Chapter 18

1. The Genetics of Virusesand Bacteria Chapter 18

2. Virus Bacterium Animalcell 0.25 m Animal cell nucleus Figure 18.2 Microbial Model Systems • Recall that bacteria are prokaryotes • With cells much smaller and more simply organized than those of eukaryotes • Viruses • Are smaller and simpler still

3. 0.5 m Figure 18.1 Viruses • Viruses called bacteriophages • Can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli

4. Obligate Intracellular Parasites • A virus has a genome but can reproduce only within a host cell

5. Figure 18.3 The Discovery of Viruses: Scientific Inquiry • Tobacco mosaic disease • Stunts the growth of tobacco plants and gives their leaves a mosaic coloration

6. TMV • In the late 1800s • Researchers hypothesized that a particle smaller than bacteria caused tobacco mosaic disease • In 1935, Wendell Stanley • Confirmed this hypothesis when he crystallized the infectious particle, now known as tobacco mosaic virus (TMV)

7. Viruses • Are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope • Viral genomes may consist of • Double- or single-stranded DNA • Double- or single-stranded RNA

8. RNA DNA Capsomere Glycoprotein 70–90 nm (diameter) 18  250 mm 20 nm 50 nm CAPSID (b) Adenoviruses Capsids and Envelopes • A capsid • Is the protein shell that encloses the viral genome TMV & Adenovirus

9. Capsids and Envelopes Envelopes • Membranous coverings derived from the membrane of the host cell

10. Viral Envelopes • Many animal viruses • Have a membranous envelope • Viral glycoproteins on the envelope • Bind to specific receptor molecules on the surface of a host cell

11. Bacteriophages • A.K.A. phages • Have the most complex capsids found among viruses

13. General Features of Viral Reproductive Cycles • Viruses are obligate intracellular parasites • They can reproduce only within a host cell • Each virus has a host range • A limited number of host cells that it can infect

14. VIRUS DNA Capsid HOST CELL Viral DNA mRNA Viral DNA Capsid proteins General Features of Viral Reproductive Cycles • Viruses use enzymes, ribosomes, and small molecules of host cells to synthesize progeny viruses

15. Viral Reproductive Mechanisms • Lytic cycle Is a phage reproductive cycle that culminates in the death (lysis) of the host • Lysogenic cycle Replicates the phage genome without destroying the host

17. Lytic Cycle (Viral Reproduction) DOCKING with the host receptor protein PENETRATION of the viral nucleic acid into the host cytoplasm (Restriction Endonucleases, A.K.A. restriction enzymes break up host DNA) BIOSYNTHESIS of the viral components Assembly (MATURATION) of the viral components into complete viral units RELEASE of the completed virus from the host cell

18. Phage assembly Head Tail fibers Figure 18.6 Tails

19. Lysogenic Cycle Lambda • Temperate phages Are capable of using both the lytic & lysogenic cycles of reproduction

20. Prophage When viral DNA is integrated into the bacterial chromosome (Plasmid)

21. Capsid RNA Envelope (with glycoproteins) HOST CELL Viral genome (RNA) Template mRNA Capsid proteins ER Copy of genome (RNA) Glyco- proteins

22. Glycoprotein Viral envelope Capsid RNA(two identicalstrands) Reversetranscriptase HIV • Retroviruses, such as HIV, use the enzyme reverse transcriptase • To copy their RNA genome into DNA, which can then be integrated into the host genome as a provirus (Integrates into host DNA) 2 of each

23. Evolution of Viruses • Viruses do not really fit our definition of living organisms since viruses can reproduce only within cells • They probably evolved after the first cells appeared, perhaps packaged as fragments of cellular nucleic acid

24. Viral Diseases in Animals • Viruses may damage or kill cells • By causing the release of hydrolytic enzymes from lysosomes • Some viruses cause infected cells • To produce toxins that lead to disease symptoms

25. Viral Diseases in Animals • Viruses may damage or kill cells (Amount of damage depends on the ability of infected tissue to regenerate by mitosis) • -Respiratory tract epithelium repairs quickly from adenovirus infection • - Nerve tracts affected by polio virus is permanent • Find host cells using “lock & key” fit with proteins on virus & host cell receptors

26. Prions • Protein infectious particles • Contain no RNA or DNA • Long incubation period (~10 years)

27. Originalprion Prion Many prions Normalprotein Newprion Prions Prions • Are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals • Propagate by converting normal proteins into the prion version

29. Emerging Viruses • Are those that appear suddenly or suddenly come to the attention of medical scientists • 3 processes contribute to emerging viruses • Mutation of existing viruses as RNA is not corrected by proofreading e.g. SARS • Spread from one host species to another e.g. Hanta Virus • Dissemination from a small isolated population e.g. HIV

30. (b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope. (a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS. Figure 18.11 A, B SARS – Severe Acute Respiratory Syndrome

31. Emerging Viruses are NOT new • They are existing viruses that • Mutate • Spread to new host species • Disseminate more widely in the host species

33. Small Pox

34. Polio Polio

35. Herpes Simplex

36. Hepatitis

37. Varicella Zoster

38. Mumps

39. Measles - Rubeola

40. Other Viruses that affect humans • Influenza Virus • Rubella • Parvo-virus • Epstein Barr Virus • Hanta Virus • (HPV) Human Papilloma Virus • (RSV) Respiratory Syncytial Virus • Rabies • Rhinovirus • Rotavirus • West Nile Virus

41. Viral Diseases in Plants • More than 2,000 types of viral diseases of plants are known • Common symptoms of viral infection include • Spots on leaves and fruits, stunted growth, and damaged flowers or roots

42. Viral Diseases in Plants • Plant viruses spread disease in two major modes • Horizontal transmission, entering through damaged cell walls • Vertical transmission, inheriting the virus from a parent

43. Viroids -The Simplest Infectious Agent • Are circular RNA molecules that infect plants and disrupt their growth

44. Bacteria • Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria • Bacteria allow researchers • To investigate molecular genetics in the simplest true organisms

45. The Bacterial Genome and Its Replication • The bacterial chromosome • Is usually a circular DNA molecule with few associated proteins • In addition to the chromosome • Many bacteria have plasmids, smaller circular DNA molecules that can replicate independently of the bacterial chromosome

46. Replicationfork Origin of replication Termination of replication Binary Fission The Bacterial Genome and Its Replication • Bacterial cells divide by binary fission • Which is preceded by replication of the bacterial chromosome

47. Mutation and Genetic Recombination as Sources of Genetic Variation • Since bacteria can reproduce rapidly • New mutations can quickly increase a population’s genetic diversity • Genetic diversity • Can also arise by recombination of the DNA from two different bacterial cells • Remember that prokaryotes don’t undergo meiosis or fertilization

48. Recombination in Bacteria • Three processes bring bacterial DNA from different individuals together • Transformation • Transduction • Conjugation

49. Transformation Is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment

50. Transduction Phages carry bacterial genes from one host cell to another