Genetics of Viruses and Bacteria

1. Genetics of Viruses and Bacteria

2. Bacterium Virus Animal cell 0.25 µm Animal cell nucleus Virus Size • Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes • Viruses are smaller and simpler than bacteria

3. The Discovery of Viruses: Scientific Inquiry • Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration • In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease • In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)

4. Viruses: “Non-living” Entities • Small packages of nucleic acids in a protein coat • Are NOT cells—no cytoplasm and do not perform metabolic reactions • Obligate intracellular parasites—dependent upon other cells for replication

5. Types of Viruses • DNA viruses • Genome is DNA • RNA viruses • Genome is RNA • Smaller than DNA viruses • Lack of proofreading—leads to  evolution rate

6. Viruses have a Specific Host Range • Recognition by complementary fit between external viral protein and specific cell surface receptor sites http://pathmicro.med.sc.edu/mhunt/rep1.jpg

7. Bacteriophages • Viruses that infect bacteria • Set in motion a genetic takeover of bacteria, such as Escherichia coli

8. Viruses Replicate Inside Living Cells • Obligate intracellular parasites • Viruses lack enzymes needed for metabolism and have no structures to make proteins • Use cells own machinery to replicate viruses • Pathogen: agent that causes disease • Viruses damage cells during replication

9. Reproductive Cycles of Phages • Phages are the best understood of all viruses • Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

10. Lytic Cycle Virulent Viruses-reproduce only by lytic cycle

11. Temperate Viruses-reproduces by lytic cycle or lysogenic cycle

12. Reproductive Cycles of Animal Viruses • Two key variables in classifying viruses that infect animals: • DNA or RNA? • Single-stranded or double-stranded?

17. Animal Viruses • Glycoproteins on viral envelope recognize/bind specific receptors on host cell • Viral envelope fises with cell’s plasma membrane, and the capsid and viral genome enter the cell • Cellular enzymes remove capsid • Viral genome serves as template for replication of RNA strands • Templates for new RNA • Serve as mRNA for protein synthesis • Vesicles transport glycoproteins to cell’s plasma membrane • Capsid forms around viral genome • Virus buds from the cell

18. After entering the cell, viral DNA uses host nucleotides and enzymes to replicate itself It uses host materials and machinery to produce capsid proteins Viral DNA and capsid proteins assemble into new virus particles, which leave the cell

19. RNA Protein Figure 10.19 Plant Viruses • Plant viruses are serious agricultural pests • Most plant viruses • Have RNA genomes • Enter their hosts via wounds in the plant’s outer layers • Injuries, insects feeding, contaminated farming tools • Once infected, virus spreads through plasmodesmata

20. RNA as Viral Genetic Material • The broadest variety of RNA genomes is found in viruses that infect animals • Retroviruses use reverse transcriptase to copy their RNA genome into DNA • HIV is the retrovirus that causes AIDS

21. Genetic flow : • RNA  DNA • 2 identical strands ofRNA • Infects white blood cells

22. HIV virus

23. Vaccinations • Antibiotics don’t work—no metabolic reactions to interfere with • Vaccines—harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen • Parts of viruses, modified or killed viruses are injected into the body • Allows immune system to make antibodies against specific markers on the viral coat • HIV mutates too fast for immune system to keep up with

24. Influenza Vaccine • Influenza, also known as the flu, is a contagious disease that is caused by the influenza virus.  It attacks the respiratory tract in humans (nose, throat, and lungs).  The flu is different from a cold.  Influenza usually comes on suddenly and may include these symptoms: • Fever • Headache • Tiredness (can be extreme) • Dry cough • Sore throat • Nasal congestion • Body aches

25. Emerging Viruses • Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists • Severe acute respiratory syndrome (SARS) recently appeared in China • Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

26. LE 18-11 The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glyco-protein spikes protruding form the envelope. Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.

27. Emerging Viruses • RNA viruses have unusually high mutation rate • Spread of virus from one host species to another • Dissemination of a virus from a small, isolated populations to widespread epidemics

28. Global View of HIV epidemicas of 2008 http://www.who.int/hiv/facts/en/hiv_global2003sm.jpg

29. Viruses and Cancer • Tumor viruses can transform cells into cancerous cells

30. Viruses and Cancer • Virus inserts viral nucleic acids into host cell DNA • Insertion is permanent-provirus never excises • Insertion for DNA tumor viruses straightforward • Oncogenes-genes found in viruses or as part of normal eukaryotic genome; trigger transformation of a cell to a cancerous state • Usually more than one must be activated to transform a cell

31. Viroids • Smaller and simpler than viruses • Small, naked, circular RNA molecules that do not code for proteins • Disrupt normal plant metabolism, development, and growth by causing errors in gene regulation • Affect many commercial plants—tomatoes, potatoes, chrysanthemums • Thought to have originated from escaped introns—sequences similar to self-splicing introns

32. Chrysanthemum with chrysanthemum chlorotic mottle viroid Green tomato infected with tomato spotted wilt virus

33. Prions • Pathogens that are proteins • Cause several degenerative brain diseases (Scrapie in sheep, “Mad Cow” disease, Creutzfeldt-Jakob disease)

34. Prions: Hypothesis for Propagation

35. INFECTIOUS PRION PROTEINS have a different shape, which they impose on normal prion proteins in a chain reaction that ends in sickness and death.

36. A hypothesis of how infectious protein particles, or prions, cause disease: PrPSc - an abnormal protein - communicates with its normal twin - PrPc - creating an abnormal form, that will eventually harm neurons. (Adapted by Leigh Coriale Design and Illustration, with permission, Science [July 12], 1996, American Association for the Advancement of Science.)

37. Normal Brain Kuru Infected Brain It exists only among a single tribe in Papua New Guinea. The afflicted tribe - the Fore Highlanders - describe it as the "laughing death", because it leads to loss of coordination accompanied by dementia.

38. Kuru Brain

40. Genetics of Bacteria

41. The Bacterial Chromosome • One double-stranded, circular molecule of DNA • Located in nucleoid region, so transcription and translation can occur simultaneously • Many also contain extrachromosomal DNA in plasmids

42. Binary Fission

43. Genetic Recombination Produces New Bacterial Strain • Transformation • Transduction • Conjugation Gene transfer occurs separately from bacterial reproduction

44. Transformation • Alteration of bacterial cell’s genotype by uptake of naked, foreign DNA from the environment

45. Transformation • Biotech companies use this technique to artificially introduce foreign genes into bacterial genomes (human insulin, human growth hormone)

46. Transduction • Gene transfer from one bacterium to another by a bacteriophage

47. Plasmids • Short, circular DNA molecules outside the chromosome • Carry genes that are beneficial but not essential • Replicate independently of chromosome • Episomes—plasmids that can be incorporated into chromosome

48. “female” “male” F- F+ Sex pili Conjugation • Direct transfer of genetic material between bacterial cells that are temporarily joined (bacterial sex) “Maleness” results from presence of F factor—segment of DNA in chromosome or in F plasmid

49. Conjugation