1. Ch 23 Viruses
AP Biology

2. Virus is one type of Pathogen
Noncellular

3. Tobacco Mosaic Disease
Spotted appearance on leaves
Plant sap
Ivanowsky
Agent passed through filters, SO not bacteria
Beijerinck
Found that agent could only reproduce inside living cell

4. RESULTS Extracted sap from tobacco plant with tobacco mosaic disease
Fig. 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 What causes tobacco mosaic disease? 4
Healthy plants
became infected

5. Infectious particle (NOT CELLS) Obligate intracellular parasite
Viruses are NOT LIVING
Infectious particle (NOT CELLS)
Obligate intracellular parasite
Nucleic acid + protein coat (capsid)
Some have outer membrane
DNA OR RNA
Shape

6. Virus SHAPE Determined by proteins that make capsid Usually Helical
Ex: Tobacco mosaic
Polyhedral
Ex: Adenovirus
Combo of both helical and polyhedral
Ex: T4

7. RNA DNA Membranous envelope Head RNA Capsomere DNA Capsid Tail sheath
Fig. 19-3
RNA
DNA
Membranous
envelope
Head
RNA
Capsomere
DNA
Capsid
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

8. On the Origin of Viruses
OLD hypothesis
“escaped gene hypothesis”
Species-specific
NEW Hypothesis
Viruses arose before 3 domains
Evidence = similarities in proteins of capsids to Archaea and Eubacteria

9. Bacteriophages “bacteria eaters” DNA (usually) inside Polyhedral
Tail fibers – attachment
Treat bacterial infections
Reproduce by lytic and lysogenic cycles

10. 2 types of Bacteriophage lifecycles: Lytic Cycle Virulent Viruses
Virus destroys host cell
Virulent = virus with only a lytic cycle

11. Steps of the Lytic cycle
1. Attachment (Absorption)
Virus attaches to receptors on host cell wall
2. Penetration
Viral nucleic acid enters
Capsid remains outside host

12. Steps of the Lytic cycle- continued
3. Replication
Copy viral nucleic acid and proteins
4. Assembly
New virus parts assembled
5. Release
Phage makes lytic enzymes to destroy host
New viruses released

13. Fig 1
Attachment
Figure 19.5 The lytic cycle of phage T4, a virulent phage

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

15. Attachment Entry of phage DNA and degradation of host DNA
Fig 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

16. Attachment Entry of phage DNA and degradation of host DNA
Fig 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

17. Attachment Entry of phage DNA and degradation of host DNA Release
Fig 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

18. 2 types of Bacteriophage lifecycles: Lysogenic Cycle Temperate Viruses
Do not always destroy host
Lysogenic – viral and host genomes become ONE  prophage (Lysogenic cell)
Bacteria replicates all genetic material

19. The phage injects its DNA.
Fig. 19-6
Daughter cell
with prophage
Phage
DNA
The phage injects its DNA.
Cell divisions
produce
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
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 λ, 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.

20. How do temperate viruses cause symptoms?
STRESS
UV light, X-rays
Spontaneous
NOW lytic cycle – destroy host

21. Lysogenic Conversion
Lysogenic cells exhibit new properties
Ex: Clostridium botulinum
Usually harmless
Produce toxin when infected with specific phage

22. Viruses that Infect Animals
Similar to a bacteriophage
Methods may differ

23. Table 19-1a
Table 1

24. Table 19-1b
Table 1

25. Viruses that infect Animal Cells
Attachment
Species specific, cell specific
Ex: Herpes, influenza, rabies – lipoprotein envelope with projecting glycoprotein spikes for attachment
Measles, Pox viruses – many cell types
Polioviruses – digestive tract, motor neurons

26. A Couple Ways to Penetrate Animal Cells
1. Membrane Fusion
2. Endocytosis
Release of Animal Viruses
1. Budding
2. Lysis

27. Capsid and viral genome enter the cell Capsid
Fig. 19-7
Capsid and viral genome
enter the cell
Capsid
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA
Capsid
proteins ER
Copy of
genome (RNA)
Glyco-
proteins
Figure 19.7 The reproductive cycle of an enveloped RNA virus
New virus

28. DNA OR RNA?????
DNA Viruses
Making viral DNA/proteins easy because process similar to host’s
RNA Viruses
RNA replication and transcription need RNA-dependent RNA polymerase

29. Retroviruses RNA Reverse transcriptase: RNADNA intermediate (cDNA)
cDNA  dsDNA
dsDNA into host genome
RNA polymerases transcribe incorporated DNA into copies of viral RNA
Viral proteins made
Capsid produced
New viruses assembled
EX: HIV

30. Viral envelope Glycoprotein Capsid RNA (two identical strands) Reverse
Fig. 19-8a
Glycoprotein
Viral envelope
Capsid
RNA (two
identical
strands)
Reverse
transcriptase
HOST CELL
HIV
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS
mRNA
New virus

31. Membrane of white blood cell HIV HIV entering a cell
Fig. 19-8b
Membrane of
white blood cell
HIV
Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS
0.25 µm
HIV entering a cell
New HIV leaving a cell

32. harmless derivatives of pathogenic microbes
Viral Treatment
No antibiotics
Antiviral drugs
Vaccines – Prevention
harmless derivatives of pathogenic microbes
stimulate the immune system

33. New or previously uncommon SARS –China, AIDS, West Nile
Emerging Viruses
New or previously uncommon
SARS –China, AIDS, West Nile
Outbreaks - why?
Extend range
High mutation rates

34. (a) The 1918 flu pandemic Fig. 19-9a
Figure 19.9 Influenza in humans and other animals
(a) The 1918 flu pandemic

35. (b) Influenza A H5N1 virus 0.5 µm Fig. 19-9b
Figure 19.9 Influenza in humans and other animals
(b) Influenza A H5N1
virus

36. (c) Vaccinating ducks Fig. 19-9c
Figure 19.9 Influenza in humans and other animals
(c) Vaccinating ducks

37. H1N1

38. Viruses can Infect Plant Cells
Spread – insects (aphids, leafhoppers)
Entrance – damaged cells; infected seeds/asexual propagation
Once infected – virus passes between cells by plasmodesmata
Most RNA
Symptoms:
Reduced size; spots/streaks/mottled patterns
Prevention – virus resistant strains

39. Fig
Figure Viral infection of plants

40. Smaller than Viruses Viroids Prions Naked RNA RNA polymerases – copy
Viroid = template
Infect nuclei of plants/animals
Interfere with gene regulation
Protein-like
No nucleic acid
Linked to TSEs
Transmissable spongiform encephalopathies

41. TSEs
Brain becomes spongelike
Radiation useless
No RNA/DNA
Mammals – have gene to encode prion protein
Normally harmless
GE mice – lack prion protein gene – immune to TSE infection
Creutzfeldt-Jakob disease

42. Scrapie (a TSE)
Sheep/goats
Lose coordination
Irritable
Severely itch

43. Bovine Spongiform Encephalopathy (BSE)
“mad cow”
UK 1990s epidemic
Cattle ate feed mixed with sheep's brains+; (contained prions)
Cow  human
Similar to CJD

44. You should now be able to:
Explain how envelopes are formed
Distinguish between the lytic and lysogenic reproductive cycles
Explain why viruses are obligate intracellular parasites
Describe the reproductive cycle of an HIV retrovirus
Describe viroids and prions