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Pathogens . . .
They’ve Gone Viral !!!
Part One
Applied Ag Biology
Page 1
What is a Virus?
• Segments of nucleic acids
contained in a protein coat
• Smaller than bacteria
• Only seen w/ electron microscope
• Replicate by infecting cell & using
that cell to make more viruses
Page 2
Is a Virus a Living Organism?
• NO!!! Biologists don’t consider
them to be alive.
• They do not have all properties
of life – no metabolism, no
homeostasis, no growth
• They DO cause diseases in
living organisms
Page 3
Discovery of Viruses
• In late 1800’s, scientists were
trying to determine cause of
Tobacco Mosaic Disease
• Disease was caused by
something smaller than a
bacteria
• Called the agent a VIRUS,
the Latin word for poison
• Assumed to be tiny cells
Page 4
Discovery of Viruses
• In 1935, tobacco mosaic virus
(TMV) was purified
• Had a crystal structure, which
is a property of chemicals
• Its structure allowed it to
infect healthy tobacco plants
• Conclusion = TMV was a
chemical, not an organism
Page 5
Virus’s Shape Determined by Its Parts
• Capsid = the virus protein coat;
contains DNA or RNA (not
both)
• Examples of DNA viruses:
warts, chickenpox,
mononucleosis
• Examples of RNA viruses:
HIV, influenza, rabies
Page 6
Most Viruses Have an Envelope
• Envelope = membrane which
surrounds capsid in many
viruses; helps virus enter cells
Page 7
Common Shape of Viruses
1. Helical – rod-like in appearance,
with capsid proteins winding
around the core in a spiral
Ebola Virus
Page 8
Common Shape of Viruses
2. Polyhedral – many sides and
is roughly spherical
Page 9
How Do Viruses Replicate?
• Lack enzymes needed for
metabolism
• No structures to make proteins
• Rely on living hosts to replicate
• Step 1 – Virus infects host cell.
• Step 2 – Virus either go into the
LYTIC CYCLE or LYSOGENIC
CYCLE
Page 10
Lytic Cycle
• Lytic Cycle: the cycle of viral
infection, replication, and cell
destruction
• Viruses cause damage when
they replicate inside cells
• Virus replicats 100’s of times
and breaks out – destroying cell
Page 11
Lysogenic Cycle
• During infection, some
viruses stay inside cells but
don’t make new viruses
• Lysogenic Cycle: cycle in
which the viral genes
replicate without destroying
host cell
Page 12
Lysogenic Cycle
• In animal cells, viruses replicate
slowly so host cell is not
destroyed
• Example: virus that causes
cold sores hides deep in the
nerves of the face; when body
becomes stressed, the virus
begins to cause tissue damage
(cold sore/fever blister)
Page 13