VIRUSES

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VIRUSES

• CHAPTER 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
TMV, M13

• Helical
• Icosahedral

T4 and adenovirus
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Some viruses
T-even
lambda
HIV
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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 cells molecular machinery.
Structure of T2 phage DNA
protein
Hershey-Chase Bacteriophage Experiment - 1953
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Lytic cyle
Fig. 2.5 Life cycle of virulent T2 phage
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(No Transcript)
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Composition and Structure

• Composition
• Nucleic acid
• Genome size
• Modified bases
• Protein
• Protection
• Infection

• Structure (T4)
• Size
• Head or capsid
• Tail

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Infection of Host Cells

• Adsorption
• LPS for T4

lamB for l

• Irreversible attachment

• Sheath Contraction

• Nucleic acid injection

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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

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Assay for Lytic Phage
Phage

• Plaque assay
• Method
• Plaque forming unit (pfu)
• Measures infectious particles

Bacteria

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. 19-6
Function
Gene designation
Left end
Early promoters
Inhibits host restriction
0.3
0.7
Protein kinase
Transcribed by host RNA polymerase
Bacteriophage T7
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T7 RNA polymerase Unknown
1.1
1. Replication cycle requires 25 minutes
Promoter
Origin of DNA replication
1.3
DNA ligase
2. Genome is linear double-stranded DNA of
39,737 bp
1.7
Nonessential
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Inactivates host RNA polymerase
Proteins for DNA replication and host lysis
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Endonuclease
3. T7 encodes all of its own proteins for DNA
replication and transcription
3.5
Lysozyme
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Helicase, primase
4. Time to complete 100 T7 genome copies from
a single copy 5 minutes
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DNA polymerase
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Exonuclease
5. Burst size (Escherichia coli host) about
300 virions/cell
Promoter
Virion protein
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Head protein
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6. Head size, 45 nm
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Head assembly protein
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Major head protein
7. Forms large plaques
Promoter
Tail protein
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Transcribed by T7 RNA polymerase
8. T7 promoters are unique and widely used in
biotechnology
Tail protein
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Promoter
Virion protein
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Head protein
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Phage structural components and maturation
proteins
Head protein
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Head protein
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Promoter
Tail protein
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DNA maturation
DNA maturation
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Events Leading to Lysogeny

• Circularization of the phage chromosome
• Cohesive ends

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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

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Termination of Lysogeny

• Induction
• Adverse conditions
• Role of proteases
• recA protein
• Destruction of repressor

• Gene expression

• Excision

• Lytic growth

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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 must cross 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 effects

Damage to cells
Japanese word for little monsters mutations
that affect
tissue growth
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Viruslike Agents PLANTS

• Satellites
• Viroids

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Viruslike Agents

• Prions

Kuru Creutzfeld-Jacob BSE Scrapie
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
Kaposis 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 hemaglutinin N neuraminidase
RNA virus mutates rapidly animal
reservoirs can cross species lines