Viruses

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Viruses

• Chapter 13

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Viruses, Viroids and Prions Ch. 13
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Study of Viruses - Virology

• 5 Kingdoms
• 1. Plantae
• 2. Animalia
• 3. Fungi
• 4. Protista
• 5. Monera

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5 Characteristics of Life

• 1. Cells
• 2. Grow and maintain their structure by taking up
  chemicals and energy from the environment
• 3. Respond to their external environment
• 4. Reproduce and pass on their organization to
  their offspring
• 5. Evolve and Adapt to their environment

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Viruses are

• 1. Acellular
• 2. Obligate intracellular parasites
• 3. No ATP generating system
• 4. No Ribosomes or means of Protein Synthesis

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Typical Virus 2 Parts

• 1. Nucleic Acid
• DNA or RNA (But never both)
• 2. Capsid (Coat Protein)
• Some Viruses
• A. Envelope
• B. Enzymes

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Host range

• Spectrum of host cells that a virus can infect
• Some viruses only infect
• plants
• invertebrates
• protists
• fungi
• bacteria (Bacteriophages)

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Host range

• Most viruses have a narrow host range
• Polio virus - nerve cells
• Adenovirus - cells in upper Respiratory Tract

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Host range is determined by Viruses ability to
interact with its host cell

• Binding Sites match Receptor Sites
• Binding Sites - on viral capsid or envelope
• Receptor Sites - on host cell membrane

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Viral Size
20 nm to 1,000 nm
.02 u to 1 u
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Viral Structure

• 1. Nucleic Acid
• 2. Capsid (Coat Protein)
• Nucleic Acid
• DNA or RNA (But never both)
• ssDNA
• ds DNA
• ss RNA
• ds RNA

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Viral Structure

• Capsid (Coat Protein)
• protects viral genome from host endonucleases
• capsomeres
• Binding Sites
• Envelope
• derived from the host cell
• Binding Sites

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Viral Morphology
1. Helical
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Viral Morphology
2. Polyhedral
icosahedral
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Viral Morphology
3. Enveloped
A. Enveloped Helical B. Enveloped Polyhedral
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Viral Morphology
4. Complex
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Viral Classification

• 1. Nucleic Acid
• 2. Morphology
• 3. Strategy for replication

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Growing Viruses

• 1. Bacteriophages
• Lawn of Bacteria on a Spread Plate
• Add Bacteriophages
• Infection will result in Plaques
• Clear zones on plate

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Growing Viruses

• Animal Viruses
• A. Living Animals
• mice, rabbits, guinea pigs
• B. Chicken Embryos (Eggs)
• used to be most common method to grow viruses
• Still used to produce many vaccines (Flu
  Vaccine)
• C. Cell Cultures
• Most common method to grow viruses today

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Cell Cultures

• 1. Primary Cell Lines
• die out after a few generations
• B. Diploid Cell Lines
• derived from human embryos
• maintained for up to 100 generations
• C. Continuous Cell Lines
• Transformed Cells (Cancerous Cells)
• may be maintained indefinitly
• HeLa Cells
• Henrietta Lax 1951 (Cervical Cancer)

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Viroids and Prions

• Viroids
• Naked RNA (no capsid)
• 300 400 nucleotides long
• Closed, folded, 3-dimensional shape (protect
  against endonucleases ?)
• Plant pathogens
• Base sequence similar to introns

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Prions

• Proteinaceous infectious particle
• 1982
• Diseases
• Scrapie (sheep)
• Creutzfeldt-Jacob disease (CJD)
• Kuru (Tribes in New Guinea)
• Bovine Spongiform Encephalopathy (BSE)
• Mad Cow Disease

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Viral Replication

• Bacteriophage
• 1. Lytic Cycle
• 2. Lysogenic Cycle

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Lytic Cycle

• 1. Attachment- binding sites must match receptor
  sites on host cell
• 2. Penetration - viral DNA is injected into
  bacterial cell
• 3. Biosynthesis
• Genome replication
• Transcription
• Translation

Virus uses Host Cells enzymes and machinery
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Lytic Cycle

• 4. Assembly (Maturation)
• viral particles are assembled
• 5. Release
• Lysis

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Lysogenic Cycle

• 1. Attachment
• 2. Penetration
• 3. Integration
• Viral Genome is integrated into Host Cell Genome
• Virus is Latent
• Prophage

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Lysogenic Cycle

• 4. Biosynthesis - Viral Genome is Turned On
• Genome replication
• Transcription
• Translation
• 5. Assembly
• 6. Release
• Lysis

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Lysogenic Convergence

• 1. Corynebacterium diphtheriae
• 2. Streptococcus pyogenes
• Scarlet Fever
• 3. Clostridium botulinum

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Animal Virus Replication(non-enveloped virus)

• 1. Attachment
• Binding Sites must match receptor sites on host
  cell
• 2. Penetration
• Endocytosis (phagocytosis)
• 3. Uncoating
• separation of the Viral Genome from the capsid

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Animal Virus Replication(non-enveloped virus)

• 4. Biosynthesis
• Genome Replication
• Transcription
• Translation
• 5. Assembly
• Virus particles are assembled
• 6. Release
• Lysis

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Enveloped Virus Replication

• 1. Attachment
• 2. Penetration
• 3. Uncoating
• 4. Biosynthesis
• 5. Assembly
• 6. Release
• Budding

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Retro Viruses (1975)

• DNA ---------gt mRNA ------------gt Protein
• Central Dogma of Molecular Genetics

Normal Virus
RNA -------gt DNA --------gt mRNA -------gt
Protein
Retro Virus
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Reverse Transcriptase (Retro)
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Retro Viruses

• 1. Many Cancer causing viruses
• 2. HIV
• Human Immunodeficiency Virus
• AIDS
• Acquired Immunodeficiency Syndrome

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HIV (Human Immunodeficiency Virus)

• AIDS
• Acquired Immune Deficiency Syndrome
• results in failure of the immune system
• Death usually results from an Opportunistic
  Infection
• HIV discovered in 1984
• By who ?
• Luc Montagneir - Pasteur Institute

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HIV Structure
Retro Virus Nucleic acid - RNA (2
strands) envelope (gp 120 binding
sites) Reverse Transcriptase
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HIV Infection (Cellular Level)
1. Attachment
HIV gp 120 binding sites must match CD4 receptor
sites
CD4 Receptor Sites 1. Macrophages 2. Some
cells of CNS 3. T4 Helper Cells (CD4 Cells)
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HIV Infection
2. Penetration
Viral membrane and host cell membrane merge
(fusion)
3. Uncoating
Capsid is removed and Viral Genome is exposed
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HIV Infection
4. Integration
Once Viral Genome is integrated - 2 possibilities
1. Nothing - Virus is Latent
Virus may be latent for days, weeks, months or
years
Median latency time 10 years
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Latent HIV provirus
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2. HIV Genome can be expressed or Turned On

• Once HIV Genome is turned on death usually
  results within 2 years
• What causes the HIV Genome to be turned on?
• Other infections
• Stress or shock to the system
• Drug abuse
• Alcohol abuse
• Nutrition
• Exercise (Lack of or too much?)
• Sunburn ?
• (Herpes Simplex 1)

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Once HIV Genome is turned on

• 5. Biosynthesis
• Genome replication
• Transcription
• Translation
• 6. Assembly
• Virus particles are put together
• 7. Release
• Budding

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Modes of HIV Transmission

• HIV is transmitted by exposure to infected body
  fluids
• 4 Body Fluids
• 1. Blood
• 2. Semen
• 3. Vaginal Secretions
• 4. Breast Milk

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How are these fluids transferred from one person
to another?

• 1. High Risk Sexual Contact
• unprotected vaginal sex
• unprotected oral sex
• unprotected anal sex
• 2. Needles
• Intravenous Drug Abuse (sharing dirty needles)
• accidental needle sticks

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How are these fluids transferred from one person
to another?

• 3. Blood to Blood Contact
• open sores or wounds
• Transfusions
• Organ Transplants
• Artificial Insemination
• 4. Mother to Child
• placenta
• as baby passes thru the birth canal
• breast milk

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HIV and the Immune System

• 1. Cellular Immune System
• cells phagocytize microbes
• 2. Humoral Immune System
• antibodies to destroy or inactivate microbes

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Clinical Stages of an HIV Infection

• 1. Acute Infection
• Initial infection of HIV (exposure to infected
  body fluids)
• Viremia
• Fever
• Headaches
• Weakness
• Muscle and joint aches
• May last for a couple of weeks
• Normal CD4 cell count 1200mm3

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2. Asymptomatic Disease

• CD4 cell count lt 1000mm3
• Virus is latent inside CD4 cells
• Median latency period - 10 yrs.
• No signs or symptoms of illness (asymptomatic)
• HIV Positive - antibodies can be detected in your
  blood
• Seroconversion
• 6 to 8 weeks

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3. Symptomatic Disease

• CD4 cell count lt 600mm3
• Viral Genome is turned on, Symptoms begin to
  appear
• What causes HIV Genome to be turned on?
• Other infections
• stress
• shock to the system
• alcohol
• drug abuse
• nutrition
• exercise ?

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3. Symptomatic Disease

• Symptoms
• chronic fatigue
• low-grade fever
• night sweats
• diarrhea
• weight loss
• Susceptible to Infections
• bacterial pneumonia
• meningitis
• oral and vaginal yeast infections
• tuberculosis

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4. Advanced Disease (AIDS)

• CD4 cell count lt 200mm3
• Severe Opportunistic Infections
• Pneumocysitis carinii pneumonia (PCP) Fungi
• Kaposis Sarcoma ( Cancer - Skin and Blood
  vessels)
• Toxoplasmosis (Brain) Protozoan
• Cryptosporidiosis (G.I. Tract) Protozoan
• Other Bacterial, Fungal and Viral Infections

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HIV Infection and Immune Response (Graph)
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Blood Test - ELISA

• Enzyme Linked Immunosorbant Assay
• tests for HIV Antibodies
• If ELISA is positive, same sample is tested again
• If ELISA is positive again, then a Western Blot
  Test is done.
• Western Blot - test for Viral antigens

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Treatment for HIV Infection

• No Cure
• AZT ( Azidothymidine)
• Thymine analog
• lacks a 3 OH
• Chain Terminator
• Inhibits Reverse Transcriptase

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AIDS Cocktail (Combination Therapy)

• AZT
• 3TC ( 2-deoxy-3-thiacytidine)
• Protease Inhibitor

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Vaccine for HIV ?

• HIV mutates too rapidly
• Reverse Transcriptase causes at least 1 mutation
  each time it is used
• 1 million variants during Asymptomatic Disease
• 100 million variants during Advanced Disease
  (AIDS)

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