BIOL 191 Introductory Microbiology
Chap. 13 Viruses, Viroids, and Prions Study Guide
KEY
I.
General Characteristics of Viruses
a. Intro
i. Table 13.1 Comparing Viruses and Bacteria
Are viruses… Are typical bacteria…
Intracellular parasites? Yes, No
Do they have a plasma membrane? No, Yes
Do they contain ribosomes? No, Yes
Do they reproduce by binary fission? No, Yes
Do they possess both RNA and DNA in the same
structure? No, Yes
Are they sensitive to antibiotics? No, Yes
Are they sensitive to interferons? Yes, No
ii. *Obligatory*intracellular*parasites*: What does this
mean?
In order to reproduce, they have to be inside of a living
cell
iii. Define ‘virus’ Entities that contain a single type of
nucleic acid in the virion, have a protein coat (capsid),
multiply inside living cells by using the machinery of the
cell, and cause the cell to synthesize structures that will
allow the virus to infect another cell
b. Host Range. What does the host *range* depend on? What is
a bacteriophage? The host range includes the species that a
particular virus can infect. It is usually very specific. It is
determined by attachment capabilities and the availability of
cellular components that are required for viral multiplication.
A bacteriophage (phage for short) is a virus that infects only
bacteria.
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c. Viral Sizes The typical viral size is 20-1000 nm (nanometer 10-9
m) in length
II.
Viral Structure - Define ‘Virion’ The complete viral particle at the
moment of attachment (or the moment it leaves the cell). It is the
only structure capable of transmission from one host to another.
Will a virus survive outside of a host cell and be able to transmit
to another host if it is not in this form? No
a. Nucleic Acid –
i. Genetics
Chap. 8 Fig. 8.2 The Flow of Genetic Information Know
the basics of DNA expression and replication. These
processes are necessary for viral multiplication.
ss = single stranded
ds = double stranded
ii. What types of nucleic acids may viruses have?
Depending on the viral species, a virus may have- in the
virion- one of the following: ssRNA, dsRNA, ssDNA,
dsDNA
b. Capsid – Outer protein shell (coat)
c. Envelope – A structure that surrounds viruses that bud out
of the host cell. It consists of a portion of the host cell
membrane that ‘sticks’ to the virus as it pushes through the
host cell membrane to leave the cell
d. Spikes - Structures that project out of the viral envelope
Examples: influenza virus (H and N stand for subtypes of these
spikes). See discussion on p. 374-375.
1. H (Hemagglutinin) proteins
Hemagglutinin is one of two virally-coded envelope protein spikes of the influenza virus.
Hemagglutinin is responsible for host cell binding and subsequent fusion of viral envelope
and host membrane.
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2. N (Neuraminidase) proteins
Projections from surfaces of influenza viruses containing neuraminidase are involved in
the release of viruses from infected cells.
Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier.
e. General Morphology- Be able to identify and label the
morphological structures on the last page of the Study Guide
1. Helical (Capsomeres arranged in a helix, but they
resemble a rod)
2. Polyhedral (Many sided)
3. Enveloped (Contain an envelope)
Examples:
Herpesvirus- Polyhedral enveloped
Influenza-Helical enveloped
4. Complex viruses: A virus with additional structures
and/or a combination of the basic morphological
shapes
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III.
Taxonomy of Viruses
a. What is ‘taxonomy’?
The study of categorizing (grouping) organisms
b. How does the International Committee on Taxonomy of
Viruses group (classify) viruses? Nucleic acids and Structure
c. What is a viral ‘species’? A group of viruses sharing the same
genetic material and host range
IV. Isolation, Cultivation and Identification of Viruses
a. How are bacterial and animal viruses grown in the lab?
-Bacteriophages: Are grown in bacterial cells (for example in a
petri plate).
Plaques are clear areas where a phage has infected and killed
the bacteria. Viruses that have been released from these now
dead cells are most likely now in nearby bacteria cells
Be able to identify the type of
virus grown in a bacterial culture
such as the one in the illustration
to the left. The ‘dots’ are
plaques. Fig. 13.6
Animal viruses: Live animals (some, such as mice, may have
been altered to produce human cells in which human viruses
can grow), chick embryos, animal cell cultures (similar to
bacteria in a petri plate and plaques)
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b. What are some ways viruses are identified?
i. Characteristics of spikes (p. 371)
ii. Reaction to antibodies
iii. Cytopathic effects (characteristics of a cell after
infection)
iv. Characterization of nucleic acids
Viral Multiplication: First bacteriophage, then animal viruses
In Chap. 8 Microbial Genetics- be sure you know what DNA replication, protein
synthesis (transcription/translation), mRNA, tRNA, and rRNA refer to.
What genes do viruses have?
Viruses are very small and cannot contain all of the genes
needed for viral multiplication. The host cell must have the other
necessary genes and the molecules and structures coded for by
those genes that are required for that specific virus to infect that
specific cell.
The genes viruses do contain include those that code for
capsid proteins and other surface molecules (such as those for
spikes) and possibly a few of the enzymes required to continue the
viral life cycle. The virus generally does not contain genes for
proteins the host cell already has.
What enzymes do virions contain?
Some do not contain any enzymes in their virions. Other may
contain one or a few which function to aid the virus in entering the
cell or replicating its genetic material once inside.
V.
Multiplication of Bacteriophages: Lytic cycle or Lysogenic cycles
a. T-Even Bacteriophage Lytic Cycle
Know general info about the T-even bacteriophages- Large, with a
complex morphology, infect E. coli
“T” stands for ‘type’. T-even (T-2, T-4, etc.) bacteriophages are dsDNA bacteriophages that
have the complex morphology shown in Fig. 13.11.
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Fig. 13.11 The lytic cycle of a T-even bacteriphage
The following illustration is not on the exam, but students should know the order
of the steps, what they mean, and the basic concepts of the lytic cycle- The phage
infects the cell, immediately takes over the cell and forces the cell to make more
phage- the cycle ends with the cells lysing (thus, dying), and the new phage virions
are released to infect another nearby bacterial cell
- Attachment
-Penetration
-Biosynthesis
-Maturation
-Release
i.
ii.
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b. Bacteriphage Lambda Lysogenic Cycle
These viruses may go through a lytic cycle, but also may enter a latent
(inactive) stage where the viral dsDNA is incorporated into the host cell
DNA. If the bacterial cell is not harmed by this, the bacterial cell will
function normally and divide, thus replicating not only its own DNA but also
that of the PROPHAGE (the viral DNA while it is incorporated in the
bacterial DNA). Both bacterial daughter cells will therefore be infected and
contain prophage in their DNA.
BE ABLE TO LABEL THE PROPHAGE AND RECOGNIZE WHICH SIDE OF
THE ILLUSTRATION IS THE LYTIC OR LYSOGENIC STAGE
Fig. 13. 12 The lysogenic cycle of bacteriophage lambda in E. coli
-Attachment
-Phage DNA circularization
-If the phage enters the lysogenic cycle: See above for answers
 Define prophage.
 What happens when the bacterium reproduces?
 Can lysogenic viruses be lytic?
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 What are important possible results of lysogeny?
-The bacterial cell cannot be infected by another of the
same type of phage (however, other types of phage may
infect this cell)
-The bacterial cell may exhibit new genetic properties from
the DNA brought into the bacterial cell by the phage (see
discussion of phage conversion causing diphtheria,
streptococcal toxic shock, and botulism on p. 384)
-Specialized transduction, in which only DNA on either side
of the prophage DNA can be transferred. Fig. 13.13.
--This is different from generalized transduction, in which
any bacterial DNA can be transferred from one bacterial cell
to another. Chap. 8 Fig. 8.29.
VI.
Multiplication of Animal Viruses
a. How do animal viruses differ from phages? Table 13.3
Know this example: Rather than injecting the genetic
material into the bacterial cell (leaving the phage capsid
behind), the capsid of an animal virus enters the host cell,
and, therefore, the virus must uncoat (digest the viral
capsid) to release the viral genetic material into the cell.
b. Why might some people be resistant to a specific virus but not
others? Receptor sites on the surfaces of human cells are
inherited and different from one person to another.
Therefore, a virus may be able to attach to one person’s
receptors but not another’s.
c. How is ‘attachment’ related to drug development against
viruses? Drugs may be developed that block the receptor of
the animal or the viruses’ attachment sites.
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d. Animal DNA virus multiplication
Fig. 13.15 Fig. Replication of a DNA Animal Virus
i.
ii.
iii.
iv.
Attachment
Entry
Uncoating
(Bio)synthesis of early viral proteins (needed for viral
DNA replication), synthesis of viral DNA, and then
synthesis of late proteins (for example, the capsid)
v. Maturation
vi. Release – through budding or rupture
e. Animal RNA viral multiplication
Fig. 13.17 Pathways of multiplication used by various RNAcontaining viruses
i.
ii.
iii.
iv.
Know what a
complementary
strand means and
why it is necessary for
synthesis of any
strand of DNA or RNA.
Attachment
Entry
Uncoating
Biosynthesis of viral RNA in
1. ssRNA + (sense) stranded viruses (+ strand is
equivalent to mRNA)…Know what mRNA is.
2. ssRNA – (antisense) stranded viruses (– strand is
complementary to the + strand-it does not
contain the correct genetic code)
3. dsRNA viruses
4. Retroviruses (see discussion on next page)
v. Synthesis of viral proteins
vi. Maturation
vii. Release- through budding or rupture
 Know what is needed in order to replicate any strand of nucleic acid
(DNA or RNA)- The complementary strand is required, along with
enzymes capable of creating the new strand from the complementary
strand
 Understand what is required to synthesize proteins and the name of the
processes: transcription, translation, ribosomes, free RNA nucleotides
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RETROVIRUSES: includes HIV
Define retrovirus: ss+RNA viruses that also contain the enzyme reverse
transcriptase in the virion
Be able to label Fig. 13.19 below and describe what is happening at (and inbetween some) the cells. See arrows for important parts.
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 Be able to recognize what is happening in this figure.
VII. Viruses and Cancer
A. Define
a. Oncogenes: Cancer causing genes
b. Oncogenic viruses (oncoviruses): Viruses that induce
tumors by a variety of methods – They may bring in cancer
causing oncogenes with them or mutate or otherwise
change ‘silent’ oncogenes previously in the individual
before infection
c. Transformed cells have become cancerous
B. What % of cancers is known to be virus-induced? ~10%
C. What is the OUTSTANDING FEATURE of all oncogenic viruses?
The virus’ DNA must integrate into the host cell DNA
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D. DNA Oncogenic Viruses: Examples
a. HPV (human papillomavirus – cervical and anal cancer)
b. EB virus causes Burkitt’s lymphoma (this disease also
showed researchers that a virus can be transferred through
tissue transplants)
c. HBV (hepatitis B - liver cancer)
E. RNA Oncogenic Viruses
-Only RNA viruses that are retroviruses can cause cancer. Why?
(See the outstanding feature of oncoviruses above)
Only retroviruses can create dsDNA from their RNA that then can
integrate into the host cell chromosome, a required characteristic of
oncoviruses.
VIII. Latent and Persistent Viral Infections: Define and know examples from
text discussion
Fig. 13.21 Latent and persistent viral infections
Latent viral infections can remain in the host for an indefinite time
without producing new viruses or causing symptoms. Some type of
STRESS may ‘trigger’ the virus into becoming ‘active’ and forcing
the cell to make more viruses. This is analogous to lysogeny in
bacteriophage.
An example is shingles (chickenpox [varicella herpes] in nerve cells).
A persistent viral infection produces viruses, but at low levels for
long periods of time. These are usually fatal.
Examples: measles that years later develops into a form of
encephalitis, HIV, HBV liver cancer, HPV cervical cancer
IX. Prions
See Fig. 13.22 How a protein can be infectious
A. Know examples and how they are different from viruses
Prions are pure protein (no nucleic acid) that are infectious. They
are able to ‘replicate’ when the abnormal prion form of this
normal mammal brain protein transforms the normal brain
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protein into the abnormal prion form. They cause spongiform
encephalitis (large vacuoles/plaques develop and destroy the
brain).
B. Nervous System Diseases caused by Prions Chap. 22 p. 629-632
Mad cow disease, scrapie (in sheep), kuru and Creutzfeldt-Jakob (CJD) in
humans
Since this brain protein is encoded in the DNA of all mammals, the disease
may also be caused by genetic mutation or inheritance.
X. Plant Viruses and Viroids
A. Why are plants somewhat protected against many diseases?
Plants have a hard cell wall made of cellulose so usually a viral
infection must involve wounds to the cell wall or be brought in
through other parasites (such as insects).
B. Define viroid
Short pieces of naked RNA (no DNA or proteins) that (so far) are
only know to infect plants
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Polyhedral
Helical
Complex
Enveloped
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