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