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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.