Biology, Seventh Edition
Solomon • Berg • Martin
Chapter 23
Viruses and
Prokaryotes
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Viral structure
• DNA or RNA genome
• Capsid
–Protein coat
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Tobacco mosaic virus
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Adenovirus
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
T4 bacteriophage
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Viruses are not cells
• Cannot metabolize
independently
• Forces infected host cells to
replicate viral DNA
• Takes over cell’s translation and
transcription to reproduce
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Origin of viruses
• Escaped gene hypothesis
–Viruses are often host speciesspecific
–Similarity of virus genome to host
genome
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Origin prior to divergence of the
three domains
• Similarities between protein
structures of capsids
• Genetic similarities between
viruses that infect the Eubacteria
and the Archaea
• Convergent evolution unlikely
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Phages infecting E. coli bacterium
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Lytic cycle
• Destroys the host cell
–Attachment
–Penetration
–Replication
–Assembly
–Release
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Lytic cycle
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Lysogenic cycle
• Usually does not kill the host
• Viral genome replicated along
with host DNA
–Attachment
–Penetration
–Integration
–Replication
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Lysogenic cycle
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Viral infection of animal cells
• Surface attachment proteins bind
to specific cell receptors
• Fuse with plasma membrane
• Endocytosis
–Invaginates plasma membrane to
form a vesicle inside the cell
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Membrane
Fusion
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Endocytosis
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Viral infection of plant cells
• Cannot penetrate cell walls
unless they are damaged
• Spread by insects that feed on
plants or by infected seeds
• Spreads throughout the plant via
plasmodesmata
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Retrovirus reproductive cycle
• Reverse transcriptase catalyzes
synthesis of DNA complementary
to the viral RNA
• Integrase integrates DNA into the
host chromosome
• Viral DNA used to transcribe viral
RNA and synthesize proteins
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Life cycle
of HIV
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Viroids
• Short RNA strands with no
protective coat
• Cause plant diseases
• Prions
• Only protein
• Cause transmissible spongiform
encaphalopathies such as BSE
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Prokaryotes
• No membrane-enclosed
organelles such as nuclei or
mitochondria
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Common shapes
• Cocci
• Bacillus (rod-shaped)
• Spiral
–Spirillum (rigid helix)
–Spirochete (flexible helix)
• Vibrios (comma shaped)
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Micrococcus
coccus bacteria
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Salmonella
bacilli bacteria
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Spiroplasma
spirilla bacteria
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Cell walls in eubacteria
• Gram-positive
–Very thick peptidoglycan
• Gram-negative
–Thin layer of peptidoglycan
–Outer membrane
• Capsule
–Surrounding the cell wall
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Gram-positive cell wall
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Gram-negative cell wall
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Pili
• Protein structures that extend
from the cell
• Help bacteria adhere to surfaces
• Flagella
• Produce a rotary motion
• Basal body, hook, and filament
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Bacterial flagellum
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Genetic material
• Circular DNA molecule
• Plasmids
• Asexual reproduction
• Binary fission
• Budding
• Fragmentation
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Transformation
• Intake of DNA fragments
• Transduction
• Phage carries bacterial DNA
between cells
• Conjugation
• Cells of different mating types
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Transduction
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Heterotrophs
• Photoheterotrophs
• Chemoheterotrophs
–Feed on dead organic matter
• Autotrophs
• Photoautotrophs
• Chemoautotrophs
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Most bacteria are aerobic
• Faculatative anaerobes use
oxygen if it is available
• Obligate anaerobes carry on
metabolism only anaerobically
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Domain Eubacteria
• Domain Archaea
• Cell walls do not have
peptidoglycan
• Translation mechanisms similar
to eukaryotes
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
Eubacteria and Archaea
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Methanogens
• Produce methane gas
• Anaerobic environments
• Extreme halophiles
• Inhabit saturated salt solutions
• Extreme thermophiles
• Inhabit environments over 100°C
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Biology, Seventh Edition
CHAPTER 23 Viruses and Prokaryotes
• Vital ecological functions
• Decomposers
• Fixing nitrogen for plants
• Pathogens
• Koch’s postulates
• Exotoxins
• Endotoxins
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