Transcript Viruses
Chapter 18.2 Pages 525 - 531 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 Bacterial Cell Protein Coat (Capsid) Tail Fibers Viral DNA 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”) – 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 Animation for Lytic and Lysogenic Cycles • http://highered.mcgrawhill.com/sites/0072995246/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.