viruses

1. viruses Campbell and Reece Chapter 19

2. Virus • an infectious particle incapable of replicating outside of a cell, consisting of RNA or DNA genome surrounded by a protein coat (capsid) & for some viruses a membranous envelope

3. SEM of Phage Attack on E. coli

4. Discovery of Viruses • 1883: German scientist, Mayer, discovered that he could transmit tobacco mosaic disease (stunts growth of tobacco plant) by rubbing sap of affected plant on healthy plant

5. Discovery of Viruses • Mayer then hypothesized there was an unusually small bacteria that could not be seen with microscope • 10 yrs later Ivanowsky put sap of infected plant thru filter designed to remove all bactreria ….filtrate still caused healthy plants to get disease

6. Discovery of Viruses • next hypothesis: bacteria produce a toxin that causes disease so it would have been in filtered sap ….Beijerinck proved the filtered sap contained infectious agents that reproduced but only inside host it infected • Beijerinck credited with being 1st to describe concept of virus • 1935: Stanley crystallized the virus

7. Tobacco Mosaic Virus

8. Viral Genomes • DNA: dbl stranded • DNA: single stranded • RNA: single stranded • RNA: dbl stranded

9. Viral Genomes • smallest have 4 genes • largest several 100 • named as DNA virus or RNA virus

10. Capsid • protein shell enclosing genome • shapes: • rod • polyhedral • icosohedral

11. Viral Envelopes • membranous accessory structure • derived from host cell membranes • + proteins & glycoproteins of viral origin

12. Bacteriophages • virus that infects bacteria • 1st 7 identified infected E. coli • named Type 1 –Type 7 • Types 2, 4, & 6 very similar:

13. Structure of Viruses • smallest virus 20 nm in diameter (smaller than a ribosome) • largest virus several 100 nm barely visible with light microscope

15. Host Cells • each particular virus can only infect cells of limited # of host species called the host range of the virus • specificity result of viral recognition system • most: protein on viral surface fits into receptor of host cell

16. Simplified Viral Replicative Cycle • 1. virus enters cell & is uncoated releasing viral DNA & capsid proteins • 2. host enzymes replicate the viral genome • 3. different host enzymes transcribe the viral genome into viral mRNA  translated by host ribosomes to make viral proteins • 4. viral genomes & capsid proteins self-assemble into new virus particles which exit cell

18. Replication of RNA Viruses • use virally encoded RNAP that can use RNA as a template

19. Lytic Cycle

20. Lytic Cycle • culminates in death of host cell • phages that replicate only by lytic cycle called virulent phage

21. Bacterial Defenses • natural selection will favor mutants having receptors that no longer allow phage to attach • viral DNA may be recognized as foreign  cut up by bacterial enzymes called restriction enzymes (restrict ability of phage to infect bacterium)

22. Phage Natural Selection • allows phage mutants that can bind to altered receptors or are resistant to particular restriction enzyme • so this parasite-host relationship in constant evolutionary flux

23. The Lysogenic Cycle

24. Lysogenic Cycles • does not destroy the host • phages capable of using both modes of replication called temperate phages • Λ phage (lambda) used widely in biological research

25. Prophage • viral DNA that has been integrated into bacterial loop of DNA • when that bacterium replicates the viral DNA is passed on to all daughter cells & so on & so on • when λ genome induced to leave the loop of bacterial DNA  lytic cycle & cell (bacterium) dies

26. Phage Genes in Bacteria • diptheria, botulism, & scarlet fever would not be so harmful to humans w/out certain prophage genes that cause the host bacteria to make toxins • difference between E.coli that lives in out GI tract (no problem) & the one that’s found in food poisoning: presence of prophages

27. Animal Viruses • nature of viral genome basis for classification of viruses that infect animals: • DNA: dbl stranded • dsDNA • DNA: single stranded • ssDNA • RNA: dbl stranded • dsRNA • RNA: single stranded • ssRNA • RNA: template for mRNA synthesis • ssRNA template for mRNA synthesis • RNA: template for DNA synthesis • ssRNA template for DNA synthesis

28. Classes of Animal Viruses

29. Animal Viruses vs. Phages Animal Viruses Phages • many have both envelope & RNA • some with DNA also have envelope • few have envelope or RNA

30. Viral Envelopes • outer membrane around capsid • used to enter host cell • viral glycoproteins protrude that will bind to specific receptors on surface of host cell

31. Replicative Cycle of an Enveloped RNA Virus

32. RNA as Viral Genetic Material • includes most plant viruses & some RNA viruses • broadest variety infect animals

33. RNA Viruses • Class IV: • genomes can serve directly as mRNA  immediately after infection can translate viral proteins • Class V: • genome serves as template for mRNA synthesis • C’ strands of RNA made which serve as templates for both mRNA & new RNA strands

35. RNA Viruses • Class VI: • retroviruses • have enzyme: reverse transcriptase transcribes RNA template  DNA (opposite normal direction of information flow) • HIV a retrovirus: enveloped with 2 identical molecules ssRNA & 2 reverse transcriptase

36. Replication Cycle of HIV

37. Replication of HIV 1. envelope glycoproteins allow virus to bind to specific receptors on certain WBCs 2. virus fuses with cell’s plasma membrane & capsid proteins removed  viral RNA & proteins 3. reverse transcriptase catalyzes synthesis of a dsDNA strand c’ to the virus’s RNA

38. Replication of HIV 4. reverse transcriptase catalyzes synthesis of 2nd DNA strand c’ to the 1st 5. dsDNA incorporated as a provirus into host cell’s genome 6. proviral genes transcribed into ssRNA which are genomes for next generation & as mRNAs for translation into viral protein

39. Replication of HIV 7. viral proteins include capsid proteins & reverse transcriptase (made in cytosol) & envelope glycoproteins (made in ER) 8. vesicles transport the glycoproteins to host cell plasma membrane 9. capsids assemble around viral genome + reverse transcriptase molecules 10. new viruses bud off from host cell

41. Evolution of Viruses • there are viruses that infect every known form of life • use same universal genetic code as all living things • probable that viruses evolved after the 1st cells appeared

42. Evolution of Viruses • most accepted hypothesis: evolved from naked bits of nucleic acids that moved from 1 cell  another • possibly plasmids or tranposons  1st virus • transposon: a transposable element that moves w/in a genome by means of a DNA intermediate

43. Plasmids • small, circular DNA molecules found in bacteria & in unicellular yeast (eukaryotic) • replicate independently of genome • occasionally transferred between bacteria

44. Viruses, transposons, & plasmids are all mobile genetic elements • virus may have more in common genetically with its host cell than with other viruses that infect same species

45. Mimivirus • largest virus to date (size of small bacterium) • dsDNA • mimi: mimicking microbe • ~1,000 genes: some code for proteins used in translation, DNA repair, protein folding, & polysaccharide synthesis

48. Viruses, Viriods, & Prions • viruses cause disease in all life forms • viriods cause disease in plants • prions cause disease in animals

49. Viral Disease in Animals • How viruses cause disease: • damage or kill cells by release of hydrolytic enzymes from lysosomes • cause host cell to produce toxins • have molecular components (in envelope) that are toxic to host • many of symptoms ass’c with viral infection result of immune system reacting to infection (fever, chills, aches)

50. Vaccines • harmless variant or derivative of a pathogen that stimulates a host’s immune system to mount defenses against the pathogen