VirusesVirology – study of viruses

Virologist – scientist that studies viruses

Introduction to Viruses:

• In 1898, Friedrich Loeffler and Paul Frosch: found evidence that the cause of foot-and-mouth disease in livestock was an infectious particle smaller than any bacteria.

• This was the first clue to the nature of viruses, genetic entities that lie somewhere in the grey area between living and non-living states.

• Viruses depend on the host cells that they infect to reproduce.

• When found outside of host cells, viruses exist as:

• Protein coat or capsid, sometimes enclosed within a membrane.

• The capsid encloses either DNA or RNA which codes for the virus elements. In this form outside the cell, the virus is metabolically inert (dormant)

• Examples of Viruses: Influenza and Tobacco Mosaic Virus

Discovery of Viruses

• Beijerinck (1897) coined the Latin name "virus"  meaning poison for  the substance infecting tobacco plants

• Edward Jenner (1796) developed smallpox vaccine using milder cowpox viruses – the first ever vaccine

• 1900 Walter Reed showed that an agent so small it could pass through a filter, which trapped bacteria, caused the human disease yellow fever

• 1918 a pandemic Spanish flu kills 25 million people, more deaths than caused by the first World War

• Wendell Stanley (1935) crystallized sap from tobacco leaves infected with Tobacco Mosaic Virus (TMV) & found virus was made of nucleic acid & protein

• Viruses couldn't be seen until electron microscope invented in the 1930s

• 1950s widespread use of the Salk polio vaccine – killed vaccine

• 1978 last naturally occurring case of smallpox in the world

• 1982 recognition of new virus Human Immunodeficiency Virus (HIV-1)

• 2009 H1N1 Influenza virus – Swine Flu

Characteristics of Viruses

• Not living organisms; Noncellular

• Consist of a nucleic acid core (DNA or RNA) and a protein coat called the capsid

• Cannot grow or replicate on their own (inactive particles); Can only reproduce inside of a living host cell using its raw materials & enzymes

• Some can cause disease (smallpox, measles, mononucleosis, influenza, colds, AIDS, Ebola Virus

• Some may also cause cancers such as leukemias

• Highly host specific (only infect certain cells)

• Viruses are classified into 2 main groups by their nucleic acid --- DNA or RNA Viruses

Bacteriophage – a virus that infects a specific bacteria

• T4 Bacteriophage – virus that attacks the bacteria E. coli

Lytic & Lysogenic Cycles: 2 ways viruses reproduce in a host cell

Retroviruses

• Contains RNA – RNA enters the cells and makes DNA

– RNA is copied backward – RNA to DNA (usually DNA to RNA)

• Virus DNA becomes part of host cell’s DNA

• Hides in the cell and copies of the virus can be made at any time.

• Causes some cancers and AIDS

Viroid – infect plants, single stranded RNA molecules that have no capsid – disrupt metabolism and destroy plants

Prion- contain only protein (no DNA or RNA), forms protein clumps in nervous tissue (mad cow disease)

Comparing Viruses & Living Cells

http://www.chm.bris.ac.uk/webprojects2006/Kelly/influenzafigure1.jpg