Chapter 13-Viruses
General Characteristics of all viruses
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Contain a single type of nucleic acid
Contain a protein coat
Obligate intracellular parasites
Are viruses the only obligate intracellular
parasites?
History began with the Tobacco
Mosaic Virus (TMV)
• 1886 Aldolf Mayer
showed that a virus
was transmissable
between plants
• 1892 Iwanowski tried
to isolate it by filtering
with porcelain filter
Sizes of viruses
Polyhedral virus
• Capsid coat made of
capsomeres
• Nucleic acid inside
Helical virus with an envelope
• The shape is a long
rod
• Rabies and Ebola are
helical viruses
• Influenzae virus is
helical with an
envelope
Bacteriophage: Complex virus
Bacterial viruses
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Known as bacteriophages or phages
Two different life cycles
1. Lytic cycle-results in lysis of the cell
2. Lysogenic cycle-may result in lysis of the cell
or the virus becomes a permanent part of the
chromosome by integrating
Lytic Cycle
Lytic Cycle
Growth curve of bacteriophage
Lysogenic Cycle
How can you study
bacteriophages?
• Plaque assay
-pour agar with
bacteria and phage on
top of an agar plate
-“plaque” develops
where virus infected
bacterial cell
-each plaque is
counted as one virus
How do animal viruses differ
from bacterial viruses?
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Attachment
Replication of nucleic acid
Penetration
Uncoating
DNA Animal Viruses
• Adenoviridae
– dsDNA, non-enveloped
– First isolated in the adenoids
– Cause upper respiratory infections
DNA Animal Viruses
• Poxviridae
– dsDNA, enveloped
– Cause small pox (variola)
DNA Animal Viruses
• Poxviridae
– dsDNA, enveloped
– small pox virus
(variola)
DNA Animal Viruses
• Herpesviridae (dsDNA, enveloped virus)
-simplex 1(cold sores)
-simplex 2 (genital herpes)
-chicken pox, shingles
-epstein barr
Herpes simplex-1
• HHV-1 causes fever
blisters, HHV-2
genital herpes
• Symptoms: fluid filled
skin lesions
• Treatment: Acyclovir
Varicella (chickenpox) and
Herpes Zoster (Shingles)
• HHV-3 causes chicken
pox and latent
activation known as
shingles
• Acquired by
respiratory route, 2
weeks later see
vesicles on skin
• Vaccine established in
1995 for chickenpox
Epstein Barr
• Causes infectious
mononucleosis
• Acquire by saliva,
incubation period is 4-7
weeks
• Identify by
-lobed lymphocytes
-heterophile antibodies
-fluorescent antibody tests
Hepadnaviridae
• dsDNA, enveloped
• Hepatitis B
-passes through
intermediate stage (RNA)
-three particles in blood
Dane
filamentous
sphericle
-exposure through
blood/body fluids
Hepatitis B
• Incubation period is ~12 weeks
• 10% of cases become chronic, mortality rate
is less than 1%
• About 40% of the chronic cases die of liver
cirrhosis
RNA animal viruses
• Is there an enzyme in animal cells to
replicate RNA?
• What does RNA polymerase do?
RNA animal viruses
• (+) single stranded RNA viruses
– RNA serves as mRNA
• (-) single stranded RNA viruses
– RNA does not code for proteins
Picornaviridae (+) ssRNA
• Poliovirus
• Virus ingested then travels throughout the body
• In some cases it impairs the upper motor neurons,
less than 1% of all cases
• Vaccines
– Salk vaccine (IPV)
– Enhanced-inactivated polio (E-IPV)
– Sabin vaccine
Cases of Poliomyelitis in US
Picornaviridae (+) ssRNA
• Rhinovirus
-causes the common cold
-100 or more serological types
-virus grows best in the nose and
conjunctiva
Picornaviridae (+) ssRNA
• Enterovirus responsible for 90% of viral
gastroenteritis
– Rotavirus
• Most common cause of viral gastroenteritis
– Norwalk-like virus
• Responsible for local epidemics
Rotavirus
• Note the shape which
gave it the name
rota=wheel
Picornaviridae (+) ssRNA
• Hepatitis A
-obtain through fecal-oral route, enters GI tract
and multiplies
-incubation period is ~4 weeks
-symptoms include: anorexia, malaise, nausea,
diarrhea, abdominal discomfort, fever, and chills
lasting 2-21 days
Flaviviridae (+) ssRNA, enveloped
• Hepatitis C virus
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Obtain from blood/body fluids
Incubation period averages 6 weeks
Hard to screen blood for the virus
85% of all cases become chronic
Rhabdoviridae (-)ssRNA, enveloped
• Rabies virus
-enters the skin and multiplies in skeletal
muscle and connective tissue
-virus travels along nerves to the CNS
causing encephalitis
Pathology of rabies
Orthomyxoviridae-multiple
strands of (-)RNA
• Influenza virus
– Consists of 8 segments of RNA
– Envelope has H spikes (hemagglutinin) and N
spikes (neuraminidase)
– Incubation is 1-3 days
– Symptoms include: chills, fever, headache,
muscle aches, may lead to cold-like symptoms
Influenza virus
Retroviruses: convert RNA to
DNA
• HIV, Hepatitis B
Retroviridae-multiple strands of ()RNA
• HIV
-infects Helper T cells
-requires the enzyme
reverse transcriptase
-integrates as a provirus
-is released by budding,
or lyses the cell
Proteinaceous infectious
particles: PRIONS
• 1982 Stanley Prusiner proposed that there
were infectious proteins
• Caused the disease “scrapie” in sheep
• Caused the “mad-cow”disease in 1987
• Human forms suggest a genetic component
Prions: How do they replicate?
Can viruses cause cancer?
• Michael Bishop and Harold Varmus
received the 1989 Nobel Prize for cancercausing genes carried by a virus from
animal cells
• Oncogenes:genes that can be transformed to
cause cancer
• 10% of cancers have been found to be due
to oncogenic viruses