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