13-a Viruses13-a Virusespp. 387-415

H1N1

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VirusesViruses

Size, Structure, Morphology

Taxonomy

Growth

Identification

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SMALL size

One type of nucleic acid, either DNA or RNA

Protein coat, surrounds the NA

Few or no enzymes of their own

Characteristics of Viruses

Obligate intracellular parasite

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Multiply inside living host cells by using synthesizing machinery of the cell

Proteins, generate ATP

Specialized structures to transfer viral NA to other cells

Inert Outside Living Host Cells

Significant for development of antiviral drugs

Because most drugs would also interfere with functioning of host cell = toxic

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1. Protein coat (capsids)Capsomeres (make up capsids)

The number varies

The types of protein very

2. Nucleic acids

3. Size

4. Morphology

Can’t use biochemical tests or disease symptoms as for classification

Criteria used to ‘Classify’ Viruses

A virion contains:

6Figure 13.1

SizesSizes

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Viral StructureViral Structure

Nucleic acids

– DNA or RNA is

– Either single or double stranded

– Linear or circular

– In segments

• Example: influenza virus

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Capsid– Protein coat protects nucleic acid

Capsomere – Each capsid composed of protein subunits– Arrangement of capsomeres is characteristic

of a particular type of virusEnvelope (on some viruses)

– Covers the capsidSpikes

– CHO-protein, project from envelope surface for attachment

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1. Helical

2. Polyhedral

3. Enveloped

4. Complex

‘Group’ into several different morphological types

Viral MorphologyViral Morphology

…based on their capsid architecture

• Long rods• Rigid / flexible• NA in center• Diseases:– Ebola hemorrhagic fever– Rabies

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Helical

Fig. 13.4a, b

• Non-enveloped• ‘many sided,’ 20

‘faces’• Icosahedron• Diseases:– Adenovirus– Poliovirus

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Polyhedral

Fig. 13.2

• Covered• Roughly spherical• Enveloped helical• Enveloped polyhedral• Spikes• Diseases:

– Influenza (EH)– Herpes simples (EP)– Flu, fever blisters,

chickenpox, shingles, mononucleosis

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Enveloped

Fig. 13.3

• Bacteriophage• Capsids with

additional structures• Head contains NA• Tail sheath, fibers,

plate, pin• Diseases:– Infect bacteria

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Complex – 1

Fig. 13.5

• Poxvirus• No clearly identifiable

capsids• Have several coats• NA inside the coats• Disease:

– Smallpox

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Complex – 2

Fig. 13.5

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Viral TaxonomyViral Taxonomy

Names Ending

Family -viridae

Genus -virus

Viral species Common names are used for species

Subspecies Designated by a number

Viral species: A group of viruses sharing the same genetic information and ecological niche (host)

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Taxonomy Examples

Names Ending

Family Herpesviridae

Genus Simplexvirus

Names Ending

Family Retroviridae

Genus Lentivirus

Species Human Immunodeficiency Virus

Subspecies HIV-1, HIV-2

1. Provide living cells

2. Viruses that infect bacteria useful for understanding growth

3. Pathogenic viruses difficult to cultivate

1. Can’t use bacteriological media

2. Bacteriophages infect bacteria and grow cultures on plates

3. Grow only in higher primates & humans– Use animal cell culture

techniques

Viral GrowthViral Growth

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• Bacteriophages form plaques on a lawn of bacteria– It’s a virus that infects

bacteria– Plaque is a ‘clearing’

in the bacterial ‘lawn’– Count, PFU’s– PFU: plaque forming

units

Fig. 13.6

Bacteriophage

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• Animal viruses grown in living animals or in embryonated eggs

• Vaccine production

Figure 13.7

Viral Growth in Living Cells

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Animal and plant viruses grown in cell culture

Figure 13.8

Viral Growth in Cell Culture

– Continuous animal cell lines maintained indefinitely

– Famous cell line: HeLa

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Viral Identification – Not EasyViral Identification – Not Easy

1. Too small to see

2. Western blotting

3. Virus kills host cell

4. Restriction fragment length polymorphism

5. Polymerase chain reaction

1. Observe with electron microscope

2. Serological methods

3. Cytopathic effects

4. RFLP’s

5. PCR

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Uninfected (mo) cells Infected (mo) cells, 24 hrs later

Figure 13.9

Cytopathic effect (CPE): a visible effect on a host cell, may result in host cell death; ‘host’ cells deteriorate

Viral Identification – Cytopathic EffectViral Identification – Cytopathic Effect

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Q:Q:

1.Which of the following statements is NOT true?

a.Viruses contain DNA or RNA

b.The nucleic acid of a virus is surrounded by a protein coat

c. Viruses multiply inside living cells using viral mRNA, tRNA and ribosomes

d.Viruses cause the synthesis of specialized infectious elements

e.Viruses multiply inside living cells

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Q:Q:1. In which of the following ways do

viruses differ from bacteria?

a.Viruses are filterable

b.Viruses are obligate intracellular parasites

c. Viruses don’t have any nucleic acid

d.Viruses are not composed of cells

e.All of the above

2. Virus spikes are used for attachment to the host cell. True False

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Q:Q:1. What type of cell is the virus, seen

in the figure, capable of infecting?

a.A liver cell

b.A plant cell

c. A bacterial cell

d.Many different cell types

2. Viruses range in size from 20-1000 nm in length and are easy to see in a compound light microscope.

True False