VIRUSES Section 20.1

Key Players  Dmitri Ivanovski  Russian biologist in the 1890’s  Noticed tobacco plants were diseased but only knew it was because of something in the liquid secreted by the diseased plant. Called the disease Tobacco Mosaic Disease (1892)  Martinus Beijernick  Dutch biologist in the 1890’s  Suggested that tiny particles in the liquid were causing the disease. Named the particles “viruses” after the Latin word for poison (1897)

Key Players Cont.  Wendell Stanley  American Biochemist in the 1930’s  Isolated crystals of the “Tobacco Mosaic Virus”  This is important because living things DO NOT CRYSTALLIZE which means viruses are not truly LIVING THINGS.

What is a virus then?  A virus is a nonliving particle made of proteins, DNA, and sometimes fats.  They can only reproduce by infecting other living cells.

Structure and Composition  The way viruses look varies greatly  Most are so small they can only be seen under an electron microscope  Have a protein coat that surrounds them called a capsid.  Simple viruses have only a few genes, while more complex ones have thousands

Structure and Composition  Proteins on the capsid help them to enter host cells.  Most viruses only can infect a specific kind of cell  Example:  Bacteriophage – viruses that infect bacteria

What happens when a virus infects a cell?

Lytic Infections 1. Virus enters bacteria cell 2. Virus copies itself 3. Too many copies cause the cell to burst (lyse) 4. The extra copies go out an infect more bacteria cells and the process happens over and over and over and over and over……etc.

Lysogenic Infection Do not happen as quickly as lytic infections 1. Virus enters cells 2. Inserts a copy of the virus DNA into the bacteria DNA 3. Cell unknowingly copies the virus DNA when it copies its own DNA. 4. At some point the virus will take its DNA back, create more viruses like in a lytic infection and then the cell will burst and this process will happen over and over and over and over and over….etc.

A closer look at two viruses The common cold and HIV

The Common Cold 1. A capsid enters a host cell usually in the hosts nose. 2. The host inhales the capsid/cell deeper into nasal cavity 3. The cold virus is a lytic infection so it quickly copies itself and then when the cell bursts, more copies of the virus go out an infect other cells.

HIV  Causes AIDS later in life 1. This is an infection called a retroviral infection. a) It is similar to a lysogenic infection meaning it takes a long time for the virus to infect the host. 2. The virus is transmitted through blood or body fluid.

CharacteristicVirusCell Structure Reproduction Genetic Code Growth & Development Obtain & Use Energy Response to Environment Change Over Time DNA or RNA in a capsid Cell membrane, cytoplasm, organelles Only within a host cell Independent cell division DNA or RNADNA No Yes