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

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

• Wendell Stanley, 1935 – Isolated tobacco mosaic virus

[INSERT FIGURE 13.4]

• 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

• Virion

– complete, fully developed, infectious viral particle– Living or non-living entities?????

• 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

Viral Classification

• 3 primary means of classification:– Nucleic acid

– Replication strategy

– Morphology of protein coat (capsid)

Helical Viruses

Polyhedral Viruses

• Viral Envelope– Acquired from host cell– Phospholipids and proteins– Some glycoproteins are virally coded spikes – Often play role in host recognition

Critical Swine Flu prevention tip:

Don't DO this!

Complex Viruses

[INSERT FIGURE 13.5]

Viral Taxonomy

• Order –virales

• Family –viridae

• Genus – virus

• Species– Common names– Subspecies designated by a number

Viral Taxonomy

• Retroviridae– family

– Lentivirus – genus • Human Immunodeficiency Virus– species

• Herpesviridae – Simplexvirus

• Human herpesvirus 1, HHV 2, HHV 3

Isolation and Cultivation of Viruses

• Viruses must be grown in living cells– Cytopathic effects

• Animal viruses may be grown in living animals or in embryonated eggs

• Animal & plants viruses may be grown in cell cultures– Primary cell lines – Continuous cell lines (transformed cells )

• Serological tests– Detect antibodies against viruses in a patient

• Nucleic acids– RFLPs– PCR

Virus Identification

Multiplication of Bacteriophages

• Lytic cycle Phage causes lysis and death of host cell

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

• 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

Lytic Cycle

[INSERT FIGURE 13.8]

One-step Growth Curve

Lysogenic Cycle

• 3 Important results of lysogeny– Immunity to re-infection – Phage conversion– Specialized transduction

Specialized Transduction

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

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

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

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2

3

Prophage

gal gene

gal gene Bacterial DNA

Galactose-positive donor cell gal gene

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

4

Galactose-negative recipient cell

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

5

Lysogenic cell can now metabolize galactose.

6

Galactose-positive recombinant cell

• AttachmentViruses attaches to cell membrane• PenetrationBy 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

Multiplication of Animal viruses

• Attachment of animal viruses– Chemical attraction– No tails or tail fibers– Glycoprotein spikes or other attachment molecules

• Replication of Animal Viruses– 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 VirusVirion attaches to host cell

Virion penetrates cell and its DNA is uncoated

Viral DNA penetrates host nucleusEarly transcription and translation; enzymes are synthesized

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2

3

DNA

Late transcription; DNA is replicated

4

Late translation; capsid proteins are synthesized

5

Virions mature6

Capsid

Papovavirus

Host cell

DNA

Cytoplasm

Virions are released7

Capsid proteins

mRNA

[INSERT FIGURE 13.13]

Multiplication of RNA Virus

Multiplication of a Retrovirus

Retrovirus penetrates host cell.

Virion penetrates cell and its DNA is uncoated

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.

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2

3

DNA

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

4

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

5

CapsidReverse transcriptase

Virus Two identical + stands of RNA

DNA of one of the host cell’s chromosomes

Provirus

Host cell

Reverse transcriptase

Viral RNA

RNA

Viral proteins

Identical strands of RNA

• Assembly and release of animal viruses• 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

• 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

Viruses and Cancer

•Oncoviruses•Viral DNA integrated into host DNA•Induces tumors

• Oncogenic DNA Viruses– Adenoviridae– Herpesviridae– Poxviridae– Papovaviridae– Hepadnaviridae

Oncogenic Viruses

• Oncogenic RNA viruses– Retroviridae

• DNA• HTLV 1• HTLV 2

• Latent Viral Infections– 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)

• Plant Viruses– Plant viruses

enter through wounds or via biting insects

– May be transmitted in pollen

• Viroids– infectious naked

RNA

Potato Spindle Tuber Viroid

• Proteinaceous infectious particle• Inherited and transmissible diseases• Spongiform encephalopathies

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

Prions

PrPC, normal cellular prion proteinPrPSc, scrapie protein

[INSERT FIGURE 13.23]

scrapie proteins accumulate in brain cells forming large vacuoles