1. Pathogens . . . They’ve Gone Viral !!!
Part One
Applied Ag Biology

2. What is a Virus? Segments of nucleic acids contained in a protein coat
Smaller than bacteria
Only seen w/ electron microscope
Replicate by infecting cell & using that cell to make more viruses

3. Is a Virus a Living Organism?
NO!!! Biologists don’t consider them to be alive.
They do not have all properties of life – no metabolism, no homeostasis, no growth
They DO cause diseases in living organisms

4. Discovery of Viruses
In late 1800’s, scientists were trying to determine cause of Tobacco Mosaic Disease
Disease was caused by something smaller than a bacteria
Called the agent a VIRUS, the Latin word for poison
Assumed to be tiny cells

5. Discovery of Viruses In 1935, tobacco mosaic virus (TMV) was purified
Had a crystal structure, which is a property of chemicals
Its structure allowed it to infect healthy tobacco plants
Conclusion = TMV was a chemical, not an organism

6. Capsid = the virus protein coat; contains DNA or RNA (not both)
Virus’s Shape Determined by Its Parts
Capsid = the virus protein coat; contains DNA or RNA (not both)
Examples of DNA viruses: warts, chickenpox, mononucleosis
Examples of RNA viruses: HIV, influenza, rabies

7. Most Viruses Have an Envelope
Envelope = membrane which surrounds capsid in many viruses; helps virus enter cells

8. Common Shape of Viruses
1. Helical – rod-like in appearance, with capsid proteins winding around the core in a spiral
Ebola Virus

9. Common Shape of Viruses
2. Polyhedral – many sides and is roughly spherical

10. How Do Viruses Replicate?
Lack enzymes needed for metabolism
No structures to make proteins
Rely on living hosts to replicate
Step 1 – Virus infects host cell.
Step 2 – Virus either go into the LYTIC CYCLE or LYSOGENIC CYCLE

11. Lytic Cycle
Lytic Cycle: the cycle of viral infection, replication, and cell destruction
Viruses cause damage when they replicate inside cells
Virus replicats 100’s of times and breaks out – destroying cell

12. Lysogenic Cycle
During infection, some viruses stay inside cells but don’t make new viruses
Lysogenic Cycle: cycle in which the viral genes replicate without destroying host cell

13. Lysogenic Cycle
In animal cells, viruses replicate slowly so host cell is not destroyed
Example: virus that causes cold sores hides deep in the nerves of the face; when body becomes stressed, the virus begins to cause tissue damage (cold sore/fever blister)