Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 19.1 0.5 mm...

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 19.1

0.5 mm

Viruses: A Borrowed Life


The Discovery of Viruses: Scientific Inquiry

• 1935, Wendell Stanley crystallized tobacco mosaic virus (TMV)

© 2011 Pearson Education, Inc.


Figure 19.2

Extracted sapfrom tobaccoplant withtobacco mosaicdisease

RESULTS

Passed sapthrough aporcelain filterknown to trapbacteria

Healthy plantsbecame infected

Rubbed filteredsap on healthytobacco plants

1 2 3

4


Figure 19.2b


Figure 19.2c


Structure of Viruses

• Not cells

• Consists of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope



Viral Genomes

– Double- or single-stranded DNA, or

– Double- or single-stranded RNA



Figure 19.3

Capsomereof capsid

RNA CapsomereDNA

Glycoprotein Glycoproteins

Membranousenvelope RNA

CapsidHead

DNA

Tailsheath

Tailfiber

18 250 nm 80 225 nm70–90 nm (diameter) 80–200 nm (diameter)

20 nm 50 nm 50 nm 50 nm(a)Tobacco

mosaic virus(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4


• Some viruses have viral envelopes

• Derived from the host cell’s membrane



• Bacteriophages (phage) viruses that infect bacteria



Lytic Cycle

Attachment1


Figure 19.5-2

Attachment

2

1

Entry of phageDNA anddegradation of host DNA


Figure 19.5-3

Attachment

2

1

3


Synthesis ofviral genomesand proteins


Figure 19.5-4

Attachment

2

1

43



Assembly

Phage assembly

Head Tail Tailfibers


Figure 19.5-5

Attachment

2

1

5

43


Release


Assembly

Phage assembly

Head Tail Tailfibers


The Lysogenic Cycle

• Replicates the phage genome without destroying the host


Animation: Phage Lambda Lysogenic and Lytic Cycles


Figure 19.6

New phage DNA and proteinsare synthesized and assembledinto phages.

The cell lyses, releasing phages.

Phage

PhageDNA

The phageinjects its DNA.

Bacterialchromosome

Lytic cycle

lytic cycleis induced

or

Phage DNAcircularizes.

Certain factorsdetermine whether

lysogenic cycleis entered

Lysogenic cycle

Prophage

Daughter cellwith prophage

Occasionally, a prophageexits the bacterial chromosome,initiating a lytic cycle.

Cell divisionsproduce apopulation ofbacteria infectedwith the prophage.

The bacterium reproduces,copying the prophage andtransmitting it to daughtercells.

Phage DNA integrates intothe bacterial chromosome,becoming a prophage.


New phage DNA and proteinsare synthesized and assembledinto phages.

The cell lyses, releasing phages.

Phage

PhageDNA

The phageinjects its DNA.

Bacterialchromosome

Lytic cycle


or




Figure 19.6a



or




Lysogenic cycle

Prophage

Daughter cellwith prophage

Occasionally, a prophageexits the bacterial chromosome,initiating a lytic cycle.

Cell divisionsproduce apopulation ofbacteria infectedwith the prophage.

The bacterium reproduces,copying the prophage andtransmitting it to daughtercells.

Phage DNA integrates intothe bacterial chromosome,becoming a prophage.

Figure 19.6b


Table 19.1a


Table 19.1b


RNA as Viral Genetic Material

• Retroviruses use reverse transcriptase to copy their RNA genome into DNA

• HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)



Glycoprotein

Reversetranscriptase

HIV

Viral envelope

Capsid

RNA (twoidenticalstrands)

HOSTCELL

Viral RNAReversetranscriptase

RNA-DNAhybrid

DNA

NUCLEUSProvirusChromosomal

DNA

RNA genomefor thenext viralgeneration

mRNA

New virus

Figure 19.8a


Figure 19.8c

HIV

Membraneof whiteblood cell

HIV entering a cell


Figure 19.8d

HIV entering a cell0.25 m


Figure 19.8e

New HIV leaving a cell


Figure 19.8f



Figure 19.8g



Evolution of Viruses

• Probably evolved as bits of cellular nucleic acid

• e.g. Plasmids, transposons, and viruses are all mobile genetic elements



Viral Diseases in Animals

• Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes

• Some viruses cause infected cells to produce toxins that lead to disease symptoms

• Others have molecular components such as envelope proteins that are toxic



• Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the harmful pathogen



Emerging Viruses

• e.g. influenza virus H1N1 Flu epidemics



• Viral diseases in a small isolated population can emerge and become global

• Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity



• Can cause pandemics, global epidemics

• The 2009 flu pandemic was likely passed to humans from pigs; for this reason it was originally called the “swine flu”



Figure 19.9a

2009 pandemic H1N1influenza A virus

(a)

1 m


Figure 19.9b

(b) 2009 pandemic screening


Figure 19.9c

(c) 1918 flu pandemic


Viral Diseases in Plants

• Most plant viruses have an RNA genome



Figure 19.10a


Figure 19.10c


Viroids and Prions: The Simplest Infectious Agents

• Viroids are small circular RNA molecules that infect plants and disrupt their growth

• Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals

• Convert normal proteins into the prion version

• e.g. mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions



Figure 19.11

Prion

Normal protein

Originalprion

Newprion

Aggregatesof prions

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 19.1 0.5 mm...

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