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VIRUSES
Is a virus alive?
• Viruses are not cells;
they are not made up of
cells.
• Viruses ARE small
segments of nucleic acid
that are surrounded by a
protective protein coat.
• Viruses ARE 10 to 100
times smaller than
bacteria.
• Viruses ARE
pathogens—they cause
disease in living
organisms.
• Viruses can replicate
by infecting cells and
using the cell to make
more viruses.
• Viruses do not possess
all the properties of life,
therefore they are not
considered living.
• But because they
cause great harm to
living organisms, we try
to learn all we can
about them.
Who Discovered the First Virus?
• In the early 1900s, scientists were trying to
figure out what was killing the tobacco plants.
• They isolated a chemical in the sap of the plant
and crystallized it to show some sort of poison.
• In Latin, “virus” means poison
• Because of this discovery, many other viruses
could be identified and studied.
Viral Structure
• The capsid is made
of protein and holds
either DNA or RNA
• The tail fibers attach
to the cell
• The plug makes an
opening in the
membrane of the
living cell
• The sheath injects
the nucleic acid into
the cell
Viruses come in all shapes and
sizes, making them very unique…
Viral Replication
Viruses must rely on living cells (host cells) for replication.
An animal virus enters the cell by endocystosis.
A bacterial virus punches a hole in the cell wall.
Comparison of the two cycles:
•
•
•
•
Lytic Cycle
Viral genes enter the cell
and duplicate using
proteins inside of the host
cell
New viruses are made
inside of the host cell
The host cell is flooded
with new viruses and
bursts open
The host cell is killed
•
•
•
•
Lysogenic Cycle
Sometimes viruses inject
their genes but do not
duplicate inside of the host
cell
Instead, the virus’ DNA (or
RNA) combines with the
host cell’s DNA
The host cell divides
normally through mitosis
The viral genes are
replicated without killing the
host cell.
Prions
• In 1982 an American
scientist named
Prusiner discovered a
new class of pathogens
called prions.
• Prions are composed of
proteins that have no
nucleic acid.
• Mad cow disease is
associated with prions.
• It is believed that the
disease spreads when
cows eat feed
containing remnants of
infected cattle.
What are vaccines?
• Vaccines are killed or
weakened viruses that
are introduced into the
body to build immunity.
• Immunity is your
body’s defense
mechanism that
recognizes harmful
viruses when they try to
attack and prevents
them from making you
sick.
Why give vaccines?
• Immunizations
help a child’s
immune system
do its work. The
child develops
protection against
future infections,
the same as if he
or she had been
exposed to the
natural disease.
What else do vaccines do?
1. Protect a child’s friends, schoolmates, and others
from those same diseases. Some children can’t get
certain vaccines for medical reasons, or some children
are not able to respond to certain vaccines. For these
children, the immunity of people around them is their
only protection.
2. Protect your grandchildren, their grandchildren, and
future generations from diseases. If we stopped
vaccinating, diseases that are under control would
eventually come back to cause epidemics. This has
happened in several countries.
3. Help rid the world of diseases that have been
crippling and killing children for centuries.
Immunization allowed us to eradicate smallpox. Today
polio is nearly gone, and in the future measles and other
diseases will follow.