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Viruses
&
Prions
• Virus – miniscule, acellular, infectious agent
having one or several pieces of either DNA
or RNA
• No cytoplasmic membrane, cytosol,
organelles
• Cannot carry out any metabolic pathway
• Neither grow nor respond to the environment
• Cannot reproduce independently
• Obligate intracellular parasites
Viruses
Viruses
• Viruses contain DNA or RNA
• And a protein coat - capsid
• Some are enclosed by an envelope
• Some viruses have spikes
• Infect only specific cells in a specific
host
• Obligate parasites – need living cells
History of Virology
• 1892 – viruses 1st mentioned
– Russian Bacteriologist – Dimitri Iwanowski
– TMD – tobacco mosaic disease • Filtered plant sap
• Liquid still infectious
• 1935 – TMV isolated and purified
– American Chemist – Wendell Stanley
• Virus = Latin for poison
Virus Sizes
Figure 13.1
To Study Needed
to Develop:
EM (TEM & SEM)
Ultracentrifuge
& tissue culture
Most 20nm -200nm
Growing Viruses
• Viruses must be
grown in living cells.
– Bacteriophages form
plaques on a lawn of
bacteria.
Growing Viruses
• Viruses 1st
cultivated in live
animals
• or in embryonated
eggs
Figure 13.7
Growing Viruses • Next Tissue culture techniques were
developed.
– Continuous cell lines may be maintained
indefinitely.
Figure 13.8
How do you see growing virus in living cells?
Virus Identification
Figure 13.9
Cytopathic effects - CPE
Figure 13.3a
Cytopathic Effects
TMV
Categorizing Viruses
1. Genome compostion – RNA or DNA • dsDNA
• ssRNA
• ssDNA
• dsRNA
Categorizing Viruses
2. Capsid – made up of capsomeres
(proteins) differentiated by symmetry
Helical viruses
• capsomeres arranged in a tube, nucleic
acid inside
Polyhedral viruses
• Icosahedral (20 faces, 12 corners)
Complex viruses
• capsid + tail + tail fibers + pins + face plate
Helical Viruses
Figure 13.4a, b
Polyhedral Viruses
Figure 13.2a, b
Complex
Viruses
Figure 13.5a
Categorizing Viruses
3. Presence of Envelope
• Is the capsid surrounded by a phospholipid bilayer
derived from the hosts plasma membrane
Viruses and Host Range
• Viruses have specific hosts
– Plant
– Animal
– Bacteria
• Host Range
– Determined by specific receptors on the host
cell surface
– What are these used for normally?
Viral Taxonomy • Viruses named after
– the disease they cause
• Poliovirus, mumps virus, measles virus
– the river near place of isolation
• Ebola virus
• Viruses that infect bacteria
– Are called bacteriophages
– Letters and numbers are used for names
• T4, λ phage
How are bacteria named?
Viral Life Cycles
• Bacteriophage
– Lytic cycle
– Lysogenic cycle
• Viruses
– DNA virus
– RNA virus
– Retrovirus
• Attachment Attachment of virus to bacterial cell
• Entry Entry of nucleic acid into host cell
• Synthesis Production of nucleic acid & proteins
• Assembly Nucleic acid and capsid proteins
assemble
• Release Viruses burst out of the host cell
Multiplication of Viruses in a
Bacterial Host Cell
Figure 13.8
Lytic Replication of Bacteriophages
Figure 13.8
Lytic Replication of Bacteriophages
Figure 13.9
Lytic Phage Replication Cycle
• Lytic cycle Phage causes lysis and
death of host cell
• Lysogenic cycle Prophage DNA
incorporated in host DNA (Temperate phages)
The Lysogenic Cycle
Figure 13.12
Specialized Transduction
Figure 13.13
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.
1
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
Viral Life Cycles
• Bacteriophage
– Lytic cycle
– Lysogenic cycle
• Viruses
– DNA virus
– RNA virus
– Retrovirus
• Adsorption Virus attaches to cell membrane
• Penetration By endocytosis or fusion
• Uncoating By viral or host enzymes
• Replication Production of nucleic acid & proteins
• Assembly Nucleic acid and capsid proteins
assemble
• Release By budding (enveloped viruses) or
rupture
Multiplication of Viruses in an
Animal Host Cell
• Same basic replication pathway as
bacteriophages
• Differences result from
–Presence of envelope around some viruses
–Eukaryotic nature of animal cells
–Lack of cell wall in animal cells
Replication of Animal Viruses
• Chemical attraction
• Animal viruses do not have tails or tail
fibers
• Have glycoprotein spikes or other
attachment molecules that mediate
attachment
Adsorption of Animal Viruses
Figure 13.3b
Viral Spikes
Figure 13.12ab
Penetration and Uncoating of Animal Viruses
Figure 13.12c
Penetration and Uncoating of Animal Viruses
Adsorption, Penetration, and
Uncoating
Figure 13.14
• Each type of animal virus requires different
strategy depending on its nucleic acid
• Must consider
–How mRNA is synthesized?
–What serves as template for nucleic acid
replication?
Replication of Animal Viruses
Replication & Assembly of DNA Virus
Figure 13.15
Virion attaches to host cell
Virion penetrates cell and its DNA is uncoated
Early transcription and translation; enzymes are synthesized
1
2
3
DNA
Late transcription; DNA is replicated
4
Late translation; capsid proteins are synthesized
5
Virions mature 6
Capsid
Papovavirus
Host cell
DNA
Cytoplasm
Virions are released 7
Capsid proteins
mRNA
Replication & Assembly in
RNA Viruses
Figure 13.17
Replication & Assembly of a Retrovirus
Figure 13.19
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.
1
2
3
Envelop
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
Capsid Reverse 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
Table 13.3
Summary of Replication &
Assembly of Animal Viruses
Release of an enveloped virus by
budding
Figure 13.20
Figure 13.13
Release of Enveloped Viruses
by Budding
• DNA viruses - assembly in nucleus
• RNA viruses develop solely in cytoplasm
• Number of viruses produced &released depends on
– type of virus
– Size of virus
– initial health of host cell
• Enveloped viruses cause persistent infections
• Naked viruses released by exocytosis or may
cause lysis and death of host cell
Assembly and Release of Animal Viruses
• Some viruses can remain dormant
• May exist for years with no viral activity,
signs, or symptoms
• Some latent viruses do not become
incorporated into host chromosome
• When provirus is incorporated into host DNA,
condition is permanent; becomes permanent
physical part of host’s chromosome
Latency of Animal Viruses
Summary of Bacteriophage and
Animal Virus Replication
Table 13.4
Cytopathic Effects
Figure 13.9
Cytopathic effects - CPE
Causes of Cytopathic Effects
1. Host cell DNA, RNA, and protein synthesis has been stopped by the virus
2. Fusion of the plasma membranes of many cells (Herpes viruses)
3. Inclusion bodies in host cytoplasm – Rabies = Negri bodies (viral particles)
4. Toxic effect of capsid proteins – Mumps virus & Influenza virus
5. Host Chromosomal Disruptions – Herpes virus
6. Transformation of cells into malignant cells
• Transformed/malignant cells = uncontrolled
cell division
• Oncovirus – has oncogenic effect on the
host (causes cancer)
– Genetic material of oncogenic viruses becomes
integrated into the host cell's DNA.
– Activated oncogenes transform normal cells into
cancerous cells.
– Transformed cells have increased growth, loss
of contact inhibition, and T antigens.
Cancer
Figure 13.15
Oncogene
Theory
• Ultraviolet light
• Radiation
• Carcinogens
• Viruses
Factors Involved in Activation of
Oncogenes
• Some carry copies of oncogenes as part of their
genomes
• Some stimulate oncogenes already present in host
• Some interfere with tumor repression when they
insert into host’s repressor gene
• Several DNA and RNA viruses are known to cause
~15% of human cancers
– Burkitt’s lymphoma
– Hodgkin’s disease
– Kaposi’s sarcoma
– Cervical cancer
How Viruses Cause Cancer
• Oncogenic DNA Viruses
– Adenoviridae
• Adenovirus -
adenocarcinomas
– Herpesviridae
• EBV – Burkitt’s lymphoma
– Poxviridae
• Smallpox, cowpox – misc.
– Papovaviridae
• Human papilloma virus –
cervical cancer
– Hepadnaviridae
• HBV – liver cancer
Oncogenic Viruses
• Oncogenic RNA viruses
– Retroviridae
• HTLV 1 (human T-
lymphotrophic virus)
• HTLV 2
– Causes leukemia in
adults
• Acute Viral Infections –
– Rapid onset, short duration • Influenza
• Latent Viral Infections
– Virus remains dormant and is later induced into activity
• HSV1 - Cold sores, varicella-zoster virus - shingles
• Chronic/Persistent Viral Infections
– Long duration, generally fatal • HCV, Subacute sclerosing panencephalitis (measles
virus)
Types of Infections
• Cytopathic effects
• Serological tests
– Detect antibodies against viruses in a patient
– Use antibodies to identify viruses in
neutralization tests, viral hemagglutination, and
Western blot
• Nucleic acids
– RFLPs
– PCR
Virus Identification
• 1982 purposed by American neurobiologist
– Stanley Prusiner
• Infectious proteins
• Transmissible by ingestion, transplant, & surgical
instruments
• Spongiform encephalopathies:
– Sheep scrapie, Creutzfeldt-Jakob disease, Kuru, fatal
familial insomnia, mad cow disease (bovine spongiform
encephalopathy)
– Large vacuoles in the brain
Prions
Prions
• Prion = proteinaceous infectious particle
– Reduced infectivity = proteases, not radiation
• PrPC, normal cellular prion protein
(glycoprotein), on cell surface
• PrPSc, scrapie protein, accumulate in brain
cells forming plaques
• Two stable tertiary structures of PrP
–Normal functional structure with α-helices called
cellular PrP
–Disease-causing form with β-sheets called prion
PrP
• Prion PrP converts cellular PrP into prion PrP
by inducing conformational change
Characteristics of Prions
Figure 13.21
Tertiary Structures of PrP
• Normally, nearby proteins and
polysaccharides force PrP into cellular shape
• Mutations in PrP gene result in initial
formation of prion PrP
• When prions present, they cause newly
synthesized cellular PrP to refold into prion
PrP
Characteristics of Prions
Prions
Figure 13.21
PrPc
PrPSc
1 2 3 4
5 6 7 8
Endosome
Lysosome
• All = fatal neurological degeneration,
– deposition of fibrils in brain,
– & loss of brain matter
• Large vacuoles form in brain
– = spongy appearance
• Spongiform encephalopathies – BSE, CJD,
kuru
• Only destroyed by incineration; not cooking
or sterilization
Prion Diseases
Figure 13.22
Scrapie in Sheep
Kuru