General Structure Pre-formed enzymes may be present
Polymerases – DNA or RNA
Replicases – copy RNA
Reverse transcriptase – synthesis of DNA from RNA (AIDS virus)

18. How Viruses Are Classified
Main criteria presently used are structure, chemical composition, and genetic makeup
Currently recognized: 3 orders, 63 families, and 263 genera of viruses
Family name ends in -viridae, i.e.Herpesviridae
Genus name ends in -virus, Simplexvirus
Herpes simplex virus I (HSV-I)

19. Human Viruses & Viral Diseases

20. Human Viruses & Viral Diseases

21. Modes of Viral Multiplication
General phases in animal virus multiplication cycle:
Adsorption – binding of virus to specific molecules on the host cell
Penetration – genome enters the host cell
Uncoating – the viral nucleic acid is released from the capsid
Synthesis – viral components are produced
Assembly – new viral particles are constructed
Release – assembled viruses are released by budding (exocytosis) or cell lysis

22. Animal Virus Multiplication

23. Adsorption and Host Range
Virus coincidentally collides with a susceptible host cell and adsorbs specifically to receptor sites on the membrane
Spectrum of cells a virus can infect – host range
Hepatitis B – human liver cells
Poliovirus – primate intestinal and nerve cells
Rabies – various cells of many mammals
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
Envelope spike
Host cell membrane
Capsid spike
Receptor
Host cell
membrane
Receptor
(a)
(b)

24. Penetration/Uncoating
Flexible cell membrane is penetrated by the whole virus or its nucleic acid by:
Endocytosis – entire virus is engulfed and enclosed in a vacuole or vesicle
Fusion – envelope merges directly with membrane resulting in nucleocapsid’s entry into cytoplasm

25. Variety in Penetration and Uncoating

26. Replication and Protein Production
Varies depending on whether the virus is a DNA or RNA virus
DNA viruses generally are replicated and assembled in the nucleus
RNA viruses generally are replicated and assembled in the cytoplasm
Positive-sense RNA contain the message for translation
Negative-sense RNA must be converted into positive-sense message

27. Release Assembled viruses leave the host cell in one of two ways:
Budding – exocytosis; nucleocapsid binds to membrane which pinches off and sheds the viruses gradually; cell is not immediately destroyed
Lysis – nonenveloped and complex viruses released when cell dies and ruptures
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
(b)
© Chris Bjornberg/Photo Researchers, Inc.
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
Host cell membrane
Viral nucleocapsid
Viral glycoprotein spikes
Cytoplasm
Capsid
RNA
Budding
virion
Free infectious
virion with envelope
Viral matrix
protein
(a)

28. Concept Check: Viruses commonly contain both DNA and RNA A. False
B. True
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29. Concept Check:
Viruses commonly contain both DNA and RNA.
True
False

30. Damage to Host Cell Cytopathic effects - virus-induced damage to cells
Changes in size and shape
Cytoplasmic inclusion bodies
Inclusion bodies
Cells fuse to form multinucleated cells
Cell lysis
Alter DNA
Transform cells into cancerous cells

31. Effects of Some Human Viruses

32. Persistent Infections
Persistent infections - cell harbors the virus and is not immediately lysed
Can last weeks or host’s lifetime; several can periodically reactivate – chronic latent state
Measles virus – may remain hidden in brain cells for many years
Herpes simplex virus – cold sores and genital herpes
Herpes zoster virus – chickenpox and shingles

33. Viral Damage
Some animal viruses enter the host cell and permanently alter its genetic material resulting in cancer – transformation of the cell
Transformed cells have an increased rate of growth, alterations in chromosomes, and the capacity to divide for indefinite time periods resulting in tumors
Mammalian viruses capable of initiating tumors are called oncoviruses
Papillomavirus – cervical cancer
Epstein-Barr virus – Burkitt’s lymphoma

34. Multiplication Cycle in Bacteriophages
Bacteriophages – bacterial viruses (phages)
Most widely studied are those that infect Escherichia coli – complex structure, DNA
Multiplication goes through similar stages as animal viruses
Only the nucleic acid enters the cytoplasm - uncoating is not necessary
Release is a result of cell lysis induced by viral enzymes and accumulation of viruses - lytic cycle

35. Steps in Phage Replication
Adsorption – binding of virus to specific molecules on host cell
Penetration – genome enters host cell
Replication – viral components are produced
Assembly – viral components are assembled
Maturation – completion of viral formation
Lysis & Release – viruses leave the cell to infect other cells 35

36. Multiplication of Bacteriophage
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E. coli host 7
Release of viruses
Bacteriophage
Bacterial
DNA
Viral
DNA
Lysogenic State 1
Adsorption
Viral DNA becomes
latent as prophage. 2
Penetration 6
Lysis of weakened cell
Lytic
Cycle
DNA
splits
Spliced
viral
genome 3
Duplication of phage
components; replication of
virus genetic material 5
Maturation
Viral
DNA
Bacterial
DNA molecule
Capsid
DNA
The lysogenic state in bacteria.
The viral DNA molecule is inserted at
specific sites on the bacterial
chromosome. The viral DNA is
duplicated along with the regular
genome and can provide adaptive
genes for the host bacterium.
Tail +
Tail fibers 4
Assembly of
new virions
Sheath
Bacteriophage
Bacteriophage assembly line.
First the capsomers are synthesized by the host
cell. A strand of viral nucleic acid is inserted
during capsid formation. In final assembly, the
prefabricated components fit together into whole
parts and finally into the finished viruses.

37. Comparison of Bacteriophage and Animal Virus
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Head
Bacterial
cell wall
Tube
Viral nucleic acid
Cytoplasm
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
© K.G. Murti/Visuals Unlimited

38. Concept Check:
Which of the following is a step found in animal virus multiplication but not in bacteriophage replication?
Adsorption
Penetration
Uncoating
Assembly
Release

39. Concept Check:
Which of the following is a step found in animal virus multiplication but not in bacteriophage replication?
Adsorption
Penetration
Uncoating
Assembly
Release

40. Lysogeny: The Silent Virus Infection
Not all phages complete the lytic cycle
Some DNA phages, called temperate phages, undergo adsorption and penetration but don’t replicate
The viral genome inserts into bacterial genome and becomes an inactive prophage – the cell is not lysed
Prophage is retained and copied during normal cell division resulting in the transfer of temperate phage genome to all host cell progeny – lysogeny
Induction can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis

41. Lytic and Lysogenic Lifecycles
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
E. coli host 7
Release of viruses
Bacteriophage
Bacterial
DNA
Viral
DNA
Lysogenic State 1
Adsorption
Viral DNA becomes
latent as prophage. 2
Penetration 6
Lysis of weakened cell
Lytic
Cycle
DNA
splits
Spliced
viral
genome 3
Duplication of phage
components; replication of
virus genetic material 5
Maturation
Viral
DNA
Bacterial
DNA molecule
Capsid
DNA
The lysogenic state in bacteria.
The viral DNA molecule is inserted at
specific sites on the bacterial
chromosome. The viral DNA is
duplicated along with the regular
genome and can provide adaptive
genes for the host bacterium.
Tail +
Tail fibers 4
Assembly of
new virions
Sheath
Bacteriophage
Bacteriophage assembly line.
First the capsomers are synthesized by the host
cell. A strand of viral nucleic acid is inserted
during capsid formation. In final assembly, the
prefabricated components fit together into whole
parts and finally into the finished viruses.

42. Lysogeny
Lysogeny results in the spread of the virus without killing the host cell
Phage genes in the bacterial chromosome can cause the production of toxins or enzymes that cause pathology – lysogenic conversion
Corynebacterium diphtheriae
Vibrio cholerae
Clostridium botulinum

43. Techniques in Cultivating and Identifying Animal Viruses
Obligate intracellular parasites that require appropriate cells to replicate
Methods used:
Cell (tissue) cultures – cultured cells grow in sheets that support viral replication and permit observation for cytopathic effects
Bird embryos – incubating egg is an ideal system; virus is injected through the shell
Live animal inoculation – occasionally used when necessary

44. Methods for Growing Viruses

45. Medical Importance of Viruses
Viruses are the most common cause of acute infections
Several billion viral infections per year
Some viruses have high mortality rates
Possible connection of viruses to chronic afflictions of unknown cause
Viruses are major participants in the earth’s ecosystem

46. Detection and Treatment of Animal Viral Infections
More difficult than other agents
Consider overall clinical picture
Take appropriate sample
Infect cell culture – look for characteristic cytopathic effects
Screen for parts of the virus
Screen for immune response to virus (antibodies)
Antiviral drugs can cause serious side effects

47. Prions and Other Infectious Particles
Prions - misfolded proteins, contain no nucleic acid
Extremely resistant to usual sterilization techniques
Cause transmissible spongiform encephalopathies – fatal neurodegenerative diseases

48. Prions Diseases Common in animals: Scrapie in sheep and goats
Bovine spongiform encephalopathies (BSE), a.k.a. mad cow disease
Wasting disease in elk
Humans – Creutzfeldt-Jakob Syndrome (CJS) 48

49. Other Noncellular Infectious Agents
Satellite viruses – dependent on other viruses for replication
Adeno-associated virus – replicates only in cells infected with adenovirus
Delta agent – naked strand of RNA expressed only in the presence of hepatitis B virus
Viroids – short pieces of RNA, no protein coat; only been identified in plants

50. Concept Check:
Exposure to Nucleases that degrade DNA and RNA would damage all of the following EXCEPT
Animal Viruses
Bacteriophage
Prions
Satellite Viruses
Viroids

51. Concept Check:
Exposure to Nucleases that degrade DNA and RNA would damage all of the following EXCEPT
Animal Viruses
Bacteriophage
Prions
Satellite Viruses
Viroids