An Introduction to the Viruses
Chapter Summary
This introduces viruses. It gives the student a clear picture of the structure of viral particles. It also
describes the process of infection, and replication in the host cell. The student will be exposed to two
major techniques of culturing viruses: chick embryos and tissue culture.
The student will learn how viruses are classified and how names are assigned to them.
The medically important viral-like particles are named and discussed - prions, virinos and viroids.
The treatment, or lack of treatment, for viral infections is described, with a few antiviral agents being
named.
Chapter Outline
I.
II.
The Search for the Elusive Viruses
The Position of Viruses in the Biological Spectrum
A. Obligate Intracellular Parasites
B. Infectious particles
C. Active or inactive
III. The General Structure of Viruses
A. Size Range - ultramicroscopic
B. The Viral Capsid: The Protective Outer Shell
1. Capsomers
2. Helical Capsids: tobacco mosaic virus
3. Icosahedron: adenovirus {ch24, 744} example
C. The Viral Envelope: Spikes or Peplomers
D. Functions of the Viral Capsid/Envelope: role in infection and host defense
E. Complex Viruses: Atypical Viruses
1. Complex Viruses
2. Poxviruses {ch24, p.729}
3. Bacteriophages
F. Nucleic Acids: At the Core of a Virus
1. DNA or RNA
2. Single or double-stranded
3. Viruses are genetic parasites
G. Other Substances in the Virus Particle
IV. How Viruses Are Classified and Named
A. DNA or RNA
B. Family name ends in -viridae
C. Characteristics include: capsid type, nucleic acid strand, envelope presence, viral size, host cell
specificity, and shape or size
D. Examples:
1. Rhabdovirus: bullet-shaped
2. Togavirus: Cloak-like envelope
3. Adenovirus: found in adenoids
4. Lentivirus: slow, chronic infection
5. Picornaviruses: small, RNA
6. Reoviruses: respiratory, enteric, orphan viruses
V. Modes of Viral Multiplication
A. A Note on Terminology: virus, virus particle, virion, lytic, lysogenic phases
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B. The Multiplication Cycle in Bacteriophages
1. Adsorption: Docking onto the Host Cell Surface
2. Bacteriophage Penetration: Entry of the Nucleic Acid
3. The Bacteriophage Assembly Line: eclipse stages
4. Lysogeny: The Silent Virus Infection
a. Temperate phages
b. Prophage
c. Lysogeny compared with the lytic cycles
C. Multiplication Cycles in Animal Viruses
1. Adsorption and Host Range
2. Penetration/Uncoating of Animal Viruses: endocytosis
3. Replication and Maturation of Animal Viruses: Host Cell As Factory
a. Positive sense
b. Negative sense
4. Release of Mature Viruses: budding or exocytosis
5. Damage to the Host Cell and Persistent Infections
a. Cytopathic effects
b. Inclusion bodies
c. Persistent infection
d. Chronic latent state
e. Oncogenic viruses, oncoviruses: Epstein-Barr virus {ch24, pp.737-738}
f. Transformation
VI. Techniques in Cultivating and Identifying Animal Viruses
A. Using live animal inoculation
B. Using Bird Embryos: pocks
C. Using Cell (Tissue) Culture Techniques
1. Primary cell culture
2. Cell lines
3. Plaques
VII. Medical Importance of Viruses
A. Other non-cellular infectious agents
1. Prions
a. Creutzfeldt-Jakob disease
b. Bovine spongiform encephalitis
2. Satellite viruses
3. Viroids
B. Detection and Control of Viral Infections
C. Treatment of Viral Infections
1. Azidothymidine (AZT)
2. Interferon
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Key Terms and Phrases
virology
virus
obligate intracellular parasite
ultramicroscopic
capsid
nucleocapsid
envelope
naked nucleocapsid
capsomer
helical capsid
icosahedron
spike or peplomer
complex viruses
poxviruses
bacteriophage
nucleic acids
genetic parasites
polymerases
superfamilies
rhabdovirus
togavirus
adenovirus
herpesvirus
picornavirus
virion
lytic cycle
latent
adsorption
penetration
replication
maturation
release
lyses
lysogeny
temperate
prophage
host range
tropism
endocytosis
uncoated
budding
exocytosis
cytopathic effects
oncogenic
transformation
in vivo
in vitro
embryo
pocks
cell culture
cell lines
plaques
prions
interferon
viroids
AZT
Topics for Discussion
Students will be very interested in this chapter. They are very concerned with viral infections, as well
they should be. The concept of viral infection and replication is somewhat confusing to the student. It is
worth a diagrammatic lecture demonstration to illustrate this concept. Discuss the controversy that exists
in understanding why viruses are not living.
The medically important viruses will be covered, in detail, later in the text. You need only introduce
the concepts here.
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