Transcript Viruses-TEA - KCI-SBI3U

VIRUSES
Virus (Latin): meaning poisonous slime of plant or
animal origin
Virus size: 10300 nm
•Viruses are non-living.
1. Acellular: lack cell membrane and
organelles
2. No metabolic activity
3. Lack energy generating system
4. They are obligate parasites - only
capable of reproduction in a host cell.
5. Genetic material: DNA or RNA,
housed within a protein coat called a
capsid
Virus Characteristics (cont.)

There may be a further layer referred
to as the envelope, which surrounds
the capsid - this is found in animal
viruses and is derived from the
membrane of the host animal cell.
•Protein spikes may
protrude from the
viral particle - these
are involved in the
attachment of the
particle to the host
cell.
Influenza viruses (100nm
each), with capsid and
membrane visible
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VIRUS
CLASSIFICATION
Polyhedron
Viruses are classified based on
1. Capsid’s size and shape (Cylindrical, Polyhedron,
Enveloped, Binal (complex) shape)
2. Types of genetic materials (i.e. DNA or RNA)
3. The number of strands in Nucleic Acid - either
single stranded or double stranded form
4. Nature of host (plant, animal or bacteria)
5. Presence or absence of envelope
Cylindrical/
helical
Binal (irregular) shape
http://www.youtube.com/watch?
v=7iVm1uEIyP0
Spherical
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VIRAL SHAPES EXAMPLES
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BACTERIOPHAGE STRUCTURE
Bacteriophage: is a virus that infects bacteria
capsid (protein coat)
nucleic acid
tail
tail fibres
cell membrane
viral RNA/DNA injected into
host cell
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Largest Virus Found in 2011!
•Pandoravirus (~1
m)
•Irregular shape,
•Look like a
bacterium
•DNA as genetic
material
•Infects amoebas
•Not a threat to
human
•Found in sediments
in Chile, Australia
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Virus Microviewer Lab
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Virus Reproduction
• Viruses undergo replication within a host cell
• incubation period: time between when the virus
infects a cell and when the virus ruptures the cell
• 2 possible cycles: lytic and lysogenic
1. The LYTIC Cycle
(shorter incubation
period)
E.g. Bacteriophage T4 infects E. coli
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A. Attachment
Viral DNA
Bacterial host cell
E. Lysis &
Release
Cell lysed due
to viral
enzyme;
new phage
go on infect
other hosts
D. Assembly
New viral particles are
assembled
virus
bacterial
DNA
B. Insertion
- Host’s
DNA/RNA
get
digested
- Virus
injects its
nucleic
acid into
host cell
C. Replication
The host’s metabolism
replicates the viral
RNA/DNA
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Bacteria are NOT defenceless against phage
infections
Bacteria use
restriction
enzymes that act
like scissors to cut
up phage’s DNA,
making it useless
- Bacteria with effective restriction enzymes survive and reproduce
- Any phages that develop resistance to the restriction enzymes also
have a reproductive advantage
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Bacteria and virus arms race!
Summary of lytic cycle (HW- self note)
1. Attachment & 2. Insertion:
3. Replication:
4. Assembly:
5. Lysis & Release:
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Virus Reproduction
2. The LYSOGENIC cycle (longer incubation period)

Cells undergoing this cycle can be triggered into
reproducing in the lytic cycle
virus
bacterial
DNA/RNA
A. Attachment
& Entry
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Virus Reproduction
2. The LYSOGENIC cycle (longer incubation period)
provirus
B. Integration
Viral nucleic acid becomes a part
of the bacterial nucleic acid
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Virus Reproduction
2. The LYSOGENIC cycle (longer incubation period)
C. Multiplication of Genome
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Virus Reproduction
2. The LYSOGENIC cycle (longer incubation period)
May now
undergo
the lytic
cycle
D. Cell Division
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The LYSOGENIC Cycle (longer incubation period)
• Attachment: The virus invades a host cell and injects its
DNA (or RNA) into the host.
• The viral DNA is merged with the host DNA.
• The viral DNA remains dormant for a period of time
(provirus)
• The viral DNA is extracted from the HOST by a "trigger".
• The host cell replicates the parts of the virus and other
materials in viral DNA  LYTIC CYCLE
• New viral particles are assembled in the host cell.
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Bring 2 cycles together
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Compare lytic and lysogenic cycles
Lytic cycle
Lysogenic cycle
Fate of virus
DNA/RNA after
infecting the host
Take over host
DNA
Becomes part of
host DNA
Incubation time in
host cell
Short; new viruses
released within
host’s lifetime or in
the next generation
Long; new viruses
released after
several generations
of host
Onset of viral
diseases
symptoms
Immediate
delayed
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VIRUSES and DISEASES
•Caused by herpes simplex virus
(a db stranded DNA virus).
•The sores appear during lytic
cycle
•The sores disappear (when the
viruses is in its provirus stage) on
the skin throughout the person’s
lifetime.
• Virus is periodically
reactivated in times of
physical or emotional
stress (newly added note)
COLD SORE (Kissing Disease)
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A different kind of sore: canker sore
VIRUSES and DISEASES
Retro-viruses
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VIRUSES and DISEASES
Retro-viruses
• Normal genetic information transfer is from DNA to RNA
• Retro-viruses use the reverse process (i.e. RNA to DNA):
- After injecting RNA into host cell, retro-viruses have an
enzyme called Reverse transcriptase that can then convert
their RNA into DNA
-The viral DNA merges with
host’s DNA and can replicate
for many years showing no
harm to the host.
- Can switch to lytic cycle any
time. What triggers this
switch is not yet known.
•HIV video: http://www.youtube.com/watch?v=v_AUuMbtO-E
What’s the inference for showing HIV video under retro-viruses topic?
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To sum it up…
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VIRUSES and DISEASES
•AIDS (acquired immune deficiency
syndrome):
HIV (AIDS)
•Caused by HIV- a retro virus
•The HIV destroys the body’s Tlymphocytes, which help fight off
infections from other diseases
•When leaving the host cell, new
HIV take a piece of the host cell
membrane to disguise themselves
from the host body’s immune
system.
•HIV video: http://www.youtube.com/watch?v=v_AUuMbtO-E
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VIRUSES and BIOTECHNOLOGY
•can be used to make lots of copies of desirable genes
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VIRUSES and BIOTECHNOLOGY
•Viruses well-known for
inserting their DNA/RNA into
host’s DNA
•can be used in ‘gene therapy’
to repair bad genes
•The virus must be disarmed
To correct for a genetic disorder
permanently, a stem cell is
used.
-The virus carrying the desired
gene is thus let infect the
patient’s stem cells.
- The result is the desired gene
is permanently expressed once
the stem cells are injected26
into
the bone marrow of the patient
Vaccines



Vaccines are mixtures of dead or
weakened forms/parts of
dangerous viruses
When inside a host, the antigens
on the surface of the weakened
viruses triggered white blood
cells to produce antibodies,
without causing an infection
antigens
Antibodies are chemicals
produce by your body to
recognize foreign antigens
These
antibodies has long lasting memories that allows the
immune system to react quickly if it contacts the real virus
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NPR Flu Attack video: http://www.youtube.com/watch?v=Rpj0emEGShQ
(not in your note)
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Vaccines
antibody
Foreign
Invader
White Blood
Cell
macrophage
antigen
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Viroids
• Smaller than virus, with no capsid
• Single stranded, circular RNA that do not code for
proteins (unlike viruses; viral RNA do code for
proteins)
Viroids under microscope
Mis-shaped potatoes
caused by viroids
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Prions
• Are single proteins
• Found in brain of
infected animals
• Once eaten, prions go to
the animal’s brain
• Cause diseases by
interacting with normally
shaped proteins causing
them to become
abnormal and infectious
• Responsible for brain
diseases such as mad
cow diseases in cow,
scrapie in sheep and
Creutzfeldt-Jakob
disease in human
BSE = bovine spongiform encaphelopathy
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or mad cow disease
News Update
Univ. of Cadiff- School of Med
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http://youtu.be/jFpwHe6KjDA
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