Download Lecture 06 - Viruses

Viruses
Viruses
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What is a virus?
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Defined by their inability to replicate/multiply without
utilizing a host cells reproductive mechanisms.
Only contain ONE nucleic acid, NOT both.
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DNA OR RNA
Smaller than a bacterial cell! Typically too small to be
seen under a light microscope.
What does a virus do?
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“Hijacks” the host cell and uses the hosts organelles to
synthesize components required to create new viruses.
The infected cell can produce up to thousands of new
virus particles before cell death occurs.
Viral infections cause tissue damage as a result of this
cellular death.
Viral Structure
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Viral Components:
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Nucleic Acid Core: The
location of the DNA or RNA of
the virus.
Protective Coating: Either a
capsid protein coating or a
bilayered membrane
envelope surrounds the
genetic material.
Lipid Envelope: Fatty
envelope providing extra
protection.
Spikes: Protein spikes that act
as antigens – these trigger out
immune response to the
invading virus.
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The two types are known as
hemagglutinin spikes and
neuraminidase spikes.
Viron: A complete virus
particle.
Nucleic Acids
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Genome: The genetic
information a virus uses to
replicate itself in the host
cell.
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Results in a disruption of
host cell activities and
eventually cell death.
Nucleic Acid: The total
genetic material of a virus.
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Can be DNA
(deoxyribonucleic acid) or
RNA (ribonucleic acid).
Can be Single-stranded
or Double-stranded.
Can be Linear (straight),
Circular, or Segmented
(in fragments).
Capsids
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Capsids: The outer
structure that
encloses & protects
the nucleic acid of a
viron.
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Determines the shape
of the virus.
Plays a key role in the
attachment of the virus
to the host cell.
Capsomeres: Protein
subunits that combine
to compose of the
capsid.
Envelope
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Envelope: The bilayer
membrane either outside
the capsid or acting as the
sole protector of the
nucleic acid of a virus.
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Produced as the virus
buds (moves through) the
membrane system of the
host cell.
Spikes: Glycoproteins
extending from the
envelope that help to
attach virons to receptor
sites on host cells.
Naked Virus: A virus that
contains only an envelope
& no capsid layer.
Envelope
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Advantages:
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Help protect the virus from the hosts immune
system since it is acquired from the host cell
membrane.
Help viruses infect new cells by fusion with the
host’s cell membrane.
Disadvantages:
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Can be easily damaged.
Environmental conditions can destroy the
membranes.
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E.g., extreme temperature, pH outside of normal
range, chemicals, etc.
Naked viruses tend to be more resistant to
extreme environments.
Bacteriophages
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Bacteriophages:
Viruses that
specifically infect
bacteria.
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Uses the bacteria’s
structure to replicate
new viruses.
When the bacteria is full
of bacteriophages, the
cell lyses (pops).
This releases
endotoxins & causes us
to feel ill!
Viral Virulence
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Host Range: The number and type of species
that a virus is able to infect.
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Most have a limited range of one or a small few potential
hosts.
Viral Specificity: A property of viruses that
refers to the specific kinds of cells that a virus
can infect. Determined by 3 things..
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The ability of as virus to attach to a certain cell type
based on the presence of specific receptor sites.
Whether the host cell has the proteins & machinery
needed for the virus to replicate.
Whether the replicate viruses have a means of exiting
the host cell.
Classifying Viruses
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Classification of viruses is dependent on…
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Nucleic Acid Type
Capsid Shape
Envelope Presence
Viron Size
Classifying Viruses – Nucleic Acid
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DNA Viruses: Viruses containing DNA
only.
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Single-Stranded DNA (ssDNA): Viruses with
a single strand of DNA.
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Only one known to cause human disease – human
papillomavirus (HPV).
Double-Stranded DNA (dsDNA): Viruses
with a double-stranded DNA.
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Can be linear (straight) or circular in shape.
Classifying Viruses – Nucleic Acid
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RNA Viruses: Viruses containing RNA only.
Classified by the number of strands and whether
it is segmented or in one piece.
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Single-Stranded RNA (ssRNA): Viruses with a single
strand of RNA – most common.
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Positive Sense RNA: Contains RNA that can mimic
eukaryotic mRNA – allows the host’s ribosomes to translate
the viral RNA into protein.
Negative Sense RNA: Contains RNA that is not
translatable by the host ribosomes – these viruses must
carry a special RNA polymerase to change the negative
strand into a positive strand, allowing RNA translation.
Double-Stranded RNA (dsRNA): Viruses with a
double strand of RNA.
Classifying Viruses – Retroviruses
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Retrovirus: A type of virus that can
translate its own RNA to form a
complimentary strand of DNA.
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Reverse Transcriptase: A specific enzyme
that allows RNA translation & DNA synthesis.
The produced DNA strand is typically replicated
to form dsDNA.
The dsDNA migrates to the host cell nucleus &
incorporates into the host chromosome.
Retroviruses can be treated using antiviral
medication.
 HIV is a retrovirus!
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Viral Reproduction
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Occurs two ways…
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The Lytic Cycle
The Lysogenic Cycle
Viral Reproduction – Lytic Cycle
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Lytic Cycle: The main replicative cycle of
all viruses.
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A virus takes over a host cell’s machinery and
reproduces new viruses.
The new viruses then exit the host cell & infect
new cells.
Occurs in 5 steps:
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Adsorption
Penetration
Synythesis
Maturation
Release
Viral Reproduction – Lytic Cycle
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Adsorption: The virus attaches to the host cell
using a combination of virus & host cell
recognition.
Penetration: The virus enters the cell.
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Enveloped viruses enter through fusion.
Naked viruses enter through endocytosis
(movement into the cell via a vesicle).
Synthesis: New viral genetic material &
proteins are synthesized using the host cell
mechanisms.
Maturation: Synthesized components are
assembled into complete progeny viruses
(virons).
Release: The new viruses are released from the
cell.
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Enveloped viruses can exit via budding in a
controlled manner – this does not lyse the cell.
Viruses that do not follow this controlled release
will lyse (pop) the cell, causing the clinical
symptoms of endotoxin release.
Viral Reproduction – Lysogenic Cycle
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Lysogenic Cycle: A long-term,
stable relationship between the
host cell and the virus.
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Involves the viral nucleic acid
incorporating into the host
chromosome.
Virus can stay dormant this way
for long periods.
Dorman virus genes are passed
on through each & every host
cell division!
Induction: The process of the
dormant virus genes becoming
activated and initiating a lytic
cycle.
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Can be spontaneous or in
response to an outside
stimulus.