An Introduction to the Viruses

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An Introduction to the Viruses
Chapter 6
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Virusesobligate intracellular parasites

• Virus (L. poison)
• Non-cellular
• infectious particles
• Non-living
• active or inactive
• Genetic parasites
• Use host cell machinery to replicate DNA or RNA

• Infect all types of cells
• Bacteria
• Algae
• Fungi
• Protozoa
• Plants
• Animals

Poxvirus
Ebola Virus
Vaccinia Virus
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Viruses Ultramicroscopic

• Smallest infectious agents (lt 0.2 ?m)
• Parvoviruses (respiratory infections) 20 nm
• Poxviruses (smallpox) 250 nm
• 2000 viruses/bacterial cell
• 50 million/human cell

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Viral Terminology

• Virion infectious extracellular particle
• Capsid outer coating
• Nucleocapsid nucleic acid capsid

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Naked vs Enveloped Viruses

• Envelope modified host cell membrane
• Spikes or peplomers attachment of virus
• Pleiomorphic due to flexible envelope

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Capsid Protein Coat

• Encloses protects nucleic acid (DNA RNA)
• Composed of identical protein subunits
  capsomers
• 2 types
• helical
• iscosahedral

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Helical Capsids

• Rod-shaped capsomers form helix
• DNA or RNA coiled inside
• Naked helical (rigid)
• Tobacco mosaic virus
• Enveloped helical (flexible)
• Influenza
• Measles
• Rabies

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Icosahedral Capsids

• 20-sided with 12 corners
• Vary in of capsomers
• Poliovirus (32)
• Adenovirus (240)
• Capsid sphere or cube

• Naked
• Papillomavirus (warts)
• Enveloped
• Herpes simplex (cold sores)

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Fig. 6.10
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Viral Core

• DNA or RNA
• Single or double-stranded
• Virus genetic parasite
• Hepatitis B 4 genes
• E. coli 4000 genes
• Human 40,000 genes

• Carry genes for
• Viral capsid
• Genetic material
• Host regulation
• Viral packaging

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Classifying Naming Viruses

• Latin root (descriptive) plus -viridae
• Family name ends in -viridae (Herpesviridae)
• Genus name ends in -virus (Varicellovirus)
• Common name (Chickenpox)
• Capsid type (helical or icosahedral)
• Enveloped or naked
• DNA (6 families) or RNA (13 families)
• Host cell (bacterial, plant or animal)
• Size shape

• Rhabdovirus bullet-shaped
• Togavirus cloak-like
• Adenovirus infect adenoids
• Lentivirus slow, chronic infection
• Picornavirus small RNA
• Reoviruses respiratory, enteric viruses

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DNA vs RNA Viruses

• DNA Viruses
• Poxviruses (smallpox)
• Herpesviruses
• Hepatitis B virus
• Papillomavirus (warts)
• Adenoviruses

• RNA Viruses
• Poliovirus
• Rhinovirus (colds)
• West Nile virus
• Influenza virus
• Mumps
• Measles
• Rabies
• HIV (retrovirus)

Important thing here is that both DNA and RNA
viruses exist
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Animal Virus Replication

• Adsorption (specific, mediated by receptors, much
  of the time by receptor-mediated endocytosis)
• Penetration uncoating of genome
• Duplication/synthesis
• Assembly (also called maturation)
• Release (budding, lysis)

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Host Range

• Tropism Spectrum of cells a virus can infect
• cell has specific structure (receptor) on its
  surface for viral attachment
• HIV-1 (CD4 and chemokine receptor)
• Epstein-Barr Virus (complement protein, CR2)
• SARS (ACE-2 angiotensin converting enzyme)
• cell has to contain enzymes materials needed to
  produce new virions
• May be one species or many
• HIV (only humans) vs rabies (many animals)
• May be one tissue or many within a host
• Hepatitis (liver) vs polio (intestinal nerve
  cells)

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Phage Movie
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Retrovirus Movie
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Adsorption
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Viral Penetration
HerpesvirusEndocytosis
Mumps Virus Fusion
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2 Mechanisms of EntryMovie
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Release by Budding

• Budding occurs by exocytosis
• Virions are slowly shed from cell
• Pox-infected cell releases 3000-4000 virions
• Poliovirus-infected cell releases 100,000 virions

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Cytopathic EffectsVirus-Induced Damage to Cells

• Changes in size shape
• Nuclear cytoplasmic inclusion bodies
• Cells fuse to form multinucleated cells
• Cell death lysis
• Alter DNA
• Oncoviruses transform cells into cancerous
  cells
• Epstein-Barr virus (lymphoma)
• Papillomavirus (cervival cancer)
• HTLV (leukemia)
• Persistent infections last weeks to years
• Chronic latent state
• Herpes (cold sores genital herpes)
• Varicella-Zoster (chickenpox shingles)

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Injury Induced by Viruses

• Cytopathic effect (cell damage or death)
• Shut down cell translation protein synthesis
• Induce cell death lysis
• Cell injury causes clinical symptoms
• Fever, tissue damage, pain, nausea
• Non-Cytopathic effects mediated by immune
  response to virus

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Growing Animal Viruses

• Live animals (In vivo)
• Bird embryos (chicken duck)
• intact
• self-supporting unit
• sterile
• self-nourished
• 3. Cell culture (In vitro)

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Infection Plaque Formation

• Plaques areas where virus-infected cells
  undergo lysis

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Other Noncellular Infectious Agents

• Prions misfolded proteins
• contain no nucleic acid
• cause spongiform encephalopathies holes in the
  brain
• common in animals
• Sheep goats (Scrapie)
• Cattle (Mad cow disease)
• Humans Creutzfeldt-Jakob Disease (beef) Kuru
  (cannibalism)
• Satellite Viruses Require helper virus to
  replicate
• Adeno-associated viruses
• Delta agent (RNA) needs HepB
• 3. Viroids short pieces of RNA, no protein
  coat
• only been identified in plants, so far

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Most of the time based on symptomology
diagnosis
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Importance of Viruses

• Medical infectious disease
• Colds
• Rabies
• AIDS
• Ebola
• Agricultural plant viruses
• Ecological 10 million/ml or 270 million metric
  tons of organic matter in seawater
• Research genetic tools

• Retroviruses (HIV-1)
• Treatment of rheumatoid arthritis
• Adenoviruses (common cold)
• Cystic fibrosis
• Poxviruses
• Canarypox (cancer rabies treatment)
• Herpes viruses (cold sores encephalitis)
• Infect neurons CNS
• Treat tumors in brain
• Parkinsons disease
• Vaccinia virus
• Vaccines for smallpox rabies