Viruses
I.
Introduction to the Viruses
A. History of Viruses
1. Adolf Mayer
2. Dmitri Ivanovski-tobacco mosaic disease;”filterable agent”
3. Martinus Beijernick
4. 1898-first animal virus
5. 1900-first human virus
6. 1930’s- first seen with electron microscope
B. Viruses
1. Cause infections in all types of organisms
2. Cannot carry out metabolic pathways
3. Neither grow nor respond to environment
4. Cannot reproduce independently
5. Obligate intracellular parasite
C. Characteristics of Viruses
1. Very small, acellular, infectious agent with nucleic acid
2. No cytoplasmic membrane, cytosol, organelles
3. Have extracellular and intracellular state
4. Extracellular state
a. Virion
b. Protein coat (capsid) surrounding nucleic acid
(nucleocapsid)
c. Envelope
d. Outermost layer-protection, recognition site
5. Intracellular state
a. Capsid removed
b. Virus exists as nucleic acid
D. Naming Viruses
1. No taxa above family
2. 19 families of animal viruses
3. Family name –ends in -viridae
4. Genus name-ends in –virus
E. How Viruses are Distinguished
1. Type of genetic material
2. Cells they attack
3. Size
4. Capsid coat
5. Shape
6. Envelope
F. Genetic Material of Viruses
1. More variety than in cells
2. DNA or RNA; never both
3. Main way to categorize
4. Can be dsDNA, ssDNA, dsRNA, ssRNA
5.
6.
G. Capsids
1.
2.
3.
II.
Linear with several segments or single and circular
Much smaller than genomes of cells
Protein coats that enclose and protect nucleic acid
Made of capsomers (identical subunits)
Two types
a. Helical
b. Icosahedral – 20 sided with 12 corners;some enveloped
Viral Replication
A. Viral Hosts
1. Most infect only particular host cells
2. Affinity of viral surface proteins or glycoproteins for
complementary proteins or glycoproteins on host cell surface
3. May infect particular cell in host
4. Generalists
B. Six Steps in phage replication
1. Adsorption
2. Penetration
3. Replication
4. Assembly
5. Maturation
6. Release
C. Viral Replication
1. Dependent on host’s organelles, enzymes
2. Usually results in death and lysis of host cell
3. Not all bacteriophages lyse cells
4. Temperate phages insert viral DNA into host chromosome
5. Lysogeny
D. Host Range
1. Spectrum of cells a virus can infect
2. Cell must have receptor for viral attachment
3. Cell must have all enzymes and materials needed by virus
4. May be one species or many
5. May be one tissue or many within a host
E. Differences between phage and animal virus replication
1. Animal virus replication more complex
2. Animal viruses cannot inject DNA
3. Lysogeny for phage, latency for animal viruses
F. Animal Virus replication
1. Adsorption
2. Penetration/uncoating of genome
3. Duplication/synthesis
4. Assembly
5. Release
G. Host Cell as Factory
III.
1. Generally, DNA viruses enter nucleus where they are replicated
and assembled
2. Generally, RNA viruses are replicated and assembled in
cytoplasm
3. Genetic material replicated
4. Proteins synthesized using host materials
H. Release of Mature Viruses
1. Nonenveloped and complex viruses released when cell lyses
2. Enveloped viruses released by budding
3. Budding – viruses shed gradually
4. Most viruses ultimately lethal
I. Viral envelope
1. Acquired from host cell during replication or release; portion of
membrane system of host
2. Phospholipid bilayer and proteins
3. Often play role in host recognition
Cytopathic Effects
A. Cytopathic effects- virus-induced damage to cells
1. Changes in size and shape
2. Cytoplasmic inclusion bodies
3. Nuclear inclusion bodies
4. Cells fuse to form multinucleated cells
5. Cell lysis
6. Alter DNA
7. Transform cells into cancerous cells
B. Latency of Animal Viruses
1. Animal viruses formant in host cells
2. Prolonged with no viral activity, signs, symptoms
3. Some latent viruses do not become incorporated into host
chromosome
4. When provirus is incorporated, condition is permanent
5. Examples:
C. Role of Viruses in Cancer
1. Permanently alter host genetic makeup, leads to cancer
2. These viruses are termed oncogenes
3. Effect on cell is called transformation
4. Viral DNA is consolidated into host DNA like a lysogenic phage
D. Factors Involved in Activation of Oncogenes
1. UV light
2. Radiation
3. Carcinogens
4. Viruses
E. How Viruses Cause Cancer
1. Copies of oncogenes as part of genome
2. Stimulate already present oncogenes
3. Interfere with tumor repression
IV.
.
4. Several DNA and RNA viruses known to cause about 15% of
human cancers
F. Growing Viruses
1. Live animals
2. Bird embryos
3. Cell culture
Other Noncellular Infectious Agents
A. Prions
1. Misfolded proteins; no nucleic acid
2. Causes spongiform encephalopathies
B. Viroids
1. Short pieces of RNA; no protein coat
2. Only identified in plants so far
3. Extremely small
C. Characteristics of Prions
1. Proteinaceous infectious agents
2. Single protein PrP
3. All mammals contain gene that codes for primary sequence of
amino acids in PrP
4. Two stable tertiary structures of PrP
a. Normal form with alpha helices-cellular PrP
b. Disease form with bets sheets-prion PrP
5. Prion PrP converts cellular PrP into prion PrP by inducing
conformational changes
6. Normally, nearby proteins and polysaccharides force PrP into
cellular shape
7. Excess PrP production or mutations in PrP gene result in initial
formation of prion PrP
8. When prions present, they cause newly synthesized cellular PrP
to refold into prion PrP
D. Prion Diseases
1. Neurological degeneration, deposition of fibrils in brain, loss of
brain matter
2. Large vacuoles form in brain
3. BSE, CJD, kuru
4. Only destroyed by incineration; not cooking or sterilization
E. Diagnosis of viral Diseases
1. More difficult than other agents
2. Consider overall clinical picture
3. Take appropriate sample