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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
