BIO 208 Microbiology Unit 3 Viruses
This outline is intended to facilitate your preparation for lecture. This web outline will NOT
substitute for regular lecture attendance.
Lectures 17-19 - we will discuss the following topics:
II. Viruses (Ch. 13)
A. General Characteristics of Viruses
1. Origin and Evolution – where did they come from? The main theories
a. Descendants of primitive pre-cellular life forms
b. Evolved from pre-cellular saprophytes facultative  obligate parasitism
c. Developed from normal constituents of cells
d. Degradation of cells of ancient origin & co-evolved w/ host organisms
2. Distinctive features of viruses
a. nucleic acid (na) – DNA or RNA, not both
b. protein coat – surrounds & protects na
may also have an envelope – lipid + carbo + protein
c. multiply inside living cells using machinery of host
 obligate intracellular parasites
d. cause the synthesis of specialized structures that can transfer viral na to other cells
3. Host range – range of species whose cells a virus may infect (Ex. polio virus infects
humans, not fruit flies) - determined by attachment
a. animal cells – receptor for virus is on the plasma membrane of the host cell
Tissue Tropism – the cells of which tissue a virus will infect (Ex. within humans, polio
virus infects cells of intestines and nervous tissue, but not liver) – also determined by
attachment
b. bacteria – receptor for virus is on cell wall, fimbriae, or flagella
4. Sizes (Fig. 13.1)
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BIO 208 Microbiology Unit 3 Viruses
B. Viral Structure - view w/ EM (pp. 370-373)
1. Virion – complete, fully developed viral particle
2. Nucleic acid – DNA or RNA; ss or ds
3. Capsids and envelopes
capsid – the protein coat
capsomeres – protein subunit that makes up the capsid
envelopes – lipids + carbos + protein
naked vs. enveloped
spikes – carbo-protein complexes on surface of virus
Ex. Hemagglutinin of Influenzavirus
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BIO 208 Microbiology Unit 3 Viruses
4. Morphology - used in classification
a. helical – long rod
capsid is helical
na is helical
Ex.
b. polyhedral – many sided
capsid is icosahedron
20 triangles
12 corners
na is packed within
Ex.
c. complex
Ex. bacteriophages
5. Taxonomy
a. Family: -viridae
Ex. Herpesviridae
b. Genus: -virus
Ex. Simplexvirus
c. Species - group of viruses sharing genetic information and ecological niche
d. Common names
Simplexvirus
herpes simplex virus
HSV
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BIO 208 Microbiology Unit 3 Viruses
C. Viral Multiplication
1. Bacteriophages
a. Lytic cycle – results in lysis and death of infected host bacterium
Ex. T-even in E. coli
b. Lysogenic cycle – the infected host remains alive
Ex. bacteriophage lambda () in E. coli
Lytic and lysogenic cycles (Fig. 13.11, 13.12)
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BIO 208 Microbiology Unit 3 Viruses
2. Animal Viruses (pp. 382-389); Fig. 13.19
a. attachment to receptors on host plasma membrane
b. penetration – 2 mechanisms
i. pinocytosis (or endocytosis) – host plasma membrane engulfs and internalizes the
entire virus
ii. fusion – viral envelope fuses with host plasma membrane, only viral na enters cell
c. uncoating – removing the capsid
d. transc. of some viral genes followed by their transl. using the host RNA pol. Genes
transc. and transl. are enz. nec. for viral DNA repl.
e. replication of viral nucleic acid – will depend on the type of na the virus has
DNA viruses – may have either ss or ds DNA molecule. In either case, DNA is repl. in
nucleus of host cell using enz. encoded by the virus.
RNA viruses – gets more complicated. The RNA can be similar to a mRNA, in which
case it is called a sense strand or a (+) strand. Or it can be opposite in orientation to a
mRNA, in which case it is called an antisense strand or a (-) strand. Some RNA
viruses are ss and will have either a (+) strand or a (-) strand. If they have a (-) strand
RNA they must first complement this RNA and create a (+) strand to serve as a
mRNA using a special RNA pol that can read an RNA template (rather than a DNA
template). There are also viruses that are ds RNA having 1 (+) and 1 (-) RNA. Finally
there are Retroviruses. Retroviruses will copy their RNA genome into a DNA copy
using an enz. called reverse transcriptase.
f. transcription & translation (expression) of viral capsid proteins using a true mRNA or a
(+) RNA and the protein synthesis apparatus of the host cell
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BIO 208 Microbiology Unit 3 Viruses
g. assembly of virions
h. release of mature virions
i. lysis
ii. budding
i. effects of viruses on animal cells (not in your text)
a. lytic - kills
Ex
b. persistent – slow shedding over long periods of time
Ex.
c. latent – dormant and then triggered
Ex.
d transformation – turns cancerous
Ex.
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BIO 208 Microbiology Unit 3 Viruses
D. Working with Viruses in the Lab
Biosafety Levels (BL)
BL1 – no disease risk – general lab safety protocols; lab coats, handwashing
BL2 – moderate potential health hazards – training, sign posting, eyewear, gloves, masks
BL3 – serious high individual risk – high level training, vaccinations, clean suits, respirators,
UV lights
BL4 – lethal, no vaccines or treatments – fully protective suits, airlocks, decontamination
showers
U.S. BL4 Units
1. Centers for Disease Control and Prevention (CDC)
2. US Army Medical Research Institute of Infectious Disease (USAMRID)
3. National Institutes of Health (NIH)
4. Southwest Institute for Biomedical Research
video
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BIO 208 Microbiology Unit 3 Viruses
E. Survey of viruses and viral infections
1. DNA Viruses
4 families to know
a. Papovaviridae (p, 385) - ds, nonenveloped
 Include viruses that cause warts
 More than 50 types
 Genital warts = #1 STD - epidemic - 500,000 new cases every year
 Associated with cancer of cervix and penis
b. Poxviridae (p. 385) - ds, enveloped
 Orthopoxvirus - smallpox, cowpox, monkeypox
 Smallpox - humans only, ancient
 Immunization history
 Eradication of smallpox
 Gone but not gone
Bioterrorism
Changing ecology of pox viruses
Monkeypox
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BIO 208 Microbiology Unit 3 Viruses
c. Herpesviridae (p. 385)- ds, enveloped
 Nearly 100 known, include viruses that cause cold sores, genital herpes,
chickenpox, and infectious mononucleosis (mono)
 Latency
 No cure - "Herpes is for Life"
i. Herpes - Simplexvirus - transmitted by direct contact
 HSV-1 (or HHV-1)- cold sores, fever blisters - 90%
 Wrestlers
 HSV-2 (or HHV-2) - sexually transmitted
ii. Chickenpox - Varicellovirus (HHV-3)
 3rd most common reportable infectious disease in U.S.
 Transmitted by aerosol or by direct contact
 High morbidity but low mortality
 Reactivates as shingles
 Vaccine approved in 1995
iii. Epstein-Barr Virus (EBV) - Lymphocryptovirus (HHV-4)
 Infectious mononucleosis (mono)
 Developing countries vs U.S.
 Burkitt's lymphoma and relationship to malaria
d. Hepadnaviridae (p.386)- ds, enveloped
 Hepatitis B virus - transmitted by blood, needles (incl. Tattoo needles), saliva,
sexual contact - can survive up to 1 week in dried fluids.
 100X more contagious than HIV
 Hepatitis - inflammation of the liver - 2nd most commonly reported infectious
disease in the U.S.
 300,000 young adults infected each year
 5,000 deaths each year
 Symptoms - loss of appetite, low-grade fever, joint pain, later jaundice
 10% chronic carriers (1 million U.S.) - associated with liver disease, incl. cancer
 Vaccine - 3 shots
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BIO 208 Microbiology Unit 3 Viruses
2. RNA viruses – 6 families to know
a. Picornaviridae (p.387) - ss, positive (+) strand, no envelope
i. Enterovirus - acquired by ingestion, replicate in the intestinal tract.
 Include poliovirus - ancient
 Vaccine developed in 1950s and 1960s
 Worldwide eradication predicted
ii. Rhinovirus - common cold
 Transmitted by aerosols, direct contact
 More than 100 strains
 Incubation period is 24 hours
 No effective treatment
iii. Hepatitis A virus - hepatitis
 Acquired by ingestion of contaminated food (oysters) and water
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BIO 208 Microbiology Unit 3 Viruses
b. Flaviviridae (not in text) – ss, (+) strand, enveloped
 Includes Yellow Fever, Dengue, West Nile
 All are vector-borne (mosquitoes, etc)
 West Nile – Uganda 1957, U.S. 1999
c. Coronaviridae (not in text) – ss (+) strand, enveloped
 Includes SARS-CoV – Severe Acute Respiratory Syndrome
 Newly evolved strain, 1st appeared in China in 2002
 Spread in large respiratory droplets
 Pneumonia
d. Filoviridae (not in text) - ss, negative (-) strand, enveloped
 Includes Marburg and Ebola
 Indigenous to Africa, but not confined to Africa
e. Orthomyxoviridae (not in text) - (-) strand RNA, segmented
 Includes - Influenzavirus - the flu
f. Retroviridae (p.387) - RNA viruses that produce DNA using the enzyme reverse
transcriptase, enveloped
 Includes Lentivirus or HIV
F. Prions - Proteinaceous Infectious Particles (pp. 392-393)
A normal host cellular protein is refolded incorrectly, becoming abnormal and infectious
 Scrapie (sheep)
 Kuru (humans)
 Creutzfeldt-Jacob Disease (CJD) (humans)
 Bovine Spongiform Encephalopathy (BSE = mad cow disease; nvCJD in humans)
So, what am I to know about viruses?
1. Describe the physical structure of an enveloped and a nonenveloped (naked) virus
2. Understand how viral families, genera, and species are named
3. Describe lytic and lysogenic cycles of bacteriophages
4. Know the steps in multiplication of animal viruses
5. Discuss the relationship of viruses to cancer
6. Provide examples of latent infections
***7. Know characteristics of the 10 virus families listed, including diseases
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