CHAPTER 13 VIRUSES
I. Characteristics
A. noncellular, nonmetabolicinfectious particles
B. size:fig. 13.4
C. nucleic acid
1. ss or ds DNA or RNA = genome
2. chromosome circular, linear, or
fragmented
D. capsid
1. protein - determines shape:
a. helical, polyhedral, complex
2. how virus attaches – naked virus
E. in addition, some viruses have an envelope
1. surrounds outside of capsid
2. composition
a. derived from previous host cell’s memb.
b. often modified w/ viral proteins (spikes)
3. attachment in enveloped viruses
F. enzymes not found in host cell
1. lysozyme in bacteriophages
2. reverse transcriptase in retroviruses
3. RNA-dependent RNA polymerase in ss
RNA viruses
G. obligate intracellular “parasites”
1. replicate only inside living host cell
a. cell types infected
b. specificity
c. determined by
2. directs host cell to make viral components
a. viral n.a.
b. viral proteins
II. Viral replication – in general
A. 5 steps
1. attachment
2. entry
3. synthesis
4. assembly
5. release
III. Bacteriophages - recall chap 7
A. lytic cycle: fig. 13.8
1. attachment via tail fibers to bact. cell wall receptors
2. entry – DNA injected into weakened cell wall
3. synthesis
4. assembly – opportunity for generalized transduction
5. release with lysozyme – cell lyses
B. lysogeny – temperate phages fig. 13.11
1. prophage inserts into host chromosome
2. replicated when host chrom. replicated during
binary fission
3. host stress causes excision of prophage which
enters lytic cycle
4. transduction - specialized
IV. Animal virus replication
A. attachment - capsid or envelope
B. entryfig. 13.12
1. naked - direct penetration
2. enveloped – fusion or endocytosis, uncoating
C. synthesis - depends on:
1. DNA viruses
2. RNA viruses - depends on if ss, + or -, or
ds, or retrovirus.
D. assembly
E. release
1. enveloped - budding
2. naked – exocytosis or lysis
V. Latent viruses
A. proviruses remain dormant in host cell; may be
reactivated
B. Herpesviruses
1. types
2. cells latent in:
C. retroviruses
VI. Oncogenic viruses
A. disrupt proto-oncogenes or repressor genes in host
B. examples
1. hepatitis B and C viruses
2. Epstein-Barr
3. HPV
4. herpesvirus 8
VII. Culturing viruses -
A. Purpose
1. research
2. vaccine production
3. diagnosis of viral disease
B. must grow w/in cells
C. bacteriophages
D. embryonated eggs
1. problems
2. advantages
E. cell cultures
1. types of cells used
a. diploidcell cultures
b. continuouscell cultures
2. CPEs – cytopathiceffects
a. cellshape changes, syncytia, lysis
b. transformations
c. inclusions
VIII. Otherdisease-causingparticles
A. viroids
B. prions
1. description
2. spongiform encephalopathies
OBJECTIVES FOR CHAPTER 13
1. Describe the minimum requirements for a virus
2. Describe the viral envelope
3. Describe the lytic and lysogenic life cycles of bacteriophages
4. Know the steps of viral replication
5. Know what determines the steps of the synthesis step in animal viral replication
6. Distinguish between enveloped and naked viruses in terms of attachment, entry, and
release
7. List the viruses that are latent and oncogenic
8. Know why viruses are cultured, and define cytopathic effects
9. Define viroid and prion, and give examples of diseases caused by each
10. Define and describe retroviruses, including the enzyme involved
11. Know the names of and any information given about microbes presented in this chapter
12. Do multiple choice questions 2-5, 8, 10; matching; short answer questions 1-5, 7, 8; and
criticalthinking question 2
SAMPLE QUESTIONS FOR CHAPTER 13
1. A viroid is
a. a complete, infectious virus particle
b. a naked, infectious piece of RNA
c. a capsid without a nucleic acid
d. a provirus
e. an infectious protein
2. Retroviruses
a. infect cells by entering the cytoplasm in reverse
b. contain the enzyme reverse transcriptase
c. make DNA copies from their RNA nucleic acid
d. all of the above
e. b and c only
3. The mechanism whereby an enveloped virus leaves a host cell is called
a. transduction
b. budding
c. teratogenesis
d. lysogeny
e. penetration
4. Viral envelopes
a. are made of polysaccharide
b. are found between the capsid and nucleic acid
c. are derived from the host cell’s plasma membrane
d. are added to the nucleocapsid during adsorption
e. prevent attachment to host receptor sites
5. During bacteriophage assembly, occasionally a piece of bacterial DNA is inserted into a phage
capsid. If this new particle infects a new host cell, this can lead to
a. transcription
b. latency
c. transduction
d. transformation
e. all of the above
6. Prions
a. are infectious pieces of protein
b. are infectious pieces of RNA
c. are also called viroids
d. have capsids but not envelopes
e. is the name given to latent viruses
7. Temperate phages enter the lytic cycle
a. when the host cell is exposed to UV light
b. when the host cell is stressed
c. whenever the host cell divides
d. when reverse transcriptase is present
e. a and b only
8. Epstein-Barr virus
a. causes mononucleosis
b. is a latent virus
c. is a member of the Herpesvirus family
d. can be oncogenic
e. all of the above
9. HIV
a. is oncogenic
b. is a naked DNA virus
c. infects a wide variety of human cells
d. is a retrovirus
e. a and d only
10. Which of the following are found inside some bacteriophages?
a. ribosomes
b. lysozyme
c. reverse transcriptase
d. lysosomes
e. envelopes