MCB100 INTRODUCTORY MICROBIOLOGY Spring 2019 Viruses Reading Assignment: Chapter 13

1. MCB100 INTRODUCTORY MICROBIOLOGYSpring 2019 Viruses Reading Assignment: Chapter 13 - Lytic vs. Lysogenic (Latent) Lifestyle – Bacteriophage l - Life cycle of a + strand RNA virus: polio virus - Life cycle of a retrovirus: HIV - Some significant Human Viral Diseases

2. LON-CAPA in MCB100 Homework Set 9 is due: Friday, April 26 at 11 PM (read chapter 14) Homework Set 10 is due: Wednesday, May 1 at 11 PM (read chapters 15 and 16) Homework Set XI “make-up” due: Wed. May 1, 11 PM Extra Credit Set – 3 due: Wed. May 1, 11 PM The make-up homework set (XI), is not “extra credit”. There are 10 regular homework sets and the make –up folder. Each set is worth up to 10 points. Of these 11 homework folders, I drop your lowest score. If you missed a homework set, you should definitely do the“make-up” set. If you got 10s on all 10 regular homework sets you can’t gain more points by doing the “make-up” set, but it’s okay to do the problems for practice.

3. MCB100 Final Exam Monday, 5/6/19 8:00 – 11:00 AM Last name A - N: Room 112 Gregory Hall Last name O – Z: Room 149 NSRC Conflict Exam IF you have a legitimate excuse, contact Dr. Chapman to sign up. Friday, 5/3 at 8:00 – 11:00 AM Monday 5/6 at 1:30 – 4:30 PM or 7 - 10 PM Tuesday, 5/7 at 8:00 – 11:00 AM in Room 242 Burrill Hall

4. Viruses cannot reproduce unless they are inside a suitable host cell. A suitable host cell must have a receptor that the virus can attach to so it can enter the cell. A suitable host cell must also have all of the enzymes that the virus needs for replicating it’s genetic material and making viral proteins. Viruses are very specific in their host range. Every type of cellular organism can be infected by some type of virus, but a given virus will infect only specific hosts. Viruses always have a limited host range. Viruses that infect plants don’t bother animals. Viruses that infect bacteria (bacteriophages) don’t invade eukaryotic cells.

7. Left Viruses are much smaller than Bacteria or Protozoa. Right Virus capsids often, but not always, Have an icosahedral shape. This is a 20-sided figure with each side being an equilateral triangle. The capsid of the polio virus consists of 60 moleculesof coat protein, each face is made of 3.

8. Some viruses have an envelope that covers the capsid. (Others don’t.) The viral envelope is a lipid bilayer with embedded proteins but it is not an active cytoplasmic membrane. Viruses do not absorb nutrients or excrete wastes when they are outside a host cell. Viruses that are outside host cells have no metabolism and can’t repair themselves if they are damaged. Naked viruses include: Polio Virus Rhino Viruses Enveloped viruses include: HIV Influenza Yellow Fever Fig. 13.02

9. Tobacco Mosaic Virus, which causes a disease in plants, is a helical RNA virus, the coat proteins cover the genomic RNA.

10. Bullet shaped rabies virus

11. A bacteriophage, or phage, is a virus that attacks a bacterial host. In this example the bacteria are spread thickly across the surface of the mediumcreating a lawn of cells rather than isolated colonies. A phage kills bacteria creating a hole in the lawn, called a plaque. “Phago-“ means to eat or swallow . Bacterial viruses are called phages because it looks like something is eating the bacteria making plaques.

12. Bacteriophage T4 attacks Escherichia coli.See figures 13.6 and 13.8 in your textbook.

13. Viruses 1 – General Virology Which one of the following options describes a trait of ALL viruses? A. All viruses are parasites of humans or animals. B. All viruses contain 70S ribosomes within the capsule. C. For all viruses, the capsule is surrounded by an envelope that is a lipid bilayer. D. For all viruses, the genetic material is DNA. E. At some point during their life cycle virus-specific mRNAs must be translated by host cell ribosomes to make viral proteins.

14. Viruses 1 – General Virology Which one of the following options describes a trait of ALL viruses? A. All viruses are parasites of humans or animals. B. All viruses contain 70S ribosomes within the capsule. C. For all viruses, the capsule is surrounded by an envelope that is a lipid bilayer. D. For all viruses, the genetic material is DNA. E. At some point during their life cycle virus-specific mRNAs must be translated by host cell ribosomes to make viral proteins. The genetic material within a virus particle is surrounded by a capsid of proteins.

15. Figure 13.8 When T4 reproduces, the progeny phage particles exit the old host cell by causing the cell to lyse (break open), hence the term “lytic cycle”.

16. Bacteriophage lambda (l) can reproduce and kill E.coli (lytic cycle) or it can become dormant within the host cell (lysogeny). Lysogenic Replication Cycle

18. In the lytic cycle the virus actively grows and will kill the cell when progeny viruses break out.

19. In the lysogenic cycle, the virus remains dormant and the host cell continues to grow and divide.

20. The Life Cycle of Bacteriophage lambda Lysogeny vs. Lytic Cycle How is the decision made? When the lambda DNA first gets into an E. coli cell, both regulatory factors that are required to activate transcription of phage genes and the lambda repressor protein are made. It’s a race to see if the phage DNA is first repressed and enters the lysogenic cycle or is actively transcribed so it enters the lytic cycle. When a lambda lysogen encounters harsh conditions, the lambda repressor protein may be degraded. This causes activation of the prophage genes. Viral DNA is excised from the host chromosome and lambda enters the lytic cycle.

21. Viruses 2 - Bacteriophages Which one of the following statements about bacteriophages is TRUE? A. A bacteriophage is a bacterium that eats viruses. B. All bacteriophages have a latent (lysogenic) cycle. C. A bacteriophage is a virus that invades a bacterial cell and uses it as a host. D. The genetic material in all bacteriophages is double-stranded DNA. E. Bacteriophages are much smaller and simpler than the viruses that attack human or animal cells.

22. Viruses 2 - Bacteriophages Which one of the following statements about bacteriophages is TRUE? A. A bacteriophage is a bacterium that eats viruses. B. All bacteriophages have a latent (lysogenic) cycle.C. A bacteriophage is a virus that invades a bacterial cell and uses it as a host. D. The genetic material in all bacteriophages is double-stranded DNA. E. Bacteriophages are much smaller and simpler than the viruses that attack human or animal cells.

23. Human and Animal Viruses Some human viruses are always in a lyticcycle. These viruses kill human cells. Examples include: Rhinoviruses – small RNA viruses that cause the common cold Enteroviruses – small RNA viruses that cause viral diarrhea Hepatitis A – an RNA virus that attacks liver cells Hemorrhagic fever viruses, such as Yellow Fever and Ebola virus Some viruses may become latent (dormant) within human cells. These viruses have a stage in their life cycle that includes a DNA copy of their genome. They may disrupt the regulation of cell division and possibly cause cancer. Examples include: Retroviruses – RNA viruses that replicate via DNA intermediates Human Immunodeficiency Virus (HIV) Some DNA viruses – Herpes viruses, Chickenpox, Hepatitis B

24. Exiting the old host cell by lysis. Some animal viruses exit a host cell by lysis. These viruses generally do not have an envelope.

25. Exiting the old host cell by budding. Some animal viruses exit a host cell by budding. Unlike lysis, budding doesn’t necessarily kill the host cell. The viral envelope is derived from the cytoplasmic membrane of the old host cell, with some viral proteins embedded within it. Viral glycoproteins mark an infected cell with foreign antigens.

26. An Example of a Human Virus that exits a host cell by lysis Polio Virus - Classified in the Picornaviridae family - non-enveloped, + strand RNA virus - enterovirus (entero = intestines) - Life cycle is similar to the T4 lytic cycle The Life Cycle of the Polio Virus 1. Host Encounters the Pathogen 2. Attachment to host cell 3. Entry into host cell 4. Synthesis of viral components, mRNA, proteins, genome 5. Assembly of new virus particles 6. Release from the old cell

27. Steps in a Viral Infection 1. Host Encounters the Pathogen The virus is ingested in contaminated water, or food, or otherwise enters the mouth. 2. Attachment The primary tissue tropism for polio is intestinal epithelial cells. Virus coat proteins stick to receptors on the surface of the host cell. 3. Entry Host cell cytoplasmic membrane encircles the virus enclosing it within a vesicle. Viral capsid breaks open depositing viral genomic RNA into the cytoplasm.

29. 4. Synthesis - Viral genomic RNA is translated by host ribosomes, making viral proteins. - The viral RNA-dependent RNA polymerase in the cytoplasm copies virus RNA. - A negative sense copy of the viral RNA is made. - The – sense strand of viral RNA is used as a template by the viral RNA polymerase to make new copies of the viral + strand RNA. 5. Assembly - Viral coat proteins spontaneously associate with viral + strand RNA. 6. Release - When the cell is full of virus particles it breaks open. New virus particles infect other cells within the host or are excreted in feces.

30. Polio Virus Life Cycle Polio is a fecal-oral cycle pathogen. Most cases show mild symptoms. Even in very mild cases an infected host sheds virus particles. The oral polio vaccine is a live, but attenuated or weakened, virus.

31. Polio – The Cause of Disease The death of infected cells is the root cause of disease. The symptoms of the primary infection are minor, possible diarrhea. Neurological symptoms that include paralysis develop when the virus is present in the body in high levels and secondary tissues are attacked. The virus prefers to attack intestinal epithelial cells but may invade nerve cells as a secondary tissue tropism. Death of nerve cells can lead to a loss of muscle coordination and paralysis. Only a small fraction of infections lead to paralytic disease.

33. Viruses 3 - Polio Which ONE of the following statements about polio or the poliovirus is FALSE? A. All cases of infection by the poliovirus lead to paralysis of some muscle groups. B. Polio is an example of a viral fecal-oral cycle disease. C. The primary tissue tropism for the poliovirus is intestinal epithelial cells. D. The replication of poliovirus genetic material requires a virus-specific RNA-dependent RNA-polymerase. E. Poliovirus particles exit an old host cell by causing the cell to lyse (break open).

34. Viruses 3 - Polio Which ONE of the following statements about polio or the poliovirus is FALSE? A. All cases of infection by the poliovirus lead to paralysis of some muscle groups. False. Actuallyparalysis symptoms are seen in about 1% of polio virus infections. Mild diarrhea is common. B. Polio is an example of a viral fecal-oral cycle disease. C. The primary tissue tropism for the poliovirus is intestinal epithelial cells. D. The replication of poliovirus genetic material requires a virus-specific RNA-dependent RNA-polymerase. E. Poliovirus particles exit an old host cell by causing the cell to lyse (break open).