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

2. General Characteristics of Viruses Differentiate a virus from a bacterium.
Obligatory intracellular parasites
Contain DNA or RNA
Contain a protein coat
Some are enclosed by an envelope
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

3. Virus Sizes How could the small size of viruses have helped researchers detect viruses before the invention of the electron microscope?
p. 369
Figure 13.1

4. Virion Structure Nucleic acid Capsid Envelope Spikes DNA or RNA
Capsomeres
Envelope
Spikes
p. 372
Figure 13.2a

5. Morphology of an Enveloped Virus
Figure 13.3

6. Enveloped Viruses p. 387 Fig. 13.16 b Herpesvirus (DNA, enveloped)
Figure 13.16b

7. Morphology of a Helical Virus
Figure 13.4

8. Morphology of a Complex Virus
Figure 13.5

9. Taxonomy of Viruses Family names end in -viridae. Herpesviridae
Genus names end in -virus Herpesvirus
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.
Human herpes virus HHV-1, HHV-2, HHV-3

10. Growing Viruses Viruses must be grown in living cells
Bacteriophages form plaques on a lawn of bacteria
p. 377
Figure 13.6

11. Growing Viruses
Animal viruses may be grown in living animals or in embryonated eggs
p. 377
Figure 13.7

12. Growing Viruses Animal and plant viruses may be grown in cell culture
Continuous cell lines may be maintained indefinitely
Figure 13.8

13. Virus Identification
Cytopathic effects: Detectable changes in the host cell due to viral infection (p. 441)
Blood serum (serology) tests
Detect antibodies against viruses in a patient
Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot
Nucleic acids: Studies of the DNA/RNA

14. Virus Identification: Cytopathic Effect
a) Uninfected cells growing in a monolayer
b) Infected cells piling/rounding up
Figure 13.9

15. The 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 (analogy hypodermic syringe)
Biosynthesis: Production of phage DNA and proteins (What is the eclipse period?)
Maturation: Spontaneous assembly of phage particles
Release: Phage lysozyme breaks cell wall
Text discussion of T-even bacteriophage lytic cycle steps p. 380

16. Lytic Cycle of a T-Even Bacteriophage1 2
p. 381 3
Figure 13.11

17. Lytic Cycle of a T-Even Bacteriophage4
Continued… p. 381
Figure 13.11

18. The Lysogenic Cycle
p. 382
Figure 13.12

19. Results of Multiplication of Bacteriophages
Lytic cycle
Phage causes lysis and death of host cell
Lysogenic cycle: Prophage DNA incorporated in host DNA may result in…
Lysogenic (bacterial) cells become immune to reinfection of the same virus (but not other types of phage)
Phage conversion: Bacterial cells that have new genetic properties (Examples: Clostridium botulinium, Corynebacterium diphtheriae, Streptococcus pyogenes scarlet fever toxin genes are brought into the cell & incorporated into the cell DNA by the virus)
Specialized transduction: Bacterial DNA next to the phage DNA that is accidentally excised out of the bacterial DNA and packaged with the phage DNA (See slides 20 & 21)

20. Generalized Transduction2 3 4
Chap. 8 Fig Generalized transduction, in which any bacterial DNA can be transferred from one cell to another. p. 239 5 6
Figure 8.28

21. Specialized Transduction
Figure 13.13

22. Multiplication of Animal Viruses
Attachment: Viruses attach to cell membrane
Penetration by endocytosis or fusion (HIV)
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
Text discussion beginning p. 382

23. Attachment, Penetration, Uncoating
By pinocytosis
p. 384
Figure 13.14a

24. Attachment, Penetration, Uncoating
By fusion
p. 384
Figure 13.14b

25. Budding of an Enveloped Virus
Figure 13.20

26. Multiplication of DNA Virus
Figure 13.15

27. Sense Strand (+ Strand) RNA Virus
p. 388
Figure 13.17a

28. Antisense Strand (– Strand) RNA Virus
p. 388
Figure 13.17b

29. Double-Stranded RNA Virus
p. 388
Figure 13.17c

30. Multiplication of a Retrovirus
Figure 13.19

31. p. 385

32. p. 384

33. Prions Proteinaceous Infectious particle
Inherited and transmissible by ingestion, transplant, and 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; accumulates in brain cells, forming plaques

34. Filoviridae Single-stranded RNA, – strand, one RNA strand Filovirus
Enveloped, helical viruses
Ebola and Marburg viruses
Figure 23.21

35. Retroviridae Single-stranded RNA, 2 RNA strands, produce DNA
Use reverse transcriptase to produce DNA from viral genome
Lentivirus (HIV)
Oncogenic viruses
Includes all RNA tumor viruses
Figure 19.13