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 species-specific

• 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)

Micrococcuscoccus bacteria

Salmonellabacilli bacteria

Spiroplasmaspirilla 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