1. WHAT DO THE COMMON COLD, INFLUENZA, MEASLES, & POLIO HAVE IN COMMON? 1

2. Biology 20 Influenza VirusCold VirusHIV 2 THEY ARE ALL VIRUSES!

3. What is a virus?  Non-living strand of genetic material within a protein coat  Smallest disease- causing structures known  Does not grow, respire, or respond to stimuli but it does reproduce 3

4. Why are viruses so unique? 4  Neither living nor non-living! Outside of a host, viruses are considered non-living. Inside a host, viruses can reproduce and therefore are living. As such they are not classified in any kingdom!  They are host-specific! E.g. Tobacco mosaic virus is harmful only to tobacco plants.

5. Why are viruses so unique? 5  Smallest beings in existence! They range from 5- 300 nm (a nanometer is a one billionth of a meter). It would take 10,000 cold viruses to span the period at the end of this sentence.  Can only be seen with electron microscopes.

6. Structure  Outer core is called the capsid, which is made of proteins  Inner core is genetic material, either RNA or DNA, but not both 6

7. 7 The capsid gives the virus its shape.

8. Phylogeny 8  No fossil evidence but many theories.  One theory, now considered most likely, is that viruses came from parts of cells.  Virus genetic material is very similar to cellular genes.  These genes somehow developed the ability to exist outside the cell.

9. Viral Infection  In order to replicate, a virus must enter a host cell.  The virus attaches to the host cell using receptors.  Different types of organisms have receptors for different types of viruses, explaining why viruses cannot be transmitted between species. 9

10. Viral Infection  Once the virus attaches, the genetic material of the virus enters the cell’s cytoplasm  The virus now uses the host cell to replicate by either the lytic cycle or the lysogenic cycle 10

11. Lytic Cycle 11  The host cell makes many copies of the viral RNA or DNA, which in turn help produce many more viruses.  These new viruses cause the cell to burst, releasing new viruses that are free to infect other cells.  Viruses that replicate this way often cause active infections, meaning symptoms of the virus start to occur within 1-4 days after exposure. E.g. Common cold and influenza

12. Lysogenic Cycle 12  In this situation, the viral DNA enters the nucleus and integrates with the chromosome of the host cell.  Once integrated, the host cell will have the viral genes permanently.  The viral genes may remain dormant for months or years before becoming activated. E.g. Herpes simplex I  Activation results in the lytic cycle.

13. Retroviruses 13  Viruses are classified by their genetic material.  Viruses that have RNA, rather than DNA, are called retroviruses. E.g. human immunodeficiency virus (HIV)  Retroviruses have a complicated replication cycle.

14. Viruses & Disease  Two defenses the body has against viruses are interferon and antibodies.  Viruses are often name for the diseases they cause. 14

15. Viruses & Disease  Vaccination is the process of preventing viral disease by deliberately exposing the body to a weakened or killed pathogen, such that the body will develop an immunity to it. 15

16. Bacteria Versus Viruses  Prokaryotic cells  Carry out all life functions  Contain both DNA and RNA  Not cells, just simple molecular structures  Only capable of reproduction and can do this only inside a specific host  Contain either DNA or RNA 16 BacteriaViruses

17. Prions 17  A protein that can cause infection or disease is called a prion.  Prions normally exist in cells and are shaped like a coil. Mutated prions are shaped like a piece of paper folded many times.

18. Prion Disease  Mutated prions are associated with many diseases known as transmissible spongiform encephalopathies.  E.g. mad cow disease in cattle, Creutzfeldt-Jakob disease in humans, & chronic wasting disease in deer and elk 18