1. 1 NOTES: 20.1 - VIRUSES

2. 2  Vocabulary –Virus –Capsid –Lytic cycle –Lysogenic cycle –Retrovirus  Key Concepts –What is a virus? –How do viruses reproduce? –How do viruses make us sick? –Importance of viruses

3. 3 What is a virus?  viruses are non- cellular particles made up of genetic material (DNA or RNA… not both) and protein that invade living cells

4. 4 Are Viruses Alive?  like living things, viruses have genetic material and they can pass this on to future generations  BUT, unlike living things, viruses are NOT made of cells  outside of a host cell, they don't reproduce, feed, or grow  therefore, most biologists consider viruses NONLIVING

5. What do viruses do?  viruses can invade the cells of plants, animals, and bacteria  viruses cause illnesses like colds, flu, chicken pox, herpes, AIDS, polio, rabies, measles, mumps… Chicken pox

6. 6 How are viruses named? ● since viruses are non-living they are not named in the same way that living things are named ● many viruses are named for the disease that they cause or the organ/tissue they infect ● EX: rabies virus;poliovirus adenovirus (infects adenoid tonsils)

7. 7 What do viruses look like?  a virus is made of a core of genetic material surrounded by a protein coat (CAPSID)  viruses are SMALL!! (20-400 nanometers) Bar = 100nm

8. 8  viruses come in a variety shapes: rod- shaped; tadpole-shaped; helical; cubelike (see pg 575) T-4 bacteriophage Human Influenza virus particles

9. 9 How does a virus get into a host cell? ● the proteins on the outer coat (capsid) of the virus attach to a receptor site on the host cell’s plasma membrane ● Analogy: the 2 proteins fit together like 2 puzzle pieces ● this attachment process is SPECIFIC!  each virus can usually only attach to a few kinds of cells

10. 10 How do viruses reproduce? 1) LYTIC CYCLE:  viruses insert their genetic material into a host cell  once inside, the viral genes take over the cell’s machinery (ribosomes, etc.)  the “hijacked” cell transcribes the viral genes -uses the host cells own enzymes (e.g. DNA polymerase) & raw materials to make thousands of new viruses

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12. 12 1) LYTIC CYCLE  the cells LYSE (burst) when the concentration of new viral particles is high  the lytic cycle has its name because the host cell lyses and allows viral particles to escape  the new viruses can then infect other host cells

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14. 14 2) LYSOGENIC CYCLE: ● begins like the lytic cycle (virus injects its DNA into host cell); ● the viral DNA is integrated into the host cell’s chromosome – now called a PROVIRUS. ● the provirus may remain dormant, but every time the host cell reproduces, the provirus is also replicated and passed along to the new daughter cells.

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16. 16 2) LYSOGENIC CYCLE: ● the lysogenic phase may continue for many years; ● at any time, the provirus may be activated and enter a LYTIC CYCLE. EX: cold sores caused by herpes simplex I virus (the provirus remains in your cells; when it enters a lytic cycle, a cold sore forms)…WHY does this happen?

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18. 18 Examples of Lysogenic Viruses: ● herpes simplex I and II ● hepatitis B virus ● chicken pox (may erupt later in life as shingles)

19. 19 RETROVIRUSES:  contain RNA as their genetic material  retroviruses infect a cell and produce a DNA copy of their RNA -(retro = backward; RNA is copied in to DNA)  this DNA is inserted into the host cell's DNA  example of a retrovirus: HIV

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21. 21 Retrovirus example: HIV ● HIV = human immunodeficiency virus ● infects white blood cells (helper T cells) ● the viral genetic material becomes a provirus – infected person may not know it, but can still transmit the virus ● eventually, white blood cell count drops too low and AIDS develops

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24. 24 How Can You Protect Yourself From Viral Infections? The best way: use of VACCINES -preparation of weakened or killed virus or viral proteins -when injected, the vaccine stimulates the immune system -sometimes produces permanent immunity

25. How Can You Protect Yourself From Viral Infections? Protect yourself -stay away from known sick people -wash your hands often -safe practices (i.e. coughing / sneezing into a tissue or your sleeve; don’t share food or drink with sick people)

27. Viral Diseases: ● common cold ● influenza ● AIDS (HIV) ● chicken pox ● hepatitis B ● West Nile Virus ● human papillomavirus (HPV) – linked to forms of cancer