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PowerPoint Lecture Slide Presentation

B.E Pruitt & Jane J. Stein

Chapter 04Viruses, Viroids and Prions

REFERENCESREFERENCES

Tortora GJ, Funke BR, Case CL, 2007, Microbiologyan Introduction, 9th edition, Benjamin Cummings,San Francisco, CA 94111, USA

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History

Contangium vivum fluidum: a contagious fluid

1886: Adolf Mayer (Dutch chemist) showed TMD wastransmissible from a diseased plant to a healthy plant

1892: Dimitri Iwanowski (Russian bacteriologist)found that infectious agent had passed through theminutes pores of the filter

1930: Virus (Latin word): poison

1935: Electron microscope was found

1935: Wendell Stanley (American chemist) isolatedTMV, making it possible to be studied

Hypothesis the origin of virus:arose from independently replicating nucleic acid(plasmid) or they develop from degenerative cells

Viruses

Viruses contain DNA or RNA

Protein coat or some are enclosed by an envelope

Multiply inside the cells (obligate intracellular parasite)

Have few or no enzymes of their own metabolism

Most viruses infect only specific types of cells in onehost

Host range is determined by specific host attachmentsites and cellular factors

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Comparison between bacteria and viruses

Viral size (20 - 1.000 nm in length)

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Viral structure

Viral structure

Virion: fully developed, composed of nucleic acid andprotein coat (protection and vehicle of transmission)

Nucleic acid: DNA or RNA, single or double stranded,circular or linear or separate segment (influenza virus)

Capsid: the protein coat surrounding the nucleic acid,capsomeres (protein subunits, single or several types)

Viruses are classified by the differences in morphology

and capsid architecture Envelope: an outer covering surrounding the capsid of

some viruses (composed of CH, protein and lipid)

Spikes: a CH-protein complex that projects from thesurface of certain viruses, for identification purpose

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Helical viruses

Resemble long rods, rigid or flexible

Nucleic acid is found within a hollow, cylindrical capsidthat has a helical structure

Rabies and Ebola hemorrhagic fever

Polyhedral viruses Icosahedron: a regular polyhedron with 20 triangular

faces and 12 corners

The capsomers of each face from equilateral triangle

Adenoviruses, polioviruses

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Viral Taxonomy

1966: International Committee on Taxonomy ofViruses (ICTV)

Grouping viruses into families based on: nucleicacid type, strategy for replication and morphology

Genus names end invirus, family names end inviridae, order names end inales

Viral species: a group of viruses sharing the samegenetic information and ecological niche (host)

Specific epithet for viruses are not used, designated

by descriptive common names Subspecies are designated by a number, e.g. HIV-1

Viral Taxonomy

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Viral Taxonomy

Viral Taxonomy

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Viral Taxonomy

Virions contain only few genes needed for thesynthesis of new viruses (capsid, enzymes)

Viral enzymes are almost entirely concerned withreplication or processing viral nucleic acid

Enzymes needed for protein synthesis, ribosomes,tRNA and ATP are supplied by the host cell andare used for synthesis viral protein, including viralenzymes

Larger virions may contain one or few enzymes,helping the virus penetrate the host cell or replicateits own nucleic acid

Virus to multiply, it must invade a host cell and takeover the hosts metabolic machinery

Viral multiplication

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Phage causes lysis and death of host cell

Attachment Phage attaches by tail fibers tohost cell

Penetration Phage lysozyme opens cell wall,tail sheath contracts to force tailcore and DNA into cell

Biosynthesis Production of phage DNAand proteins

Maturation Assembly of phage particles Release Phage lysozyme breaks cell wall

The lytic cycle

Attachment:Phage attachesto host cell

Penetration:Phage pnetrateshost cell andinjects its DNA

Biosynthesis:Phage DNA directssynthesis of viralcomponents by thehost cells

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Bacterialcell wall

Bacterialchromosome

Capsid DNA

Capsid

Sheath

Tail fiber

Base plate

Pin

Cell wall

Tail

Plasma membrane

Sheath contracted

Tail core

The lytic cycle

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4 Maturation:Viral componentsare assembled intovirions.

Tail

5 Release:Host cell lyses andnew virions are

released.

DNA

Capsid

Tail fibers

The lytic cycle

One-step Growth Curve

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Prophage DNA is incorporated in host DNA

The phage remains latent (inactive)

A rare spontaneous event, UV light, certain chemicals intiation to lytic cycle

Three important results of lysogeny:

Lysogenic cells are immune to reinfection by thesame phages

Phages conversion: the host cells exhibit newproperties, e.g. synthesis of a toxin (Clostridiumbotulinum, Vibrio cholerae)

Specialized transduction: the host cells have newcharacteristic or metabolism

Lysogenic cycle / temperate phages

The lysogenic cycle

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Specialized transduction

Prophage exists in galactose-using host(containing the galgene).

Phage genome excises, carryingwith it the adjacent galgene fromthe host.

Phage matures and cell lyses, releasingphage carrying galgene.

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Prophage

galgene

galgene Bacterial DNA

Galactose-positivedonor cell galgene

Phage infects a cell that cannot utilizegalactose (lacking galgene).

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Galactose-negativerecipient cell

Along with the prophage, the bacterial galgene becomes integrated into the new

hosts DNA.

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Lysogenic cell can now metabolizegalactose.

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Galactose-positive recombinant cell

Multiplication of Animal viruses

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Generally, DNA viruses replicate their DNA in the

nucleus of the host cell by using viral enzymes

Capsid and other proteins are synthesized in thecytoplasm by using host cell enzymes

The proteins migrate into nucleus and are joined withnew DNA viruses to form virions

Virions are transported along endoplasmic reticulumto the host cells membrane for release

Adenoviridae, herpesviridae, papovaviridae,hepadnaviridae

Poxviruses are an exception because all thecomponents are synthesized in the cytoplasm

Biosynthesis of DNA viruses

Multiplication of DNA VirusVirion attaches to host cell

Virion penetratescell and its DNA isuncoated

A portion of viral DNA istranscribed, producing mRNA(using host transcriptase) thatencodes early viral proteins.Poxviruses is an exception

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DNA

Late viral DNA isreplicated and someviral proteins aremade

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Late translation;capsid proteinsare synthesized

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Virions mature6

Capsid

Papovavirus

Host cell

Nucleus

Cytoplasm

Virions are released7

Capsid proteins

mRNA

Viral DNA

Capsid proteins

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RNA viruses multiply in the host cells cytoplasm

The major differences among the multiplicationprocesses lie in producing of mRNA and viral RNA

Sense strand (+ strand): act like mRNA protein

Inhibit the host cells synthesis of RNA and protein

Form an enzymes called RNA-dependent RNApolymerase catalyzes another RNA strand

Picornaviridae

Antisense strand(- strand): template to produce

additional + strand Also contain RNA-dependent RNA polymerase

Rhabdoviridae

Biosynthesis of RNA viruses

Pathways of Multiplication for RNA-Containing Viruses

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Double-stranded (ds-RNA) see the figure

Must use mRNA (+ strand; produced in thecytoplasm) to code for protein (capsid)

Reoviridae

Two identical + strand of RNA

Reverse transcriptase: template to produce ds-DNA and degrades the original viral RNA

Integrase and protease

Provirus: viral DNA that is integrated into host cellsDNA (protected from immune system and drugs)

Mutagens: radiation can induce expression andinfect adjacent cells

Retroviridae (HIV-1 and HIV-2)

Biosynthesis of RNA viruses

Multiplication of a Retrovirus

Enter by fusion between spikesand host cells receptor

Reverse transcriptasecopies viral RNA toproduce ds-DNA

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

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Envelope

Transcription of theprovirus may also occur,

producing RNA for newretrovirus genomes andRNA that codes for theretrovirus capsid andenvelope proteins.

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Matureretrovirusleaves hostcell, acquiringan envelope asit buds out.

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CapsidReversetranscriptase

Virus Two identical + stands of RNA

DNA of one of the hostcells chromosomes

Provirus

Hostcell

Reversetranscriptase

Viral RNA

RNA

Viral proteins

Identicalstrands ofRNA

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Maturation and release

Budding, does not

immediately kill the hostcells

Non-enveloped virusescause rupture in the hostcell plasma membrane

Infectious proteins (protenaceous infectious particle)

Possible of inherited and transmissible by ingestion,transplant, & surgical instruments

Spongiform encephalopathies: Sheep scrapie,Creutzfeldt-Jakob disease, Gerstmann-Strussler-Scheinker syndrome, fatal familial insomnia, mad cowdisease

PrPC, normal cellular prion protein, on cell surface

PrPSc, scrapie protein, accumulate in brain cellsforming plaques

The PrPSc form is protease resistant, insoluble andform aggregates in neural cells leads to destructionof neural tissues and neurological symptoms

Prions

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Prions are highly resistant to denaturation processessuch as protease, heat, radiation, and formalintreatments.

Products derived from human sources or animalsincluding blood plasma products, gelatine, andpeptones may be contaminated with prions

Prions can be denatured at 134C for 18 minutes

Prions

Prions

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Short pieces of naked RNA, only 300-400 nucleotides

long with no protein coat

The RNA does not code for any proteins

Conclusively identified as pathogens only of plants

Viroids

Growing Viruses

Fact: viruses cannot multiply outside the living host

Growing in the laboratory

Bacteriophages

Animal viruses (living animals, embryonated eggsand cell culture)

Some animal viruses can be cultured only in livinganimals (mice, rabbits, guinea pigs)

Animal inoculation may be used as a diagnosticprocedure for identification and isolating a virus fromclinical specimen

Some human viruses cannot be grown in animals orcan be grown but do not cause disease, e.g. HIV-1

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Growing bacteriophages

Embryonated eggs

Fairly convenientand inexpensive

Fairly convenientand inexpensive

Viral growth issignaled by thedeath or cell

damage of theembryo, typicalpocks or lesionson the eggmembranes

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Cell culture

Homogenous and can be propagated, more convenient

Cytopathic effects: a visible effect, caused by a virusthat may result in damage or death of the host cell

Primary (tissue) and diploid (human embryos) cell lines

Continuous cell lines may be maintained indefinitely

Identification of viruses is not an easy task, EM

Cytopathic effects

Serological tests

Detect antibodies against viruses in a patient

Western blot

Nucleic acids RFLPs (Restriction Fragment Length Polymorphisms)

PCR (Polymerase Chain Reaction)

Virus Identification

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Alpha IFN & Beta IFN: Cause cells to produce antiviralproteins that inhibit viral replication

Gamma IFN: Causes neutrophils and macrophages tophagocytize bacteria

Interferons (IFNs)

Interferons (IFNs)

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Viral RNA from an

infecting virusenters the cell.

The infectingvirus replicates

into newviruses.

The infecting virus also

induces the host cell toproduce interferon onRNA (IFN-mRNA), which

is translated into alpha

and beta interferons.

Interferons released by the virus-infected host cell bind to plasmamembrane or nuclear membrane receptors on uninfected neighboring

host cells, inducing them to synthesize antiviral proteins (AVPs). These

include oligoadenylate synthetase, and protein kinase.

New viruses releasedby the virus-infectedhost cell infect

neighboring host

cells. 6 AVPs degrade viralm-RNA and inhibit

protein synthesis andthus interfere with

viral replication.