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Chapter 23

Chapter 23. Viruses and Prokaryotes. 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

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Chapter 23

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  1. Chapter 23 Viruses and Prokaryotes

  2. Viral structure • DNA or RNA genome • Capsid • Protein coat

  3. Tobacco mosaic virus

  4. Adenovirus

  5. T4 bacteriophage

  6. Viruses are not cells • Cannot metabolize independently • Forces infected host cells to replicate viral DNA • Takes over cell’s translation and transcription to reproduce

  7. Origin of viruses • Escaped gene hypothesis • Viruses are often host species-specific • Similarity of virus genome to host genome

  8. 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

  9. Phages infecting E. coli bacterium

  10. Lytic cycle • Destroys the host cell • Attachment • Penetration • Replication • Assembly • Release

  11. Lytic cycle

  12. Lysogenic cycle • Usually does not kill the host • Viral genome replicated along with host DNA • Attachment • Penetration • Integration • Replication

  13. Lysogenic cycle

  14. 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

  15. Membrane Fusion

  16. Endocytosis

  17. 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

  18. 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

  19. Life cycle of HIV

  20. Viroids • Short RNA strands with no protective coat • Cause plant diseases • Prions • Only protein • Cause transmissible spongiform encaphalopathies such as BSE

  21. Prokaryotes • No membrane-enclosed organelles such as nuclei or mitochondria

  22. Common shapes • Cocci • Bacillus (rod-shaped) • Spiral • Spirillum (rigid helix) • Spirochete (flexible helix) • Vibrios (comma shaped)

  23. Micrococcuscoccus bacteria

  24. Salmonellabacilli bacteria

  25. Spiroplasmaspirilla bacteria

  26. Cell walls in eubacteria • Gram-positive • Very thick peptidoglycan • Gram-negative • Thin layer of peptidoglycan • Outer membrane • Capsule • Surrounding the cell wall

  27. Gram-positive cell wall

  28. Gram-negative cell wall

  29. Pili • Protein structures that extend from the cell • Help bacteria adhere to surfaces • Flagella • Produce a rotary motion • Basal body, hook, and filament

  30. Bacterial flagellum

  31. Genetic material • Circular DNA molecule • Plasmids • Asexual reproduction • Binary fission • Budding • Fragmentation

  32. Transformation • Intake of DNA fragments • Transduction • Phage carries bacterial DNA between cells • Conjugation • Cells of different mating types

  33. Transduction

  34. Heterotrophs • Photoheterotrophs • Chemoheterotrophs • Feed on dead organic matter • Autotrophs • Photoautotrophs • Chemoautotrophs

  35. Most bacteria are aerobic • Faculatative anaerobes use oxygen if it is available • Obligate anaerobes carry on metabolism only anaerobically

  36. Domain Eubacteria • Domain Archaea • Cell walls do not have peptidoglycan • Translation mechanisms similar to eukaryotes

  37. Eubacteria and Archaea

  38. Methanogens • Produce methane gas • Anaerobic environments • Extreme halophiles • Inhabit saturated salt solutions • Extreme thermophiles • Inhabit environments over 100°C

  39. Vital ecological functions • Decomposers • Fixing nitrogen for plants • Pathogens • Koch’s postulates • Exotoxins • Endotoxins

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