1. VIRUSES 20.1 AHHH CHHOOO – You are SOOOO Good Looking !

2. Viruses Objectives: Describe the structure of viruses. Discuss the two methods by which viruses infect living cells.

3. Introduction to Viruses Tobacco mosaic virus –Occurred 100 years ago –Seriously threatened the tobacco crop –No visible agent could be determined Martinus Beijerinck named viruses –Discovered tiny particles in the juice Virus – comes from the Latin word for “poison”

4. What is a Virus? Wendell Stanley (1935) – American biochemist that discovered the nature of a virus and chemically isolated it (D.Ivanovski – demonstrated the cause of the plant disease was in the liquid extracted from the plants – but could not pin down the culprit) Virus – a nonliving particle made up of nucleic acids and protein that can invade cells (sometimes lipids) –Can only reproduce INSIDE a LIVING CELL

5. Structure of a Virus Typical virus is composed of: –Core – nucleic acid (either DNA or RNA) –Capsid – protein coat that protects the core (some viruses have an envelope – addition membrane that surrounds the capsid) –Several to several hundred genes

6. Bacteriophage (T4) – type of virus that infects bacteria –Head region – nucleic acid + capsid –Tail region – used to attach to host

7. Characteristics Small (10-400 nanometers) electron micro. No nucleus, cytoplasm, or membrane No cellular functions Only reproduce inside a living host cell –Specificity – viruses can only infect specific hosts (animal viruses cannot infect plants, etc.) Come in a variety of shapes –Rod, cube, helical Some have ENVELOPES, which help virus enter host

9. Viral Infections Lytic Infection – Active/Virulent Lysogenic Infection – Latent/Temperate

10. Life Cycle of a Lytic Virus Lytic Cycle– process by which host cell is invaded, burst, and destroyed by a virulent virus Virulent Virus – immediately disease- causing

11. Life Cycle of a Lytic Virus (cont’d) 1.ATTACHMENT – Virus attaches to cell wall (specific spot, “lock & key” 2.ENTRY – Viral nucleic acid enters cell through weak spot in cell wall 3.REPLICATION – viral DNA take over replication, make new viral DNA, “hijack” 4.ASSEMBLY – new virus particles are put together 5.LYSIS/RELEASE – cell wall digested, new virus is liberated Lytic Cycle

13. Life Cycle of a Lysogenic Virus Lysogenic Cycle – process by which viral DNA is incorporated into a host cell’s DNA where it lies dormant for many generations before becoming active Temperate Virus – not immediately disease causing Prophage – viral DNA that has been incorporated into host

14. Life Cycle of a Lysogenic Virus (cont’d) A phage enters a cell and remains inactive until an external stimulus (i.e. change in temperature, availability of nutrients, etc.) causes the phage to become virulent Lysogenic Cycle

15. Lytic and Lysogenic Cycle

17. Retrovirus RETROVIRUS– RNA virus that copies its information backwards (RNA into DNA), i.e. HIV Uses enzyme reverse transcriptase to make DNA from an RNA template HIV Virus