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Viruses
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A Borrowed Life

A Borrowed Life

Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli

Viruses lead a kind of borrowed life between life-forms and chemicals

The origins of molecular biology lie in early studies of viruses that infect bacteria

Slide 2

Fig. 19-1

Fig. 19-1

0.5 m

Slide 3

Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat

Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat

Viruses were detected indirectly long before they were actually seen

Slide 4

The Discovery of Viruses: Scientific Inquiry

The Discovery of Viruses: Scientific Inquiry

Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration

In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease

In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)

Slide 5

Fig. 19-2

Fig. 19-2

RESULTS

1

2

3

Extracted sap

from tobacco

plant with

tobacco

mosaic

disease

Passed sap

through a

porcelain

filter known

to trap

bacteria

Rubbed filtered

sap on healthy

tobacco plants

4

Healthy plants

became infected

Slide 6

Structure of Viruses

Structure of Viruses

Viruses are not cells

Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

Slide 7

Viral Genomes

Viral Genomes

Viral genomes may consist of either

Double- or single-stranded DNA, or

Double- or single-stranded RNA

Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

Slide 8

Capsids and Envelopes

Capsids and Envelopes

A capsid is the protein shell that encloses the viral genome

Capsids are built from protein subunits called capsomeres

A capsid can have various structures

Slide 9

Fig. 19-3

Fig. 19-3

RNA

Capsomere

Capsomere

of capsid

DNA

Glycoprotein

18  250 nm

7090 nm (diameter)

Glycoproteins

80200 nm (diameter)

80  225 nm

Membranous

envelope

RNA

Capsid

Head

DNA

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