Viruses Chapter 18.2 Pages

How were Viruses Discovered? Late 1800’s - bacteria known to causes disease Scientists found tobacco plants were becoming diseased. Scientists collected samples of the diseased plant and filtered it through pores small enough to catch bacteria. Observed that the filtered fluid still caused disease in tobacco plants Scientists infer the disease is not caused by bacteria, but by something small enough to get through the filter. This pathogen later named the “Tobacco Mosaic Virus” (TMV)

A tobacco plant suffering from “Tobacco Mosaic Virus”

Is a Virus Alive? Viruses are NOT considered living because they do not display the 8 characteristics of life: –They do not grow –They cannot reproduce on their own –They don’t carry out metabolic activities –They do not maintain homeostasis Though not alive, viruses have a major impact on the world.

Viral Structure Nucleic Acids –Can be DNA or RNA –Code for components of the virus Capsid –Protein coat that houses genetic material Envelope –Gives a virus its overall spherical shape which can help a virus enter a cell Tail Fibers –Act like a syringe which inject viral DNA into host All viruses have nucleic acid and a capsid. Only some viruses have an envelope and tail fibers.

Bacteriophage A virus that infects bacteria Protein Coat Viral DNA Bacterial Cell Tail Fibers (Capsid)

Viral Replication Viral infection begins when a virus’ genetic material enters a host cell. Then it can either enter the lytic or lysogenic cycle Lytic Cycle –Viral infection, reproduction, and cell destruction –Virus uses host cells “stuff” to make more of itself –Cell breaks open, and new viruses are released to infect more cells. Lysogenic Cycle (“gentle”)Lysogenic Cycle –When viral DNA becomes part of its host cell’s DNA (prophage) –Viral DNA replicates with host cell’s DNA –Under stressful situations, virus may enter lytic cycle Click here to see them both in action!

Animation for Lytic and Lysogenic Cycles hill.com/sites/ /student_view0/ chapter8/lambda_phage_replication_cycle.htmlhttp://highered.mcgraw- hill.com/sites/ /student_view0/ chapter8/lambda_phage_replication_cycle.html

Viruses and Humans Viruses cause most common respiratory illnesses, such as colds and coughs. They can cause a variety of infections from diarrhea to meningitis. It may take up to 2-3 weeks for your body to resolve a viral infection completely. Antibiotics DO NOT work to fight viruses! Viral infection can be prevented by vaccination.

How was Vaccination Discovered? Edward Jenner noticed that the milk-maids who contracted cow pox (not so serious) didn’t contract small pox (very serious!) He inoculated a healthy boy with cow pox (don’t try this at home!) to test his hypothesis that getting cow pox can prevent getting small pox. HE WAS RIGHT! (phew!)

How does Vaccination Work? Vaccination is the process of using one virus to immunize against another virus. Vaccines are usually a very mild dose of the disease-causing virus which has been weakened or killed. These vaccines stimulate the creation of antibodies in the body that recognize and attack a particular infection.