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VIRUSESCHAPTER 10
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What are Viruses?
Obligate intracellular parasites
Viral components– Nucleic acids
– Capsid (protein)
– Envelope(Lipid w/intg proteins)
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4 overall types of viruses
1) bacteriophages - almost always DNA with a protein capsid. Lytic and lysogenic types
2) DNA viruses of Eukaryotes - often have phospholipid envelope outside of capsid
3) RNA viruses - have RNA as genetic material. Often hypermutable
4) RNA retroviruses - have reverse transcriptase. Many can integrate into host chromosome
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Viral Shapes and Sizes Helical Icosahedral T4 and adenovirus
TMV, M13
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Some viruses
T-evenHIV lambda
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Infectious Properties
Viral Host range
Viral specificity
Viral Origins
Depends on target receptor
Depends on target receptor
Selfish DNA? Transposable elements
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Bacteriophage
Most diverse?
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Bacteriophages
Plaque counts
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Bacteriophages Replication
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Bacteriophage = Virus that attacks bacteria and replicates by invading a living cell and using the cell’s molecular machinery.
Structure of T2 phage
DNA & protein
Hershey-Chase Bacteriophage Experiment - 1953
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Fig. 2.5: Life cycle of virulent T2 phage: Lytic cyle
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1. T2 bacteriophage is composed of DNA and proteins:
2. Set-up two replicates:
• Label DNA with 32P• Label Protein with 35S
3. Infected E. coli bacteria with two types of labeled T2
4. 32P is discovered within the bacteria and progeny phages, whereas 35S is not found within the bacteria but released with phage ghosts.
Fig. 2-6: Hershey-Chase Bacteriophage Experiment - 1953
1969: Alfred Hershey
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Composition and Structure
Composition– Nucleic acid
• Genome size• Modified
bases– Protein
• Protection• Infection
Structure (T4)– Size– Head or capsid– Tail
Tail
Tail Fibers
Base Plate
Head/Capsid
Contractile Sheath
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Infection of Host Cells
Irreversible attachment
• Adsorption– LPS for T4
Nucleic acid injection
Sheath Contraction
lamB for
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Bacteriophages
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Lytic Phage Multiplication Cycle
Eclipse– Early genes– Phage DNA
synthesis– Late genes
Intracellular accumulation
Lysis and Release
Total Phage
Extracellular Phage
EclipseIntracellular accumulatio
n phase
Time after Infection
Nu
mb
er o
f In
fect
iou
s P
arti
cles
Lysis
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Assay for Lytic Phage
Plaque assay– Method– Plaque forming unit
(pfu)– Measures infectious
particles
Bacteria
Phage
+Phage
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Lytic vs Lysogenic Cycle?
Role of repressor Role of cro gene product Role of proteases
Lytic = copies and immediately lyses
Lysogenic = integrates into host chromosome
“Prophage” = the latent form of phage where viral genome is incorporated into host genome
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Bacteriophages Lysogenic
Lysogeny
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Fig. 19-6Gene designation Function
Transcribedby host RNApolymerase
Left endEarlypromoters
Promoter
Inhibits host restriction0.3
0.7 Protein kinase
11.1
T7 RNA polymeraseUnknown
Origin of DNA replication1.31.7233.5
4
5
6
DNA ligaseNonessentialInactivates host RNA polymeraseEndonucleaseLysozyme
Helicase, primase
DNA polymerase
ExonucleasePromoter
Promoter
Promoter
Promoter
Virion proteinHead protein
Head assembly proteinMajor head protein
Tail proteinTail protein
Virion proteinHead protein
Head protein
Head protein
Tail protein
7
910
1112
1314
15
16
17
18
19
DNA maturation
DNA maturation
Transcribedby T7 RNApolymerase
Proteins forDNA replicationand host lysis
Phage structuralcomponents andmaturation proteins
Bacteriophage T7
1. Replication cycle requires 25 minutes
2. Genome is linear double-stranded DNA of 39,737 bp
3. T7 encodes all of its own proteins for DNA replication and transcription
4. Time to complete 100 T7 genome copies from a single copy: 5 minutes
5. Burst size (Escherichia coli host): about 300 virions/cell
6. Head size, 45 nm
7. Forms large plaques
8. T7 promoters are unique and widely used in biotechnology
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Events Leading to Lysogeny
Circularization of the phage chromosome– Cohesive ends
Lygase
Closed Circle
Cohesive Ends
Linear Double Stranded Opened Circle
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Events Leading to Lysogeny
Site-specific recombination– Phage coded
enzyme
• Repression of the phage genome
– Repressor protein– Specific– Immunity to superinfection
gal bio
gal bio
gal
bio
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Termination of Lysogeny
Induction– Adverse
conditions
Role of proteases– recA protein– Destruction of
repressor
• Excision• Lytic growth
gal
bio
gal bio
gal bio
gal bio
• Gene expression
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Significance of Lysogeny
Model for animal virus transformation Lysogenic or phage conversion
– Definition: A change in the phenotype of a bacterial cell as a consequence of lysogeny
• Modification of Salmonella O antigen• Toxin production by Corynebacterium
diphtheriae
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Types of Bacteriophage
Lysogenic or temperate phage: Phage that can either multiply via the lytic cycle or enter a quiescent state in the bacterial cell. (e.g., )– Expression of most phage genes repressed – Prophage– Lysogen
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Viruses part II - Animals and Plants
Unique challenges. Must evade immune systems and mustcross 2 lipid bilayer barriers. (ie cross into nucleus)
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RNA Viruses
Chromosomal Arrangements– + strand (directly transcribed)
– – strand
– Double strand
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RNA Virus Families
11 RNA virus families– Picornaviridae (fmdv, polio)– Togaviridae (rubella)– Flaviviridae (hep C, west nile, yellow fever)– Orthomyxoviridae (flu)
RNA viruses more prone to mutation
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Fig. 19-18
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RNA Virus Families (cont.) Retroviridae (hep B, htlv)-retrovirus reverse transcriptase
Paramyxoviridae (measles, mumps, pneumonia) - ss strand
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RNA Virus Families (cont.)
Rhabdoviridae (rabies)
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RNA Virus Families (cont.)
Orthomyxoviridae (all influenza)
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RNA Virus Families (cont.)
Filoviridae
Bunyaviridae
Arenaviridae
Reoviridae
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DNA Virus Families Adenoviridae
Herpesviridae
Poxviridae
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DNA Virus Families (cont.) Papovaviridae
Hepadnaviridae
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DNA Virus Families (cont.)
Parvoviridae
Emerging viruses
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Viral Replication Activities
– Adsorption
– Penetration (virus or chromosome)
– Synthesis
– Maturation
– Release
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Animal Viruses DNA viruses
Envelope derives from cells own plasma membrane
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Animal Viruses
RNA viruses
Latent viruses
retroviruses
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Culturing Animal Viruses
Live animals
Eggs
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Culturing Animal Viruses
Cell Culture– Primary– Continuous
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Viral Cytopathic Effects Cytopathy
Teratogenic effectsJapanese word for “little monsters” mutations that affect tissue growth
Damage to cells
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Viruslike Agents PLANTS Satellites
Viroids
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Viruslike Agents
Prions
KuruCreutzfeld-JacobBSEScrapie
Alpha helix B-pleated sheet
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Viruses and Cancer Mechanism of
cancer causation
HPV
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Viruses and Cancer
Oncogenes/proto-oncogenes V-myc V-ras
Rous Sarcoma Virus RSV
Kaposi’s sarcoma - appears when immune system depressed probably by herpes virus 8
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Viruses to know something about
•HPV (DNA) • HIV (RNA)• Flu (RNA)• Adenovirus(DNA)• Herpes(DNA)
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Herpes Simplex
After initial infection, the viruses move to sensory nerves, where they reside as life-long, latent viruses.
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HPV human papilloma virus
• Causes warts and some strains cause cervical cancer
• teratogenic
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HIV human immunodeficiency virus
• RNA retrovirus• T-cell host (CD4+ T-killer cells)• needs protease to replicate• binds to CCR5 and CD4 receptors
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Adenovirus
• Common cold• Usually affects respiratory tract.• sometimes engineered for gene therapy• DS DNA virus
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Influenza
H = hemaglutininN = neuraminidase
• RNA virus• mutates rapidly• animal reservoirs• can cross species lines