Transcript Viruses, Viroids, and Prions

Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 20
Viruses, Bacteria, and
Archaea
Lecture Outline
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20.1 Viruses, Viroids, and Prions
•
Virus

Associated with a number of plant, animal, and
human diseases




noncellular
May have a DNA or RNA genome.
Invention of the electron microscope allowed these
infectious agents to be first seen
French chemist Louis Pasteur suggested that
something smaller than a bacterium was the cause
of rabies

•
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Viruses, Viroids, and Prions
• Viral Structure and Classification:

 Viruses are characterized by
• Size and shape
– 10–400 nm in diameter
• Type of nucleic acid core
– Single stranded or double stranded? DNA or RNA?
• Capsid or no capsid (outer layer composed of protein
subunits):
– Some are enveloped by membrane
– Others “naked”
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Viruses, Viroids, and Prions
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Capsid (protein)
Covering
Envelope (not found in all viruses)
Virus particle
Nucleic acid molecule (DNA or RNA)
Inner core
Various proteins (enzymes)
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Viruses, Viroids, and Prions
• Viruses:
 Are obligate intracellular parasites
•
• Are either active or inactive, instead of living or non-living
 Can be cultured only inside living cells
• Chicken egg
• Tissue culture
 Three hypotheses about viral origin and evolution
• Proteins and nucleic acids, organic molecules in viruses, evolved and
viruses may have arisen from these two basic polymers when cells did.
•
• Viruses may have evolved backwards from living cells.
• Viruses degenerated from living cells.
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Viruses, Viroids, and Prions
• Reproduction of Viruses:
 1. Attachment: Virus gains entry into specific host cell
based on host-specific match between virus surface
molecules and host cell receptors.
 2. Penetration: Host cell engulfs virus or virus injects
its genome into the cytoplasm.
 3. Biosynthesis: New viral components are
synthesized using host cell’s machinery and energy.
 4. Maturation:
 5. Release: New viruses exit host cell through lysis or
budding in order to infect new host cells.
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Viruses, Viroids, and Prions
• Bacteriophages – Viruses that infect bacterial cells
• There are two types of bacteriophage life cycles.
 The lytic cycle
• Viral reproduction occurs
•
• Hundreds of virus particles are released
 The lysogenic cycle
• Viral reproduction does not occur immediately but may occur in the
future.
• Virus becomes integrated into the host genome and may reenter
lytic cycle.
•
• Diphtheria is caused by a prophage-carrying bacterium, which
produces a toxin that damages the lining of the upper respiratory 7
tract, restricting breathing.
Viruses, Viroids, and Prions
• Reproduction of Animal Viruses:
 Animal virus enters the host cell
 Uncoating releases viral DNA or RNA
• Budding:
–
– Acquires a membranous envelope
• Retroviruses (HIV, the virus that causes AIDS)
– Contain reverse transcriptase
– Carries out RNA  cDNA reverse transcription
– cDNA becomes integrated into host DNA
»
» HIV may remain latent for years
– Viral DNA is transcribed; new viruses are produced
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Flu Virus
• A flu virus has an H (hemagglutinin) spike and an N
(neuraminidase) spike
 H spike allows the virus to bind to the receptor
• 16 different types
 N spike attacks host plasma membranes
• Allows mature viruses to exit the cell
• 9 different types
•
• Our immune system only recognizes H spikes and
N spikes it has been exposed to.
• Currently the H7N9 and H5N1 subtypes of flu virus
are of great concern because they can potentially
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become pandemics (global outbreaks).
Emerging Viruses
• Emerging viruses are new or previously uncommon
illnesses.
 Examples are AIDS, West Nile encephalitis, hantavirus
pulmonary syndrome (HPS), severe acute respiratory
syndrome (SARS), Ebola, hemorrhagic fever, and avian
influenza
 Several types of events can cause emergence of viruses
• A virus may extend its range.
–
• A genetic mutation may occur.
– Example: Influenza strains H5N1, H1N1, and H7N9 were created through
mutation of flu viruses which only infected animals
– It is necessary to obtain flu vaccine each year due to the rapidly mutating flu
virus
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Viruses, Viroids, and Prions
• Viruses are best known for causing infectious
diseases in plants and animals.
 Herpes, HIV, cancer
• Viruses lack metabolism; thus, antibiotics have no effect
 Viroids
• Naked strands of RNA
•
 Prions
• Protein molecules with contagious tertiary structure
• TSEs are neurodegenerative diseases which destroy nerve
tissue in the brain
– They are untreatable and fatal
•
• Some practices, including eating brains of cattle, transmitted
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the cattle disease (BSE or bovine spongiform
encephalopathy) to humans.