Chapter 20

Section 20-1: Viruses

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

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)

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

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

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

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