VIRUSES, VIROIDS

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CHAPTER 13

• VIRUSES, VIROIDS PRIONS

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General Characteristics

• Obligatory intracellular parasites - do not
  possess mechanisms to replicate outside of a host
  cell
• Contain RNA or DNA (not both)
• Protein coat - sometimes an envelope
• Cause synthesis of specialized structures that
  can transfer viral nucleic acid to other cells

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Host Range

• All living organisms can be infected by specific
  viruses
• Most viruses can infect only specific cells
  determined by
• requirements for attachment to host cell
  (receptor sites)
• and metabolic needs w/in cell in order to multiply

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Viral Size

• Length 20 - 14,000 nm
• Much smaller than bacteria - therefore filterable
  (see intro for discovery of infectious agent that
  could pass through porcelain filter also see
  that many originated in animals

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Viral Structure

• Virions classified by protein coats -capsids
• DNA, double stranded RNA, or single stranded RNA
• This is a nonenveloped virus

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Enveloped virus

• Envelope - lipid, protein, carbohydrate, may have
  attachment spikes also nonenvel-oped viruses
• Virus surface proteins interact with host
  antibodies but they may mutate! Escape detection
  by antibody

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General Morphology

• This is a helical virus - Rabies and Ebola
• Polyhedral viruses
• Enveloped viruses are spherical (or enveloped
  helical or enveloped polyhedral

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Bacteriophage

• Complex viruses w/ attachment structures
• Virus lands on bacteria injects DNA through
  sheath protein capsid remains on outside of cell
• New capsid made inside host cell

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Isolation, cultivation identification

• Growth in living organisms - difficult and
  expensive
• Bacteriophage growth in labs - in Petri plate
  with host bacteria growing viral infection gtgt
  plaques have learned much about virues in
  general from bacteriophage
• Animal virus - often use small mammals some
  cannot be growngtgt ethical issues

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Cultivation in embryonated egg

• Virus injected into appropriate region
• Method widely used for production of vaccines

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Cultivation in cell cultures
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Cultivation in cell culture

• Cytopathic effect - normal monolayer structure is
  disrupted by viral infection
• Cell lines developed from embryonic tissue
• Continuous cell lines (immortal) - HeLa
• Maintenance of cell culture lines is technically
  difficult must be kept free of microbial
  contamination.

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Viral Multiplication

• Viruses have small nucleic acid - code mainly for
  its own replication sometimes enzymes for host
  cell penetration structures
• Two alternative life cycles observed in
  bacteriophages
• lytic cycle ends with host cell lysis and death
• lysogenic - host cell remains alive as viral DNA
  incorporates with host cell DNA

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Bacteriophage lytic cycle

• Attachment - phage attaches to host cell- fibers
  interact with host cell receptors
• Penetration- lysozyme from phage tail breaks down
  host cell wall DNA passes through shaft into
  cytoplasm

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Bacteriophage lytic cycle - 2

• Biosynthesis - host DNA shut down host molecules
  used for viral DNA synthesis followed by protein
  synthesis
• Maturation - assembly of virions
• Release - lysozyme lyses cell

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Bacteriophage one-step growth curve

• Samples of phage particles removed from culture
  innoculated onto plate culture periodically to
  determine number of infective phage particles in
  culture
• of phage particles is constantgtgt burst size

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Bacteriophage lysogenic cycle

• Aka - temperate phage
• penetration, DNA circle inserts into host DNA as
  a prophage
• Replicates along with host cell DNA
• Event may cause prophage separate, reproduce
  lyse host cell (lytic cycle)

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Lysogeny results

• Host cell immunity
• Phage conversion
• Specialized trans- duction - Phage DNA picks up
  bits of host DNA ...

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Lysogeny results-2

• Phage lyses host cell and infects new cell, Phage
  transfers bacterial genes to new cell
• Animal viruses can also be latent

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Multiplication of animal viruses

• Attachment to plasma membrane receptors
• production of receptor sites is inherited some
  produce more, some less
• Penetration - endocytosis
• Enveloped viruses - fusion- envelope fuses with
  plasma membrane
• Uncoating - viral nucleic acid separates from
  protein coat - may use lysosome enzymes

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Biosynthesis of DNA viruses

• Viral enzymes used to replicate DNA inside
  eukaryotic cells nucleus
• Host enzymes synthe size viral protein - enter
  nucleus where virions are assembled
• Adeno-, papo-, herpes, hepadna viruses

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Biosynthesis of RNA viruses

• Attachment, penetra- tion and uncoating
• RNA sense strand forms complementary
  antisense strand

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Biosynthesis of RNA viruses-2

• May for mRNA for protein synthesis or
• May form new viruses

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Multiplication of Retrovirus

• Attachment, penetra- tion and uncoating
• Reverse transcriptase forms DNA from RNA
  provirus incorporates in host chromosome
• May be latent or produce new viruses

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Budding of enveloped virus

• Envelope encoded by viral genes, incorporated
  into host cell plasma membrane
• Plasma membrane forms envelope around capsid
• Organism extrudes from cell

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Viruses and Cancer

• First discovered when cell-free filtrates
  transferred leukemia to healthy chickens
• Then mouse mammary gland tumors transmitted
  through mothers milk

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Transformation of normal cells to tumor cells

• Oncogenes cause DNA alterations - cancer
• Oncogenic viruses
• Contact inhibition prevents further cell division
  once cells touch each other
• Transformed cells lose contact inhibition

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Latent persistent viral infections

• Virus inhabits host nerve cells activated by
  fever or sunburngtgt skin infection
• Virus may be present w/ no symptoms
• Chicken pox virus latent in blood - emerges as
  shingles
• Slow viral infections (persistent) - gradual
  increase in viral particlesgtgt bad diseases (Table
  13.5)

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Prions

• Proteinaceous infectious particle
• 9 neurological diseases- spongiform
  encephalopathies
• PrPC in normal DNA
• PrPSc (abnormal) can change PrPC to PrPSc on
  contact gtgt plaques

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Plant Viruses and Viroids

• Most similar to animal viruses, enter cells
  through wounds or w/ animal parasites
• Viroids - short pieces of naked RNA w/ folded 3
  dimensional structure
• Cause plant diseases - potato spindle tuber
  viroid disease