Chapter 13-Viruses. Viroids, and
Prions
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 known 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
Common Shapes
• Capsid coat made of
capsomeres
• Nucleic acid inside
Two different types of Viruses
Relationship of virus with host cell
Bacterial viruses
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Known as bacteriophages or phages
Two different life cycles
1. Lytic cycle (lytic or virulent phage)-results in
lysis of the cell
2. Lysogenic cycle (temperate or lysogenic
phage)-may result in lysis of the cell or
becomes a permanent part of the chromosome
by integrating
T4 phage
replication
Lambda Phage
replication
Lambda integrates into the chromosome
Properties conferred by prophage
Some phage are filamentous
Replication of filamentous phage
M13 is ssDNA…how does it
replicate the ssDNA?
How do bacteria protect
themselves against phage?
• Prevent phage attachment
• Attacking foreign DNA with
restriction enzymes, protecting
native DNA with methylation
• CRISPR system degrades
incoming viral nucleic acid
CRISPR defense system against
phage
Methods to study bacteriophage
• Plaque Assay used
to quantitate phage
How do animal viruses differ
from bacterial viruses?
• Attachment or entry into the cell
• Replication of viral nucleic acid (remember
eukaryotic cells have a nucleus)
• Uncoating step is required by animal
viruses
• Exit the host cell by budding or shedding
Effects of animal virus on cells
Entry of animal virus
Replication
strategies
• Watch the type of
nucleic acid
• What enzymes are
needed for the process?
Release of enveloped
viruses
Acute viral infections
• Usually short in duration
• Host develops long lasting immunity
• Infection of the virus results in a productive
infection…host cells die as a result of
infection
General Steps of Acute Viral
infection
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Attachment
Entry into host cell
Targeting where it will reproduce
Uncoating of the capsid
Synthesis of proteins, replication of nucleic acid
Maturation
Cell lysis
Can you identify some examples of viruses
that produce an acute viral infection?
Persistent infections
• Virus is continually present in the body,
released by budding
• Three categories
– Latent infections
– Chronic infections
– Slow infections
Persistent: Latent Infections
• Persistent infection with symptomless
period followed by reactivation of virus and
symptoms
• Example of latent viruses are found in the
family Herpesviridae
– Herpes simplex virus -1
– Herpes simplex virus -2
Latent Viral infections
• All of these viruses are in the Herpesviridae
family
Herpesviridae Family
• Double stranded DNA (dsDNA),
enveloped viruses
-herpes simplex virus type 1(cold sores)
-herpes simplex virus type 2 (genital
herpes)
-Varicella-zoster virus (chicken pox,
shingles)
-Epstein-Barr (infectious mono and
Burkitt’s lymphoma)
Herpes Simplex virus-1
HSV-1 reactivation
Herpes simplex-1
• HSV-1 causes fever
blisters, HSV-2 genital
herpes
• Symptoms: fluid filled
skin lesions
• Treatment: Acyclovir
Varicella (chickenpox) and
Herpes Zoster (Shingles)
• HSV-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
Chronic infections
• Infectious virus present at all times
• Disease may be present or absent
• Examples are Hepatitis Type B and Type C
viruses
Type Hepadnaviridae family: Hepatitis B
• dsDNA virus, enveloped
• Hepatitis B
-passes through
intermediate stage (RNA)
for replication
-three particles found in
blood sample
1. Dane
2. filamentous
3. sphericle
-exposure through
blood/body fluids
Hepatitis Type 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
Flaviviridae Family: Hepatitis Type C
• Hepatitis C virus
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(+) ssRNA virus, enveloped
Obtain from blood/body fluids
Incubation period averages 6 weeks
Hard to screen blood for the virus
85% of all cases become chronic
What other types of Hepatitis viruses are
known to infect humans?
• Hepatitis Type A
– Found in the Picornaviridae family (+) ssRNA
-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
Slow Infections
• Infectious agent increases in amount over a
long time during which there are no
symptoms
• Examples are HIV found in the Retroviridae
family
• Retroviruses use reverse transcriptase to
replicate ssRNA
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
HIV
replication
Viruses associated with cancers
Viruses and tumors
• dsDNA viruses are most common to cause
viral-induced tumors
• Cancer is result of integration of viral genes
into the host chromosome
• Transforming genes are called oncogenes
• Examples: papillomavirus, herpesvirus
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
If multiple forms infect one
cell…reassortment can
occur
Antigenic shift vs antigenic drift
Ways to study viruses
• Since viruses grow in living cells….need a
live cell to culture them
– Cell culture/tissue culture
– Embryonated chicken eggs
Cell Culture
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
• Contain no nucleic acid
• Abnormal protein promotes conformational
change to normal protein
• Results in damage to
neurons…transmissible spongiform
encephalopahthies
Brain with spongiform
encephalopathy
Infections caused by prions
Mechanism
of prion
replication
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