1. Viruses

2. Tiny 1/2 to 1/100 the size of smallest bacterium Nonliving Do not fulfill the criteria for life Do not carry out respiration, grow or move Can only reproduce inside a host cell Tiny 1/2 to 1/100 the size of smallest bacterium Nonliving Do not fulfill the criteria for life Do not carry out respiration, grow or move Can only reproduce inside a host cell Virus means poison in Latin

4. Viral Structure Have a nucleic acid (DNA or RNA) Surrounded by a protein coat called a capsid Shape of viral coat matches the shape on its specific host cell

5. Shape important to infection

7. Lytic Cycle

8. Lytic Cycle Cont. Virus attaches to a host cell Injects its nucleic acid into host Host DNA is destroyed Viral genes are copied Viral particles are assembled Host breaks open, viruses are released Virus attaches to a host cell Injects its nucleic acid into host Host DNA is destroyed Viral genes are copied Viral particles are assembled Host breaks open, viruses are released

9. Lytic Cycle

10. Lysogenic Cycle

11. Lysogenic Cycle Cont. Attaches to host, injects DNA, now becomes a prophage Normal activities of the cell continue Prophage replicates with host cell Emerges out of host DNA to begin lytic cycle Or, can remain in this cycle and continue to replicate with host cell Attaches to host, injects DNA, now becomes a prophage Normal activities of the cell continue Prophage replicates with host cell Emerges out of host DNA to begin lytic cycle Or, can remain in this cycle and continue to replicate with host cell

12. Lysogenic Cycle

13. Prophage Viral DNA inserted into the host cell chromosome Does not interfere with normal functioning of the cell Replicates along with host cell DNA May remain dormant for years Prophage can enter the lytic cycle at any time, and no one knows what causes this. Viral DNA inserted into the host cell chromosome Does not interfere with normal functioning of the cell Replicates along with host cell DNA May remain dormant for years Prophage can enter the lytic cycle at any time, and no one knows what causes this.

14. Examples of Lysogenic Viruses Herpes Chicken Pox HIV Herpes Chicken Pox HIV

15. Retroviruses Many viruses, such as HIV, have RNA as their nucleic acid A molecule of reverse transcriptase is injected along with the nucleic acid Reverse transcriptase copies viral RNA into DNA

16. Viroids and Prions Viroids – single stranded RNA molecules that have no surrounding capsid; causes plant diseases Prions – do not contain a nucleic acid, only a protein; causes diseases in animals Viroids – single stranded RNA molecules that have no surrounding capsid; causes plant diseases Prions – do not contain a nucleic acid, only a protein; causes diseases in animals

17. Polio

18. Influenza Virus Virus that causes the flu

19. Small Pox

20. TMV Tobbaco Mosaic Virus

21. Review Question #1 Why is a virus not considered to be alive?

22. Review Question #2 What is the difference between a lytic and lysogenic cycle?

23. Review Question #3 Why can most viruses infect only a few kinds of cells?

24. Review Question #4 Describe the state of a herpes virus in a person who had cold sores several years ago but who does not have any now.