Viruses and Prokaryotes

1. Viruses and Prokaryotes

2. Viruses • Noncellular (subcellular) infectious agents • Cannot live outside a living cell • Depend on cell’s reproductive machinery to make new viruses.

3. Discovery of Viruses • 1892 Dmitrii Ivanowsky, Russian botanist, studied the Tobacco Mosaic Disease • Plants can be infected by encountering the sap of diseased plants • Sap is infective even after being passed through a filter small enough to screen out bacteria.

4. Martinus Beijerinck, Dutch microbiologist, 1898, provided independent confirmation of Ivanowsky’s work. • Infectious agent had many characteristics of a living organism, but could reproduce only within a living cell. • Named the agent virus, which means “poison” in Latin.

5. 20th century: discovery of many infective agents, which could infect animals as well as plants. • Could not be grown in lab cultures unless living cells were present • Discovery of viruses that attack and kill bacteria “bacteriophages”

6. Viral structure DNA or RNA genome (not both) * nucleic acid may be single or double stranded Capsid ( protein coat) * helical or polyhedral, or a combination of both Some viruses have an outer membranous envelope

7. Tobacco mosaic virus

8. Adenovirus

9. T4 bacteriophage

10. Viral Classification System: • International Committee on the Taxonomy of Viruses (ICTV) classifies viruses • Non-Linnaean classification system • Not included in the 3 domains • Classified according to nucleic acids and/or presence or absence of a capsid

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

12. Origin of viruses • Escaped gene hypothesis: bits of nucleic acid that escaped from cellular organisms. May have originated as mobile genetic elements such as transposons or plasmids • Viruses are often host species-specific • Similarity of virus genome to host genome is closer than the resemblance of the genomes of different viruses.

13. Origin prior to divergence of the three domains hypothesis • Similarities between protein structures of capsids in different viruses supports this. • Genetic similarities between viruses that infect the Eubacteria and the viruses which infect the Archaea • Improbable that these similarities evolved independently. • How could they evolve independently, as all are parasitic?

14. Bacteriophages are viruses that attack bacteria • “phage” for short • Among the most complex of all viruses • May have specialized “tails” to attach to host • Over 200 types • Used before antibiotics to treat diseases; and with the growing problem of resistance to antibiotics, are again the subject of research.

15. Phages infecting E. coli bacterium

16. Viral reproduction • Two types • Lytic cycle • Lysogenic cycle

17. Lytic cycle Destroys the host cell Attachment Penetration Replication Assembly Release Viruses which only have a lytic cycle are called virulent Lytic Cycle works immediately: virus uses cell machinery to reproduct

18. Lytic cycle

19. Lysogenic cycle Usually does not kill the host Viral genome replicated along with host DNA Attachment Penetration Integration into host DNA (referred to as a prophage in this stage) Replication of prophage DNA along with host DNA These types of viruses are often called temperate viruses Lysogenic cycle works with a delay mechanism

20. Lysogenic cycle

21. Lysogenic conversion • External or internal conditions cause temperate viruses to revert to a lytic cycle and then destroy host cells.

22. Viral infection of animal cells • Surface attachment proteins bind to specific cell receptors on the host. This will determine what type of animal or what type of cell in the animal will be attacked.

23. Ways to penetrate living cells: • Fuse with plasma membrane • Endocytosis • Invaginates plasma membrane to form a vesicle inside the cell

25. Endocytosis

26. 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 • Most plant viruses are RNA viruses.

27. 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 • HIV is a retrovirus that causes AIDS

28. HIV Life Cycle • HIV infects T helper cells, specialized cells of the immune system • Virus attaches to CD4 receptors (people who don’t have these, can’t get AIDS) • Capsid is removed • Synthesis of cDNA which is transferred to the host nucleus and integrated into the host DNA

29. HIV continued • Activated viral DNA uses host enzymes to transcribe viral RNA • Viral RNA leaves the nucleus • Viral proteins synthesized on host ribosomes • Viruses assembled • Virus buds from the host cell and may attack other T-cells

31. Viroids • Short RNA strands with no protective coat • Cause plant diseases • Much smaller than viruses • RNA is copied to make more viroids • Prions • Only protein • Cause transmissible spongiform encaphalopathies such as BSE (mad cow disease) Examples from BZ Prion Diseases.

32. Prokaryotes No membrane-enclosed organelles such as nuclei or mitochondria Cellular organisms 2 domains: Archaea Eubacteria Bacteria (singular bacterium)

33. Size • Very small from 0.05 to 1 micrometer (about 1/10th length of eukaryotic cells) • Cell volume is about 1/1000th of small eukaryotic cells • Most are unicellular: some form colonies or filaments containing specialized cells

34. Common shapes • Cocci (round) (singular: coccus) • Bacillus (rod-shaped) (singular: bacillus) • Spiral • Spirillum (rigid helix) • Spirochete (flexible helix) • Vibrios (comma shaped)

35. Prefixes: • Staphlo: clumps • Strepto: chains • Diplo: groups of two • Prefixes can be placed in front of the cocci and bacilli forms (usually not used with the others)

36. Micrococcuscoccus bacteria

37. Salmonellabacilli bacteria

38. Spiroplasmaspirilla bacteria

39. Diplococcus

40. staphlococcus

41. streptococcus

42. Bacterial structure • Do not have organs which are bounded by membranes: nucleus, mitochondria, chloroplasts, ER, Golgi complex, lysosomes, etc. • Have ribosomes, storage granules • Cell membrane may be extensively folded to accommodate enzymes for respiration and/or photosynthesis

43. Bacteria may be classified as Gram-positive or Gram-negative • 1888 Christian Gram (Danish physician) developed the procedure • Gram-positive bacteria: absorb and retain crystal-violet stain • Gram-negative bacteria: do not absorb this stain—instead they are stained with an alternative stain that makes them appear reddish.

44. Cell walls in eubacteria Gram-positive Very thick peptidoglycan Gram-negative Thin layer of peptidoglycan Outer membrane Capsule Surrounding the cell wall in some bacteria: adds additional protection

45. Gram-positive cell wall

46. Gram-negative cell wall

47. Importance in treating diseases • Some antibiotics will only work against Gram-positive or Gram-negative bacteria • Penicillin attacks peptidoglycan, destroying the cell wall. It is much more effective against gram-positive bacteria.

48. Importance of capsules • Helps protect against phagocytosis by host’s white blood cells • Streptococcus pneumoniae that lacks a capsule does not cause disease, but a strain of the same bacterium with a capsule, does cause disease. • May use capsules to attach or adhere to surfaces (rocks, plant roots, human teeth)

49. Pili • Protein structures that extend from the cell • Help bacteria adhere to surfaces • Elongated pili (called sex-pili are involved in DNA exchange between bacteria • Flagella • Produce a rotary motion • Basal body, hook, and filament

50. Bacterial pili