Viruses

1. Viruses are classified as acellular pathogens. What does this mean?

Answer: They are not made of cells.

2. Why are viruses considered obligate intracellular parasites?

Answer: They must infect a host cell to replicate.

3. What microscope is required to visualize most viruses and why?

Answer: Electron microscope; viruses are too small for light microscopes.

4. A virus genome can consist of which of the following?

Answer: DNA or RNA, never both.

5. What is the function of the viral capsid?

Answer: Protects the viral genome and aids in attachment.

6. What are capsomeres?

Explanation: Capsids are constructed from repeating capsomere proteins

7. Viral spikes are composed of what molecules and what is their function?

Answer: Glycoproteins; attachment to host cell receptors.

8. What is a viral envelope and where does it come from?

Answer: A phospholipid membrane derived from the host cell.

9. What is a naked virus?

Answer: A virus without an envelope.

10. Identify the three major viral shapes.

Answer: Helical, icosahedral, complex.

11. Why are bacteriophages classified as complex viruses?

Answer: They have a head, tail, and tail fibers.

12. Why do viruses have far fewer genes than cells?

Answer: They rely on host cell machinery.

13. List the six steps of the animal virus infection cycle in order.

Answer: Attachment → Entry → Uncoating → Biosynthesis → Assembly → Release

14. What is required for viral attachment to a host cell?

Answer: Viral spikes and host cell receptors.

15. Define a susceptible host cell.

Answer: A cell with the correct receptor for the virus.

16. What is meant by broad host range?

Answer: Ability to infect many species.

17. What is tissue tropism?

Answer: Preference for specific tissues or cell types.

18. Name the two ways animal viruses enter host cells.

Answer: Receptor-mediated endocytosis and membrane fusion.

19. What occurs during uncoating?

Answer: Removal of the viral capsid.

20. Why is biosynthesis impossible without a host cell?

Answer: Viruses lack ribosomes and enzymes.

21. How are naked viruses typically released from cells?

Answer: Cell lysis.

22. How are enveloped viruses released and why is this advantageous?

Answer: Budding; it does not immediately kill the host cell.

23. Why do plant viruses enter cells differently than animal viruses?

Answer: Plant cells have rigid cell walls.

24. Why can plant viruses not be released by lysis or budding?

Answer: Presence of a cell wall.

25. What are viruses that infect bacteria called?

Answer: Bacteriophages.

26. Name the two bacteriophage replication cycles.

Answer: Lytic and lysogenic.

27. What happens during the lytic cycle?

Answer: Host cell is lysed and dies.

28. What defines the lysogenic cycle?

Answer: Viral DNA integrates into host DNA.

29. What is a prophage?

Answer: Integrated bacteriophage DNA.

30. What is induction?

Answer: Transition from lysogenic to lytic cycle.

31. What is required to grow any virus?

Answer: A host cell.

32. How are bacteriophages grown in the lab?

Answer: Using bacteria and melted agar.

33. What is a plaque and what does it indicate?

Answer: A clear area where bacteria were killed.

34. Name three ways animal viruses can be cultivated.

Answer: Living animals, embryonated eggs, cell culture.

35. Why are embryonated eggs used for influenza virus?

Answer: They provide ideal viral replication conditions.

36. What is a primary cell culture?

Answer: Cells with a finite lifespan.

37. What is a continuous cell culture?

Answer: Immortal cancer-derived cells.

38. How do you detect viral growth in cell culture?

Answer: Cytopathic effects.

39. What is a viroid?

Answer: A small circular RNA molecule without a protein coat.

40. How do prions differ from viruses?

Answer: Prions are infectious proteins with no nucleic acid.