Viruses and Prokaryotes

1. Viruses and Prokaryotes Chapter 24

2. Learning Objective 1 • What is the structure of a virus? • Contrast a virus with a living cell

3. Virus (Virion) • Subcellular particle • Consists of • DNA or RNA genome • surrounded by protein coat (capsid)

4. Virus Structure

5. RNA inside capsid Capsid 0.1 µm Fig. 24-1a, p. 502

6. Capsid with antenna-like fibers DNA inside capsid 0.05 µm Fig. 24-1b, p. 502

7. DNA inside capsid Capsid Tail Tail fibers Emerging DNA 0.1 µm Fig. 24-1c, p. 502

8. Viruses • Cannot metabolize on their own • Contain nucleic acids necessary to make copies of themselves • but must invade and use metabolic machinery of living cells in order to reproduce

9. KEY CONCEPTS • A virus is a small particle consisting of a DNA or RNA genome surrounded by a protein coat

10. Learn more about virus structure by clicking on the figure in ThomsonNOW.

11. Learning Objective 2 • According to current hypotheses, what is the evolutionary origin of viruses?

12. Origin of Viruses • Viruses may be bits of nucleic acid that originally “escaped” from animal, plant, or bacterial cells

13. Hypothesis • Viruses must have evolved before the three domains diverged • It is unlikely that similar viruses that infect archaea and bacteria evolved twice

14. Learning Objective 3 • Characterize bacteriophages (phages) • viruses that infect bacteria • What is the difference between a lytic cycle and a lysogenic cycle?

15. Viral Reproductive Cycles • Lytic cycle • virus destroys host cell • Temperate viruses • do not always destroy their hosts • Lysogenic cycle • viral genome replicated along with host DNA

16. Lytic Cycle • 5 steps: • attachment to host cell • penetration of viral nucleic acid into host cell • replication of viral nucleic acid • assembly of components into new viruses • release from host cell

17. Lytic Cycle

18. Phages 1 Attachment. Phage attaches to cell surface of bacterium. Bacterium Bacterial DNA Penetration. Phage DNA enters bacterial cell. 2 Phage protein Phage DNA Replication and synthesis. Phage DNA is replicated. Phage proteins are synthesized. 3 Fig. 24-2a (1), p. 504

19. 4 Assembly. Phage components are assembled into new viruses. Release. Bacterial cell lyses and releases many phages that can then infect other cells. 5 Fig. 24-2a (2), p. 504

20. 0.25 µm Fig. 24-2b, p. 504

21. Lysogenic Cycle • Prophage • nucleic acid of phage becomes integrated into bacterial DNA • Lysogenic cells • bacterial cells that carry prophages • Lysogenic conversion • bacterial cells containing certain temperate viruses exhibit new properties

22. Lysogenic Cycle

23. Attachment. Phage attaches to cell surface of bacterium. 1 2 Penetration. Phage DNA enters bacterial cell. Prophage 3 Integration. Phage DNA integrates into bacterial DNA. 4 Replication. Integrated prophage replicates when bacterial DNA replicates. These cells may exhibit new properties. Fig. 24-3, p. 504

24. KEY CONCEPTS • Evolution occurs rapidly in prokaryotes; natural selection acts on the genetic variation provided by mutations and genetic recombination and is facilitated by rapid reproduction

25. Insert “The two different ways that viruses replicate (lytic and lysogenic cycles)” Tbd *suggested by Mary Durant, who will review existing animations currently slated for pickup (cd)

26. Watch the lytic and lysogenic cycles by clicking on the figure in ThomsonNOW.

27. Learning Objective 4 • Compare viral infection of animals and plants • Identify specific diseases caused by animal viruses

28. Animal Viruses • Viruses enter animal cells by membrane fusion or endocytosis • Viral nucleic acid replicated in host cell • proteins synthesized • new viruses assembled and released from cell

29. Envelope proteins Virus attaches to specific receptors on plasma membrane of host cell. 1 Envelope Capsid Membrane fusion. Viral envelope fuses with plasma membrane. 2 Nucleic acid Membrane Fusion Receptors Host-cell plasma membrane Virus is released into host-cell cytoplasm. 3 Capsid Cytoplasm Nucleus Viral nucleic acid separates from its capsid. 4 Nucleic acid Ribo-somes 5 Viral nucleic acid enters host-cell nucleus and replicates. ER mRNA 6 Viral nucleic acid is transcribed into mRNA. 7 Host ribosomes are directed by mRNA to synthesize viral proteins. Viruses are released from host cell. 10 8 Vesicles transport glycoproteins to host-cell plasma membrane. New viruses are assembled and enveloped by host-cell plasma membrane. 9 Fig. 24-4b, p. 508

30. Endocytosis Host-cell plasma membrane Endosomal vesicle forms and moves into cytoplasm. 3 Virus is released into host-cell cytoplasm. 4 Host-cell plasma membrane surrounds virus. 2 Viral envelope fuses with host-cell plasma membrane (not shown). 5 Virus makes contact with plasma membrane of host cell. 1 Host-cell cytoplasm Fig. 24-4c, p. 508

31. Viral Diseases • DNA viruses cause • smallpox, herpes, respiratory infections, gastrointestinal disorders • RNA viruses cause • influenza, upper respiratory infections, AIDS, some types of cancer

32. Rubella • An RNA virus

33. Plant Viruses • Mostly RNA viruses • Spread among plants by insect vectors • Spread through plant via plasmodesmata

34. Plant Viruses

35. Learning Objective 5 • Describe the reproductive cycle of a retrovirus, such as human immunodeficiency virus (HIV)

36. Retroviruses • Use reverse transcriptase • Transcribe RNA genome into DNA intermediate • becomes integrated into host DNA • Synthesize copies of viral RNA

37. HIV

38. Nucleic acid (RNA) HIV Envelope protein HIV attaches to host-cell plasma membrane. 1 Envelope Capsid Enzymes (reverse transcriptase, ribonuclease, integrase, protease) 2 HIV enters host-cell cytoplasm. CD4 Receptors Viral nucleic acid (RNA) Capsid is removed by enzymes. Reverse transcriptase catalyzes synthesis of single-stranded (ss) DNA that is complementary to viral RNA. 3 Reverse transcriptase Host-cell plasma membrane Cytoplasm ssDNA 4 The DNA strand then serves as template for synthesis of comple- mentary DNA strand, resulting in double-stranded (ds) DNA. Nucleus dsDNA Host chromosome Viral RNA 5 dsDNA is transferred to host nucleus and enzyme integrase integrates DNA into host chromosome. 6 When activated, viral DNA uses host enzymes to transcribe viral RNA. 7 Viral RNA leaves nucleus, viral proteins are synthesized on host ribosomes, and virus is assembled. 8 Virus buds from host cell, using host-cell plasma membrane to make viral envelope. Fig. 24-5, p. 509

39. Watch the HIV life cycle by clicking on the figure in ThomsonNOW

40. Learning Objective 6 • What are viroids and prions?

41. ViroidsandPrions • Viroids • short strands of RNA with no protein coat • Prions • consists only of protein • cause transmissible spongiform encephalopathies (TSEs)

42. Prions

43. Contacts Prion Normal protein (PrP) Prion induces normal PrP to misfold, forming another prion. 1 Contacts Contacts 2 Each prion can induce additional PrP proteins to misfold. 3 Proteins aggregate. Fig. 24-7, p. 511

44. Learning Objective 7 • Describe the structure and common shapes of prokaryotic cells

45. Prokaryotic Cells • Do not have membrane-enclosed organelles • such as nuclei and mitochondria

46. Prokaryotic Cell

47. Outer membrane Pili (structures used for attachment) Cell wall Peptidoglycan layer Nuclear area (nucleoid) Storage granule Plasmid (DNA) Flagellum Ribosomes Bacterial chromosome (DNA) Capsule Plasma membrane Fig. 24-9, p. 513

48. Bacterial Shapes • Spherical (cocci)

49. Bacterial Shapes • Rod shaped (bacilli)

50. Bacterial Shapes • Spiral • rigid helix (spirillum) • flexible helix (spirochete)