1. Viruses: A Borrowed Life
Figure 19.1
Viruses: A Borrowed Life
Figure 19.1 Are the tiny viruses infecting this E. coli cell alive?
0.5 mm

2. The Discovery of Viruses: Scientific Inquiry
1935, Wendell Stanley crystallized tobacco mosaic virus (TMV)
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3. Extracted sap from tobacco plant with tobacco mosaic disease 2
Figure 19.2
RESULTS 1
Extracted sap from tobacco plant with tobacco mosaic disease 2
Passed sap through a porcelain filter known to trap bacteria 3
Rubbed filtered sap on healthy tobacco plants
Figure 19.2 Inquiry: What causes tobacco mosaic disease? 4
Healthy plants became infected

4. Figure 19.2b
Figure 19.2 Inquiry: What causes tobacco mosaic disease?

5. Figure 19.2c
Figure 19.2 Inquiry: What causes tobacco mosaic disease?

6. Structure of Viruses Not cells
Consists of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope
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7. Viral Genomes Double- or single-stranded DNA, or
Double- or single-stranded RNA
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8. Membranous envelope Capsomere RNA RNA DNA Capsid DNA Head Tail sheath
Figure 19.3
Membranous envelope
Capsomere
RNA
RNA
DNA
Capsid
DNA
Head
Tail sheath
Capsomere of capsid
Tail fiber
Glycoprotein
Glycoproteins
18  250 nm
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
Figure 19.3 Viral structure.
20 nm
50 nm
50 nm
50 nm
(a)
Tobacco mosaic virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4

9. Some viruses have viral envelopes
Derived from the host cell’s membrane
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10. Bacteriophages (phage)  viruses that infect bacteria
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11. Lytic Cycle 1 Attachment
Figure 19.5 The lytic cycle of phage T4, a virulent phage.

12. Entry of phage DNA and degradation of host DNA
Figure 1
Attachment 2
Entry of phage DNA and degradation of host DNA
Figure 19.5 The lytic cycle of phage T4, a virulent phage.

13. Entry of phage DNA and degradation of host DNA
Figure 1
Attachment 2
Entry of phage DNA and degradation of host DNA
Figure 19.5 The lytic cycle of phage T4, a virulent phage. 3
Synthesis of viral genomes and proteins

14. Entry of phage DNA and degradation of host DNA
Figure 1
Attachment 2
Entry of phage DNA and degradation of host DNA
Phage assembly
Figure 19.5 The lytic cycle of phage T4, a virulent phage. 4
Assembly 3
Synthesis of viral genomes and proteins
Head
Tail
Tail fibers

15. Entry of phage DNA and degradation of host DNA
Figure 1
Attachment 2
Entry of phage DNA and degradation of host DNA 5
Release
Phage assembly
Figure 19.5 The lytic cycle of phage T4, a virulent phage. 4
Assembly 3
Synthesis of viral genomes and proteins
Head
Tail
Tail fibers

16. The Lysogenic Cycle
Replicates the phage genome without destroying the host
Animation: Phage Lambda Lysogenic and Lytic Cycles
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17. Daughter cell with prophage Phage DNA The phage injects its DNA.
Figure 19.6
Daughter cell with prophage
Phage DNA
The phage injects its DNA.
Cell divisions produce a population of bacteria infected with the prophage.
Phage DNA circularizes.
Phage
Bacterial chromosome
Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
Certain factors determine whether
The bacterium reproduces, copying the prophage and transmitting it to daughter cells.
The cell lyses, releasing phages.
lytic cycle is induced or
lysogenic cycle is entered
Figure 19.6 The lytic and lysogenic cycles of phage , a temperate phage.
Prophage
New phage DNA and proteins are synthesized and assembled into phages.
Phage DNA integrates into the bacterial chromosome, becoming a prophage.

18. The phage injects its DNA.
Figure 19.6a
Phage DNA
The phage injects its DNA.
Phage DNA circularizes.
Phage
Bacterial chromosome
Lytic cycle
Certain factors determine whether
The cell lyses, releasing phages.
Figure 19.6 The lytic and lysogenic cycles of phage , a temperate phage.
lytic cycle is induced
lysogenic cycle is entered or
New phage DNA and proteins are synthesized and assembled into phages.

19. Daughter cell with prophage
Figure 19.6b
Daughter cell with prophage
Cell divisions produce a population of bacteria infected with the prophage.
Phage DNA circularizes.
Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.
Lysogenic cycle
Certain factors determine whether
The bacterium reproduces, copying the prophage and transmitting it to daughter cells.
Figure 19.6 The lytic and lysogenic cycles of phage , a temperate phage.
lytic cycle is induced
lysogenic cycle is entered or
Prophage
Phage DNA integrates into the bacterial chromosome, becoming a prophage.

20. Table 19.1a
Table 19.1 Classes of Animal Viruses (part 1)

21. Table 19.1b
Table 19.1 Classes of Animal Viruses (part 2)

22. RNA as Viral Genetic Material
Retroviruses use reverse transcriptase to copy their RNA genome into DNA
HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)
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23. RNA (two identical strands) Reverse transcriptase HOST CELL
Figure 19.8a
Glycoprotein
Viral envelope
Capsid
RNA (two identical strands)
Reverse transcriptase
HOST CELL
HIV
Reverse transcriptase
Viral RNA
RNA-DNA hybrid
DNA
NUCLEUS
Chromosomal DNA
Provirus
RNA genome for the next viral generation
Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS.
mRNA
New virus

24. Membrane of white blood cell
Figure 19.8c
Membrane of white blood cell
HIV
Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS.
HIV entering a cell

25. 0.25 m HIV entering a cell Figure 19.8d
Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS.
HIV entering a cell

26. New HIV leaving a cell Figure 19.8e
Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS.

27. New HIV leaving a cell Figure 19.8f
Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS.

28. New HIV leaving a cell Figure 19.8g
Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS.

29. Evolution of Viruses Probably evolved as bits of cellular nucleic acid
e.g. Plasmids, transposons, and viruses are all mobile genetic elements
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30. Viral Diseases in Animals
Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes
Some viruses cause infected cells to produce toxins that lead to disease symptoms
Others have molecular components such as envelope proteins that are toxic
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31. Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the harmful pathogen
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32. Emerging Viruses e.g. influenza virus H1N1 Flu epidemics
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33. Viral diseases in a small isolated population can emerge and become global
Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity
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34. Can cause pandemics, global epidemics
The 2009 flu pandemic was likely passed to humans from pigs; for this reason it was originally called the “swine flu”
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35. 2009 pandemic H1N1 influenza A virus (a)
Figure 19.9a
Figure 19.9 Influenza in humans.
1 m
2009 pandemic H1N1 influenza A virus
(a)

36. (b) 2009 pandemic screening
Figure 19.9b
Figure 19.9 Influenza in humans.
(b) 2009 pandemic screening

37. Figure 19.9c
Figure 19.9 Influenza in humans.
(c) 1918 flu pandemic

38. Viral Diseases in Plants
Most plant viruses have an RNA genome
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39. Figure 19.10a
Figure Viral infection of plants.

40. Figure 19.10c
Figure Viral infection of plants.

41. Viroids and Prions: The Simplest Infectious Agents
Viroids are small circular RNA molecules that infect plants and disrupt their growth
Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
Convert normal proteins into the prion version
e.g. mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions
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42. Prion Original prion Aggregates of prions New prion Normal protein
Figure 19.11
Prion
Original prion
Aggregates of prions
New prion
Normal protein
Figure Model for how prions propagate.