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Chapter 20 Section 20-1: Viruses.

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Presentation on theme: "Chapter 20 Section 20-1: Viruses."— Presentation transcript:

1 Chapter 20 Section 20-1: Viruses

2 Discovery of Viruses 1892: Dmitri Ivanovski - the cause of tobacco mosaic disease found in the liquid extracted from infected plants 1897: Martinus Beijerinck - tiny particles the juice caused the disease, he named particles viruses, Latin word for “poison” 1935: Wendell Stanley - isolated crystals of tobacco mosaic virus Living organisms do not crystallize - viruses not truly alive A virus is a nonliving particle made of proteins, nucleic acids, and sometimes lipids Viruses can reproduce only by infecting living cells

3 Structure and Composition
Vary widely Only seen with electron microscopes Protein coat called capsid Some also have lipid envelope around capsid (flu) Usually specific to plant, animal, bacteria (bacteriophage)

4 Viral Infections Lytic infection – bacteriophage T4 infects bacterium
Immediate – virus injects genes in host cell, viral genes immediately active, more viruses are made, host cell lyses releasing new viruses Lysogenic infection Host cell not immediately taken over Viral DNA incorporated into host cell DNA (prophage) and activated at a later time, then turns lytic


6 A Closer Look at 2 RNA Viruses
The Common Cold Capsid settles on a cell (usually in nose), and is brought inside, where a viral protein makes many new copies of the viral RNA Host cell’s ribosomes mistake the viral RNA for the host’s own mRNA and translate it New capsids assemble around the viral RNA copies, and within 8 hours, the host cell releases hundreds of new virus particles to infect other cells

7 A Closer Look at 2 RNA Viruses
HIV AIDS is caused by an RNA virus called human immunodeficiency virus (HIV) Special RNA virus called retrovirus The genetic information of a retrovirus is copied from RNA to DNA instead of from DNA to RNA Similar to lysogenic virus

8 Viruses and Cells All viruses are parasites
Must infect living cells in order to reproduce, use cell’s machinery Have many characteristics of living cells, but are not alive

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