18.1 Studying Viruses and Prokaryotes
KEY CONCEPT
Infections can be caused in several ways.
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18.1 Studying Viruses and Prokaryotes
Discovery of Viruses
•Beijerinck (1897)
coined the Latin
name “virus”
meaning poison
•He studied
filtered plant
juices & found
they caused
healthy plants to
become sick
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18.1 Studying Viruses and Prokaryotes
Tobacco Mosaic Virus
•Wendell Stanley
(1935) crystallized
sap from sick
tobacco plants
•He discovered
viruses were made of
nucleic acid and
protein
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18.1 Studying Viruses and Prokaryotes
Smallpox
•Edward Jenner (1796)
developed a smallpox
vaccine using milder
cowpox viruses
•Deadly viruses are
said to be - virulent
•Smallpox had been
eradicated in the
world today, but is now
coming back
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18.1 Studying Viruses and Prokaryotes
•Viruses are smaller
than the smallest cell
•Measured in
nanometers
•Viruses couldn’t be
seen until the electron
microscope was
invented in the 20th
century
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18.1 Studying Viruses and Prokaryotes
Size of Viruses
18.1 Studying Viruses and Prokaryotes
Viruses, bacteria, viroids, and prions can all cause
infection.
• Any disease-causing agent is called a pathogen.
1 nanometer (nm) = one
billionth of a meter
100 nm
eukaryotics cells
10,000-100,000 nm
viruses
50-200 nm
prokaryotics cells
200-10,000 nm
viroids
5-150 nm
prion
2-10 nm
18.1 Studying Viruses and Prokaryotes
• A virus is made of DNA or RNA and a protein coat.
– non-living pathogen
– can infect many
organisms
• A viroid is made only
of single-stranded
RNA.
– causes disease in
plants
– passed through
seeds or pollen
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18.1 Studying Viruses and Prokaryotes
Viroids
•Small, circular
RNA molecules
without a protein
coat
•Infect plants
•Potato famine in
Ireland
•Resemble introns
18.1 Studying Viruses and Prokaryotes
• A prion is made only of proteins.
– causes misfolding of other proteins
– results in diseases of the brain
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18.1 Studying Viruses and Prokaryotes
Prions
•Prions are “infectious proteins”
• They are normal body proteins
that get converted into an alternate
configuration by contact with other
prion proteins
• They have no DNA or RNA
•The main protein involved in human
and mammalian prion diseases is
called “PrP”
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18.1 Studying Viruses and Prokaryotes
Prion Diseases
•Causes neurons to rapidly
degenerate.
•Examples:
•Mad cow disease (bovine
spongiform encephalitis: BSE)
•People in New Guinea used to
suffer from kuru, which they got
from eating the brains of their
enemies
18.1 Studying Viruses and Prokaryotes
Clicker Question
Prions can be contracted by…
A: Eating leftovers from the fridge.
B: Eating the brain of an infected animal.
C: Eating meat that has come into contact
with the brain or spinal cord of an
infected animal.
D: Both B and C
18.1 Studying Viruses and Prokaryotes
Clicker Question!
• What infectious element causes
Mad Cow Disease?
A: Phages
B: Prions
C: Viroids
D: Plasma
E: Virus
18.2 Viral Structure and Reproduction
KEY CONCEPT
Viruses exist in a variety of shapes and sizes.
18.2 Viral Structure and Reproduction
Viruses differ in shape and in ways of entering host
cells.
• Viruses have a simple structure.
– genetic material
– capsid, a protein shell
– maybe a lipid envelope, a protective outer coat
enveloped
(influenza)
capsid
nucleic acid
lipid
envelope
helical
(rabies)
Surface proteins
capsid
nucleic acid
surface
proteins
lipid envelope
polyhedral
(foot-and-mouth
disease)
surface
proteins
capsid
nucleic acid
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18.2 Viral Structure and Reproduction
Viral Characteristics
• Noncellular
• Have a protein coat called the
capsid
• Have a nucleic acid core containing
DNA or RNA
• Capable of reproducing only when
inside a HOST cell
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18.2 Viral Structure and Reproduction
Characteristics
• Some viruses are
enclosed in a protective
envelope
• Some viruses may have
spikes to help attach to
the host cell
• Most viruses infect
only SPECIFIC host
cells
CAPSID
DNA
ENVELOPE
SPIKES
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18.2 Viral Structure and Reproduction
Helical Viruses
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18.2 Viral Structure and Reproduction
Polyhedral Viruses
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18.2 Viral Structure and Reproduction
Complex Viruses
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18.2 Viral Structure and Reproduction
Used for Virus
Identification
•
•
•
•
RNA or DNA Virus
Do or do NOT have an envelope
Capsid shape
HOST they infect
18.2 Viral Structure and Reproduction
• Bacteriophages infect bacteria.
capsid
DNA
tail sheath
tail fiber
18.2 Viral Structure and Reproduction
• Viruses enter cells in various ways.
– bacteriophages pierce host cells
colored SEM; magnifications:
large photo 25,000; inset 38,000x
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18.2 Viral Structure and Reproduction
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18.2 Viral Structure and Reproduction
Clicker Question - Phages
•A Bacteriophage or a
phage is:
A virus that infects a
__________ cell.
A.
B.
C.
D.
E.
Plant
Animal
Bacteria
Protista
Fungi
18.2 Viral Structure and Reproduction
• Viruses enter cells in various ways.
– viruses of eukaryotes enter by endocytosis
18.2 Viral Structure and Reproduction
• Viruses enter cells in various ways.
– viruses of eukaryotes also fuse with membrane
18.2 Viral Structure and Reproduction
Viruses cause two types of infections.
• A lytic infection causes the host cell to burst.
host bacterium
The bacterophage attaches
and injects it DNA into a host
bacterium.
The host bacterium breaks apart,
or lyses. Bacteriophages are able
to infect new host cells.
The viral DNA
forms a circle.
The viral DNA directs the host
cell to produce new viral parts.
The parts assemble into new
bacteriophages.
The virus may enter the
lysogenic cycle, in which the
host cell is not destroyed.
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18.2 Viral Structure and Reproduction
5 Steps of Lytic Cycle
• 1. Attachment to the cell
• 2. Penetration (injection) of viral DNA
or RNA
• 3. Replication (Biosynthesis) of new
viral proteins and nucleic acids
• 4. Assembly (Maturation) of the new
viruses
• 5. Release of the new viruses into the
environment (cell lyses)
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18.2 Viral Structure and Reproduction
One-step Growth Curve
18.2 Viral Structure and Reproduction
• A lysogenic infection does no immediate harm.
The prophage may leave the
host’s DNA and enter the
lytic cycle.
The viral DNA is called a prophage
when it combines with
the host cell’s DNA.
Many cell divisions produce a
colony of bacteria infected
with prophage.
Although the prophage is not
active, it replicates along with
the host cell’s DNA.
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18.2 Viral Structure and Reproduction
The Lysogenic Cycle
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18.2 Viral Structure and Reproduction
Viral Latency
•Some viruses have the ability to
become dormant inside the cell
•Called latent viruses
•They may remain inactive for long
periods of time (years)
•Later, they activate to produce new
viruses in response to some external
signal
•HIV and Herpes viruses are
examples
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18.2 Viral Structure and Reproduction
Latency in Eukaryotes
•Some eukaryotic viruses remain
dormant for many years in the
nervous system tissues
• Chickenpox (caused by the
virus Varicella zoster) is a
childhood infection
•It can reappear later in life as
shingles, a painful itching rash
limited to small areas of the body
SHINGLES
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18.2 Viral Structure and Reproduction
Latency in Eukaryotes
•Herpes viruses also becomes
latent in the nervous system
SKIN TO SKIN CONTACT
•A herpes infection lasts for a
person’s lifetime
•Genital herpes (Herpes
Simplex 2)
•Cold sores or fever blisters
(Herpes Simplex1)
PASSED AT BIRTH TO
BABY
18.2 Viral Structure and Reproduction
Clicker Question!
• Viruses replicate:
A. On their own
B. With other viruses
by joining pili
C. When ever they want
to
D. In a host cell only
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18.2 Viral Structure and Reproduction
Clicker Question!
• All Viruses ONLY contain RNA.
A. True
B. False
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18.2 Viral Structure and Reproduction
Retroviruses
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18.2 Viral Structure and Reproduction
Characteristics of Retroviruses
•Contain RNA, not DNA
•Contain enzyme called Reverse
Transcriptase
•When a retrovirus infects a
cell, it injects its RNA and
reverse transcriptase enzyme
into the cytoplasm of that cell
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18.2 Viral Structure and Reproduction
ENZYME
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18.2 Viral Structure and Reproduction
Retroviruses
•The enzyme reverse
transcriptase (or
RTase), which causes
synthesis of a
complementary DNA
molecule (cDNA) using
virus RNA as a
template
RTase
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18.2 Viral Structure and Reproduction
Retroviruses
•HIV, the AIDS
virus, is a
retrovirus
•Feline Leukemia
Virus is also a
retrovirus
18.2 Viral Structure and Reproduction
Clicker Question!
• Retroviruses contain:
A. tRNA
B. DNA
C. RNA
D. rRNA
E. mRNA
18.3 Viral Diseases
KEY CONCEPT
Some viral diseases can be prevented with vaccines.
18.3 Viral Diseases
Viruses cause many infectious diseases
• There are many examples of viral infections.
– common cold
18.3 Viral Diseases
Viruses cause many infectious diseases
• There are many examples of viral infections.
– common cold
– influenza
18.3 Viral Diseases
Viruses cause many infectious diseases
• There are many examples of viral infections.
– common cold
– influenza
– SARS
18.3 Viral Diseases
Viruses cause many infectious diseases
• There are many examples of viral infections.
– HIV
• The body has
natural
defenses
against viruses.
HIV-infected
white
blood cell
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18.3 Viral Diseases
Characteristics
•Some viruses cause
disease such as:
•Smallpox, measles,
mononucleosis,
influenza, colds, warts,
AIDS, Ebola
•Some viruses may
cause some cancers
like leukemia
MEASLES
18.3 Viral Diseases
Vaccines are made from weakened pathogens.
• A vaccine stimulates the body’s own immune response.
• Vaccines prepare the immune system for a future attack.
• Vaccines are the only way to control the spread of viral
disease.
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18.3 Viral Diseases
Herpes Virus
SIMPLEX I and II
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18.3 Viral Diseases
Adenovirus
COMMON COLD
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18.3 Viral Diseases
Influenza Virus
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18.3 Viral Diseases
Chickenpox Virus
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18.3 Viral Diseases
Papillomavirus – Warts!