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

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

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

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

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

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

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

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

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

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

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

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

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)