Viral Biology: Characteristics, Structure, Replication, and Taxonomy of Viruses - Prof. Ba, Study notes of Microbiology

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Slide 1

Viruses

Lecture 16

Slide 2

Biol 240 S

Learning Outcomes

• Characteristics of

Viruses

• Viral Taxonomy• Viral Structure• Viral Replication• DNA viruses• RNA viruses• Prions

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Biol 240 S

3

Characteristics of ALL Viruses

• 1. Viruses contain genetic material
  • Either DNA or RNA never both
    • 2. Protein coat = Virion• 3. Multiply inside living cells• 4 Cause the synthesis of structures for

their own transfer

Describe the 4 characteristics of all viruses.

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Biol 240 S

General Characteristics of Viruses •^

Most viruses infectonly specific types ofcells in one host

-^

Host range isdetermined by specifichost attachmentreceptors

-^

Polio–

Narrow range

•^

Rabies–

Broad host range

What factors determine the host range of viruses.

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7

Viral Proteins

•^

Enzymes–

DNA or RNA Polymerases

-^

protease

•^

Structural Proteins–

Capsid proteins

-^

Receptors

•^

Antigenicity due to differencein structural proteins–

Serotypes

-^

influenza

•^

Antibodies bind to structuralproteins

-^

Cytotoxic T cells

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Biol 240 S

Viral Envelope

•^

Components–

Lipoprotein derivedfrom host cell plasmamembrane

-^

Viral proteins

•^

Protein spikes–

Attach to host cellreceptors for hormones

-^

Glycoproteins

-^

Influenza

-^

Hemagglutin

-^

HIV

-^

Gp 120

•^

Less resistant thannon-enveloped viruses

Provide definitions for the following terms – capsid,nonenveloped virus, enveloped virus, and virion

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9

Viral Taxonomy

• Family names end in -viridae• Genus names end in -virus• Viral species: A group of viruses sharing the

same genetic information and ecological niche(host). Common names are used for species

• Subspecies are designated by a number

Provide an example of a family, genus, species, and acommon name of a virus.

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Biol 240 S

Viral Taxonomy

• Herpesviridae• Herpesvirus• Human herpes

virus 1, HHV 2,HHV 3

• Retroviridae• Lentivirus• Human

ImmunodeficiencyVirus 1, HIV 2

Provide an example of a family, genus, species, and acommon name of a virus.

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Biol 240 S

13

Single Strand RNA Viruses

•^

Enveloped + strand

-^

Togaviridae–

Rubivirus – rubella

•^

Flavivirus–

Yellow fever

•^

Nonenveloped

-^

Picornaviridae–

Rhinovirus

-^

Common cold

-^

Poilovirus

•^

Calciciviridae–

Norovirus

-^

Stomach Flu

•^

Enveloped - strand

-^

Rhabdoviridae–

Rabies

•^

Filoviridae–

Ebola

•^

Orthomyxoviridae–

Influenza

-^

Multiple segments ofsingle stranded RNA

Provide examples of enveloped and non-enveloped RNAviruses Which enzyme do most RNA viruses carry with theminside their capsids, and why?

Explain the difference between +RNA and –RNA viruses

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14

Culturing Viruses

•^

Viruses must begrown in living cells.

-^

Bacteria–

Phages

•^

Live animals

-^

Embryonated eggs–

Vaccine development

•^

Animal cell cultures

-^

vaccinations

List the different methods that can be used to growanimal viruses in a laboratory.

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Biol 240 S

• Cytopathic effects• Serological tests
  • Detect antibodies against viruses in a patient– Use antibodies to identify viruses in

neutralization tests,

• viral hemagglutination– Western blot
  • Nucleic acids
    • PCR– RFLPs

Virus

Identification

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18 Figure 13.10.

5

Capsid

Tail fibers

One Step Growth Curve

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The Lysogenic Cycle

Figure 13.

Explain the basic difference between the lytic cycle ofphage multiplication and the lysogenic cycle. What is a prophage?

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20

Specialized Transduction

Figure 13.

Prophage exists in galactose-using host(containing the

gal

gene).

Phage genome excises, carryingwith it the adjacent

gal

gene from

the host.Phage matures and cell lyses, releasingphage carrying

gal

gene.

1 2 3

Prophage

gal gal^ gene

gene

Bacterial DNA

Galactose-positivedonor cell

gal

gene

Phage infects a cell that cannot utilizegalactose (lacking

gal

gene).

4

Galactose-negativerecipient cell

Along with the prophage, the bacterial

gal

gene becomes integrated into the newhost’s DNA. 5

Lysogenic cell can now metabolizegalactose. 6

Galactose-positive recombinant cell

Describe a case where a bacterium owes its pathogenicityto specialized Transduction.

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• Attachment Viruses attaches to cell

membrane receptor

• Penetration By endocytosis or fusion• Uncoating

By viral or host enzymes

• BiosynthesisProduction of nucleic acid and

proteins

• Maturation

Nucleic acid and capsidproteins

assemble

• Release

Animal viruses By budding (envelopedviruses) or rupture

Describe how an animal virus like herpes simplex enteran animal cell?

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24

Replication of DNA Virus

Figure 13.

Virion attaches to host cell

Virion penetratescell and its DNA isuncoated

Early transcription andtranslation; enzymes aresynthesized

1

2

3

DNA

Late transcription;DNA is replicated 4

Late translation;capsid proteinsare synthesized Virions mature 6 5

Capsid

Papovavirus

Host cell

DNA Cytoplasm

Virions are released 7

Capsid proteins

mRNA

Following uncoating, describe the biosynthesis of a DNAvirus.

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Biol 240 S

+ RNA or SenseStrand = mRNA

•^

Viral genometranscribed

-^

“early proteins”–

Enzymes forviral replication

•^

RdRp–

Synthesizes –stand

-^

template formaking new +strands

•^

“late proteins”–

capsid proteins

Following uncoating, describe the biosynthesis of a+RNA virus.

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26

- Strand RNA Viruses - RNA dependent

RNA Polymerase

• Uses – strand to

make + strand

• • strand (mRNA)

transcribed intoviral proteins

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Retrovirus Replication

Figure 13.

Retrovirus penetrateshost cell.

Virion penetratescell and its DNA isuncoated

The new viral DNA istranported into the host cell’snucleus and integrated as aprovirus. The provirus maydivide indefinitely with thehost cell DNA.

1

2

3

DNA

Transcription of theprovirus may also occur,producing RNA for newretrovirus genomes andRNA that codes for theretrovirus capsid andenvelope proteins. 4

Matureretrovirusleaves hostcell, acquiringan envelope asit buds out. 5

Capsid

Reversetranscriptase

Virus

Two identical + stands of RNADNA of one of the hostcell’s chromosomes

Provirus

Hostcell

Reversetranscriptase Viral RNA

RNA

Viral proteins

Identicalstrands ofRNA

How does the biosynthesis of the retroviridae differ fromthat of other viruses?

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30

Latent v Persistent Infections

• Latent Viral Infections
  • Virus remains in asymptomatic

host cell for long periods

• Cold sores, shingles
  • Persistent Viral Infections
    • Disease processes occurs over a

long period, generally fatal

• Subacute sclerosing panencephalitis

(measles virus)

What is a provirus? And provide an example of a latentviral infection.

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Biol 240 S

Viral Therapy

•^

Pathogen specificbacteriophages

-^

Phages mutate at samerate as bacteria

-^

Selective

-^

Phages multiply insidepatient

-^

Viriotherapy

-^

Oncolytic virusesselectively bind to anddestroy cancer cells

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32

• Infectious proteins• Inherited and transmissible by ingestion,

transplant, & surgical instruments

• All are fatal• Spongiform encephalopathies:• Sheep scrapie,• Creutzfeldt-Jakob disease• Mad cow disease

Prions

What are prions and how are they transmitted?

Slide 33

Biol 240 S Prions

Figure 13.

PrP

c

PrP

Sc

1

2

3

4

5

6

7

8

Endosome

Lysosome

Explain how prions cause diseases like BovineSpongiform Encephalopathy?

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36

Vaccine History

• Variolation: Inoculation of smallpox

into skin (

th

century)

• Vaccination: Inoculation of cowpox

(Vaccina) into skin

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Vaccination

• Prevention better

(and cheaper)thancure

• Herd Immunity• Epidemics• Eradication
  • Small pox– Polio?– Measles?

Why is it better to vaccinate against viral disease, likepolio, that someone may never have than to treat thesymptoms if the individual contracts a viral disease?

Following the eradication of smallpox, which viraldiseases are the likely targets for the next globaleradication efforts.

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Biol 240 S

Attenuated Whole-Antigen

Vaccines

• Living, but

weakened virus

• Lifelong immunity• 95% effective• Living virus

replicates in body

• Polio - Sabin

Compare the use of attenuated whole-agent vaccines toinactivated whole-agent vaccines in terms of theireffectiveness and safety.