What do the common cold, influenza, measles, & Polio have in common?

1. What do the common cold, influenza, measles, & Polio have in common?

2. They are all Viruses! Influenza Virus Cold Virus HIV Biology 20

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

4. Why are viruses so unique? • 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? • 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

7. The capsid gives the virus its shape.

8. Phylogeny • 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.

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

11. Lytic Cycle • 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 • 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 • 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.

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.

16. Bacteria Versus Viruses Bacteria 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

17. Prions • 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