1. The Viruses: Introduction and General Characteristics
Chapter 16
The Viruses: Introduction and General Characteristics

2. General Properties of Viruses
virion
complete virus particle
consists of 1 molecule of DNA or RNA enclosed in coat of protein
may have additional layers
cannot reproduce independent of living cells nor carry out cell division as procaryotes and eucaryotes do
Obligate intracellular parasites

3. Generalized Structure of Viruses
Figure 16.1

4. The Structure of Viruses
virion size range is ~ nm in diameter with most viruses too small to be seen with the light microscope
all virions contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid)
some viruses consist only of a nucleocapsid, others have additional components
envelopes
virions having envelopes = enveloped viruses
virions lacking envelopes = naked viruses, non-envelope

5. Capsids
large macromolecular structures which serve as protein coat of virus
protect viral genetic material and aids in its transfer between host cells
made of protein subunits called protomers

6. Helical Capsids
shaped like hollow tubes with protein walls

7. Tobacco Mosaic Virus Structure
Figure 16.3

8. Influenza Virus – an Enveloped Virus with a Helical Nucleocapsid
Figure 16.4

9. Icosahedral Capsids
an icosahedron is a regular polyhedron with 20 equilateral faces and 12 vertices
it is one of nature’s favorite shapes
capsomers
ring or knob-shaped units made of 5 or 6 protomers

10. Figure 16.5

11. Figure 16.6

12. Figure 16.7

13. Figure 16.8

14. Figure 16.9

15. Viral Envelopes and Enzymes
many viruses are bound by an outer, flexible, membranous layer called the envelope
in eucaryotic viruses some envelope proteins, which are viral encoded, may project from the envelope surface as spikes or peplomers.

16. Figure 16.10

17. Virion Enzymes
it was first erroneously thought that all virions lacked enzymes
now known a variety of virions have enzymes
some are associated with the envelope or capsid but most are within the capsid

18. Viral Genome Acids
A virus may have single or double stranded DNA or RNA
the size of the nucleic acid also varies from virus to virus
genomes can be linear or circular

19. Figure 16.11

20. Generalized Illustration of Virus Reproduction
Figure 16.12

21. The Cultivation of Viruses
requires inoculation of appropriate living host

22. Hosts for animal viruses
suitable animals
embryonated eggs
tissue (cell) cultures
monolayers of animal cells
plaques
localized area of cellular destruction and lysis
cytopathic effects
microscopic or macroscopic degenerative changes or abnormalities in host cells and tissues

23. Two sites that are used to grow animal viruses are the chorioallantoic membrane and the allantoic cavity. This diagram shows a 9 day chicken embryo.
Figure 16.13

24. Two sites that are used to grow animal viruses are the chorioallantoic membrane and the allantoic cavity. This diagram shows a 9 day chicken embryo.
Figure 16.14

25. The Viruses: Bacteriophages
Chapter 17
The Viruses: Bacteriophages

26. Virulent Double-Stranded DNA Phages T-even Phages of E. coli
lytic cycle
phage life cycle that culminates with host cell bursting, releasing virions

27. Life Cycle of dsDNA T4 Phage of E. coli
adsorption to specific receptor site
penetration of the cell wall
insertion of the viral nucleic acid into the host cell
transcription  early mRNAm (before DNA is synthesizexd – proteins enzymes needed to take over the cell)
translation of early mRNA resulting in production of protein factors and enzymes involved in phage DNA synthesis

28. Phage T4 Life Cycle continued
transcription  late mRNA
translation of late mRNA resulting in synthesis of capsid proteins, proteins required for phage assembly and proteins required for cell lysis and phage release
cell lysis and phage release

29. Maturation
assembly
Figure 17.3

30. Adsorption and Penetration
receptor sites
specific surface structures on host to which viruses attach
specific for each virus
can be proteins, lipopolysaccharides,

31. Figure 17.4

32. Figure 17.5

33. Synthesis of Phage Nucleic Acids and Proteins
most double-stranded DNA viruses
use their DNA genome as a template for mRNA synthesis
the mRNA is translated to produce viral proteins

34. Replication Strategy Used by Double-Stranded DNA Viruses
Figure 17.6

35. Map of the T4 Genome early genes genes with related functions late
are usually
found
clustered
together
late
genes
Map of T4 genome. Genes of related function tend to be clustered together.
Figure 17.7

36. Assembly of Phage Particles
complex self-assembly process
involves viral proteins as well as some host cell factors

37. Figure 17.11

38. Release of Phage Particles
in T4 - E. coli system, ~150 viral particles are released
two proteins are involved in process
T4 lysozyme attacks the E. coli cell wall
holin creates holes in the E. coli plasma membrane

39. Temperate Bacteriophages and Lysogeny
temperate phages have two reproductive options
reproduce lytically as virulent phages do
remain within host cell without destroying it
done by many temperate phages by integration of their genome with the host genome in a relationship called lysogeny

40. Lysogeny prophage lysogens (lysogenic bacteria)
integrated phage genome
lysogens (lysogenic bacteria)
infected bacterial host
temperate phages
phages able to establish lysogeny

41. Distinctive characteristics of Lysogenic Bacteria
they are immune to superinfection
under appropriate conditions they will lyse and release phage particles
this occurs when conditions in the cell cause the prophage to initiate synthesis of new phage particles, a process called induction

42. Focus on lambda phage double-stranded DNA phage
linear genome with cohesive ends
circularizes upon entry into host
Figure 17.17

43. Lambda Phage DNA
the DNA contains 12 base single-stranded cohesive ends
circularization results from complementary base pairing
Figure 17.18

44. The Genome of Phage Lambda (l)
Figure 17.19

45. Infection by Lambda Phage
Two proteins appear after infection
the lambda repressor
product of cI gene (blocks lytic)
blocks transcription of the cro gene and other genes required for the lytic cycle
Cro protein (blocks lysogenic)
product of cro gene
inhibits transcription of the lambda repressor gene

46. If Lambda Repressor Wins Race with the Cro protein…
lysogeny is established
lambda genome is integrated into the host genome in a reaction catalyzed by the enzyme, integrase

47. Attachment site
On the chromosome
Figure 17.22

48. Induction triggered by drop in levels of lambda repressor
caused by exposure to UV light and chemicals that cause DNA damage

49. Fig

50. Eucaryotic Viruses and Other Acellular Infectious Agents
Chapter 18
Eucaryotic Viruses and Other Acellular Infectious Agents

51. Reproduction of Animal Viruses
adsorption
penetration and uncoating
replication of virus nucleic acids
synthesis and assembly of virions
virion release

52. Adsorption
virions attach to host cells displaying the proper receptor

53. Penetration and Uncoating
one of two mechanisms used by most viruses
fusion of envelope with host cell membrane
endocytosis
in some cases only nucleic acid enters host cell

54. Fusion with host membrane
Figure 18.4 (a)

55. Endocytosis – enveloped virus
Figure 18.4 (b)

56. Uncoating – envelope and capsid are broken down
Uncoating – envelope and capsid are broken down. Genetic material is released.

57. Endocytosis – naked virus
Naked viruses such as poliovirus, a picornavirus, may be taken up by endocytosis and then insert their nucleic acid into the cytoplasm through the vesicle membrane. It also is possible that they insert the nucleic acid directly through the plasma membrane within a coated pit.
Figure 18.4 (c)

58. Genome Replication and Transcription in DNA Viruses
early genes
encode proteins involved in take over of host and in synthesis of viral DNA and RNA
viral DNA replication
usually occurs in nucleus
early mRNA synthesis
usually by host RNA polymerase

59. e.g., herpes simplex virus I
uses host RNA
polymerase for
synthesis of viral
mRNA
uses virus-
encoded DNA
polymerase for
replication of
genome
Figure 18.6

60. Retrovirus (HIV)
RNA – genetic material
Reverse transcriptase – uses viral RNA template to make viral DNA

61. budding

62. Assembly of Virus Capsids
late genes direct capsid protein synthesis which spontaneously self-assemble to form the capsid
during icosahedral virus assembly empty procapsids form first, nucleic acid are then inserted
assembly of envelope viruses (maturation)
in most cases, similar to assembly of naked viruses

63. Virion Release
all viral envelopes are derived from host cell membranes in multistep process
naked viruses
usually by lysis of host cell
envelope viruses (budding)
formation of envelope and release usually occur concurrently
virus-encoded proteins incorporated into host membrane
nucleocapsid buds outward and is surrounded by modified host membrane

64. Release of influenza virus by budding
Viral envelope proteins (hemagglutinin and neuraminidase) are inserted into host plasma membrane. Then nucleocapsid approaches inner surface of the membrane and binds to it. At the same time viral proteins collect at the site and host membrane proteins are excluded. Finally, the plasma membrane buds to simultaneously form the viral envelope and release the mature virion.
Figure 18.11

65. HIV release by budding Figure 18.12 (a)
TEM of HIV particles beginning to bud, as well as some mature particles
Figure (a)

66. SEM view of HIV particles budding from a lymphocyte
Figure
18.12 (b)

67. Cytocidal Infections and Cell Damage
infection that results in cell death

68. Mechanisms of host cell damage and cell death
inhibition of host DNA, RNA, and protein synthesis
lysosome damage
causes release of hydrolytic enzymes into cell
alteration of plasma membrane
can lead to attack of host cell by immune system
can lead to cell fusion, forming syncytium – multinucleated cells

69. Other mechanisms… toxicity from high concentrations of viral proteins
formation of inclusion bodies
can disrupt cell structure
chromosomal disruptions
transformation of host cell into malignant cell

70. Latent viral infection
Herpes simplex virus
Dormant – nerve cells, activated under certain conditions – epithelial cells – cold sores
Herpes simplex virus 1 – oral herpes
Infancy - direct contact
Activated by fever, sunburn, stress

71. Herpes Simplex Virus 2 Genital herpes Vesicles in the area
Burning, difficulty walking
acyclovir

72. Viruses and cancer Nucleated cells have proto-oncogenes
Control (regulate) cell growth
Code for proteins – regulate cell growth
Mutation – abnormal proteins
Loss of control – uncontrolled proliferation of the cell with mutation - cancer

73. Mutations Chemicals UV light Viruses Epstein-Barr virus – DNA virus
Dormant in some B lymphocytes
Transmitted in saliva – infectious mononucleosis

74. Epstein-Barr virus DNA virus Dormant in some B-lymphocytes
Transmitted in saliva
Infectious mononucleosis

75. Burkitt’s Lymphoma Common childhood cancer in Africa
Average age 7 – malaria is common
EBV and Plasmodium cause mutation in
c-myc gene – proto-oncogene
Loss of control of cell growth
Uncontrolled proliferation
Leads to cancer – jaw bones

77. Prions Proteinaceous infectious particles – proteins Scrapie – sheep
Scrape themselves against fences
Become paralyzed and die
Mad cow disease – bovine spongiform encephalopathy (BSE) – sponge like degeneration of the brain.
Shake, shiver

78. Creutzfeldt –jakob disease
Occurs in certain families – hereditary
Transmitted by contaminated hamburgers
Dementia – die within a year

79. viroids Naked piece of RNA Plant pathogen Potato spindle tuber viroid
Damage to potato plants
Evolved from introns

81. Prions – Proteinaceous Infectious Particle
examples of degenerative diseases in animals caused by prions
scrapie
bovine spongiform encephalopathy (BSE) or mad cow disease
Creutzfeldt-Jakob disease (CJD) and varient CJD (vCJD)
kuru

82. Current Model of Disease Production by Prions
PrPC (prion protein) is present in “normal” form in host and abnormal form of prion protein is PrPSc
entry of PrPSc into animal brain causes PrPC protein to change its conformation to abnormal form.
the newly produced PrPSc molecules then convert more normal molecules to the abnormal form
interactions between PrPSc and PrPC may result in the crosslinking of PrPC molecules resulting in neuron loss

83. What about Mad Cow Disease?
prions cause bovine spongiform encephalopathy (BSE or mad cow disease)
epidemic proportions in England in 1990s
initially spread because cows were fed meal made from all parts (including brain tissue) of infected cattle

84. Variant Creutzfeldt-Jakob (vCJD) v. CJD
difference in diseases is origin
eating meat from BSE infected cattle can cause variant Creutzfeldt-Jakob (vCJD) in humans
CJD is caused by spontaneous mutation of the gene that codes the prion protein
all prion caused diseases
have no effective treatment
result in progressive degeneration of the brain and eventual death