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Chapter 13. Viruses, Viroids and Prions. Adolf Mayer, 1886 tobacco mosaic disease (TMD) transmissible Dimitri Iwanowski, 1892 Filtered sap still caused TMD contagious fluid or filterable agent Walter Reed,1901 Yellow Fever. Felix d’Herelle, 1917 Bacteriophage Suggested phage therapy

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Chapter 13

Chapter 13

Viruses, Viroids and Prions


Chapter 13

  • Adolf Mayer, 1886

    • tobacco mosaic disease (TMD) transmissible

  • Dimitri Iwanowski, 1892

    • Filtered sap still caused TMD

    • contagious fluid or filterable agent

  • Walter Reed,1901

    • Yellow Fever


Chapter 13

  • Felix d’Herelle, 1917

    • Bacteriophage

    • Suggested phage therapy

  • 1930’s, term virus introduced and electron microscopy invented

  • Wendell Stanley, 1935

    • Isolated tobacco mosaic virus



Chapter 13

  • Viral Features

    • Obligate intracellular pathogen

    • Host range

      • highly specific or generalists

    • DNA or RNA

    • Protein coat

    • Surface proteins highly susceptible to mutations

    • Cause synthesis of specialized structures to transfer viral particles to other cells


Chapter 13

  • Virion

    • complete, fully developed, infectious viral particle

    • Living or non-living entities?????


Viral classification
Viral Classification

  • Oldest system based on symptomology

  • International Committee on Taxonomy of Viruses (ICTV), 1966

    • Describe viruses as elementary bio-systems

    • Classified into orders, families, genera and species

    • Over 1,500 officially recognized species


Chapter 13

3 primary means of classification:

Nucleic acid

Replication strategy

Morphology of protein coat (capsid)


Helical viruses
Helical Viruses

Polyhedral Viruses


Chapter 13

  • Viral Envelope

    • Acquired from host cell

    • Phospholipids and proteins

    • Some glycoproteins are virally coded spikes

    • Often play role in host recognition





Viral taxonomy
Viral Taxonomy

  • Order –virales

  • Family –viridae

  • Genus – virus

  • Species

    • Common names

    • Subspecies designated by a number


Viral taxonomy1
Viral Taxonomy

  • Retroviridae– family

    • Lentivirus – genus

      • Human Immunodeficiency Virus– species

  • Herpesviridae

    • Simplexvirus

      • Human herpesvirus 1, HHV 2, HHV 3


Isolation and cultivation of viruses
Isolation and Cultivation of Viruses

  • Viruses must be grown in living cells

    • Cytopathic effects



Chapter 13


Virus identification
Virus Identification embryonated eggs

  • Serological tests

    • Detect antibodies against viruses in a patient

  • Nucleic acids

    • RFLPs

    • PCR


Multiplication of bacteriophages

Lytic cycle Phage causes lysis and death of host cell embryonated eggs

Lysogenic cycle Temperate phages incorporate DNA into host DNA (prophage)

Multiplication of Bacteriophages


Lytic cycle
Lytic Cycle embryonated eggs

  • Attachment Phage attaches by tail fibers to host cell

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

  • Biosynthesis Production of phage DNA and proteins

  • Maturation Assembly of newly synthesized phage particles

  • Release Phage lysozyme breaks cell wall


Chapter 13

[INSERT FIGURE 13.8] embryonated eggs


One step growth curve
One-step Growth Curve embryonated eggs


Lysogenic cycle
Lysogenic Cycle embryonated eggs


Chapter 13


Specialized transduction
Specialized Transduction embryonated eggs

gal gene

Bacterial DNA

Prophage

1

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

Galactose-positive donor cell

gal gene

2

Phage genome excises, carrying with it the adjacent gal gene from the host.

3

Phage matures and cell lyses, releasing phage carrying gal gene.

gal gene

4

Phage infects a cell that cannot utilize galactose (lacking gal gene).

Galactose-negative recipient cell

5

Along with the prophage, the bacterial gal gene becomes integrated into the new host’s DNA.

6

Lysogenic cell can now metabolize galactose.

Galactose-positive recombinant cell


Multiplication of animal viruses
Multiplication of Animal viruses embryonated eggs

  • Attachment Viruses attaches to cell membrane

  • Penetration By endocytosis or fusion

  • Uncoating By viral or host enzymes

  • Biosynthesis Production of nucleic acid and proteins

  • Maturation Nucleic acid and capsid proteins assemble

  • Release By budding (enveloped viruses) or rupture


Chapter 13

  • Attachment of animal viruses embryonated eggs

    • Chemical attraction

    • No tails or tail fibers

    • Glycoprotein spikes or other attachment molecules


Chapter 13

  • Replication of Animal Viruses embryonated eggs

    • Biosynthesis

      • Each virus requires different strategy depending on its nucleic acid

      • DNA viruses often enter the nucleus

      • RNA viruses typically replicate in cytoplasm

      • Must consider:

        • What serves as template for replication and how mRNA is transcribed


Multiplication of dna virus
Multiplication of DNA Virus embryonated eggs

Papovavirus

1

Virion attaches to host cell

7

Virions are released

Host cell

DNA

Capsid

2

DNA

Virion penetrates cell and its DNA is uncoated

Cytoplasm

6

Virions mature

Capsid proteins

mRNA

5

Late translation; capsid proteins are synthesized

3

Viral DNA penetrates host nucleus

Early transcription and translation; enzymes are synthesized

4

Late transcription; DNA is replicated


Multiplication of rna virus
Multiplication of RNA Virus embryonated eggs

[INSERT FIGURE 13.13]


Multiplication of a retrovirus
Multiplication of a Retrovirus embryonated eggs

Capsid

Reverse transcriptase

DNA

Virus

Two identical + stands of RNA

1

Retrovirus penetrates host cell.

Host cell

DNA of one of the host cell’s chromosomes

5

Mature retrovirus leaves host cell, acquiring an envelope as it buds out.

Reverse transcriptase

2

Virion penetrates cell and its DNA is uncoated

Viral RNA

Identical strands of RNA

4

Transcription of the provirus may also occur, producing RNA for new retrovirus genomes and RNA that codes for the retrovirus capsid and envelope proteins.

Viral proteins

RNA

3

The new viral DNA is tranported into the host cell’s nucleus and integrated as a provirus. The provirus may divide indefinitely with the host cell DNA.

Provirus


Chapter 13

  • Assembly and release of animal viruses embryonated eggs

    • Most DNA viruses assemble in and are released from nucleus into cytosol

    • Most RNA viruses develop solely in cytoplasm

    • Enveloped viruses cause persistent infections

    • Naked viruses are released by exocytosis or may cause lysis and death of host cell


Viruses and cancer
Viruses and Cancer embryonated eggs

  • Oncogenes transform normal cells into tumor cells

    • Activated by mutagenic chemicals, radiation, viruses

    • Causes increased growth, loss of contact inhibition

    • Cells tend to be misshapen and exhibit chromosomal abnormalities


Chapter 13

  • Oncoviruses embryonated eggs

    • Viral DNA integrated into host DNA

    • Induces tumors


Oncogenic viruses
Oncogenic Viruses embryonated eggs

  • Oncogenic DNA Viruses

    • Adenoviridae

    • Herpesviridae

    • Poxviridae

    • Papovaviridae

    • Hepadnaviridae

  • Oncogenic RNA viruses

    • Retroviridae

      • DNA

      • HTLV 1

      • HTLV 2


Chapter 13

  • Latent Viral Infections embryonated eggs

    • Virus remains dormant in asymptomatic host cell for long periods

      • Cold sores, shingles

  • Persistent Viral Infections

    • Disease progresses slowly over a long period, generally fatal

      • Subacute sclerosing panencephalitis (measles virus)


Chapter 13

  • Plant Viruses embryonated eggs

    • Plant viruses enter through wounds or via biting insects

    • May be transmitted in pollen

  • Viroids

    • infectious naked RNA

Potato Spindle Tuber Viroid


Prions
Prions embryonated eggs

  • Proteinaceous infectious particle

  • Inherited and transmissible diseases

  • Spongiform encephalopathies

    • Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease


Prp c normal cellular prion protein prp sc scrapie protein
PrP embryonated eggsC, normal cellular prion proteinPrPSc, scrapie protein


Chapter 13

[INSERT FIGURE 13.23] embryonated eggs

scrapie proteinsaccumulate in brain cells forming large vacuoles