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Viruses
Chapter 19
What is a Virus?
•Ultramicroscopic infectious agents that
replicates itself only within cells of living
hosts.
•Many are pathogenic;
•Composed of a piece of nucleic acid
wrapped in a thin coat of protein.
How big is a virus?
Virus Structures
1. Capsid – protein coat
2. Genetic material – DNA or RNA
Other structures that may be present:
• Surface markers – to attach to host
• Tail fibers – Bacteriophages only
Virus Shapes? No typical shape
Virus Size? Have to use an electron microscope
to see!
Viral Shapes & Structures
Virus Genetic Material
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DNA
Fairly stable
Very few mutations
Vaccines work well
Ex: polio, small pox,
chicken pox
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RNA
Retroviruses
Mutations are
common
No vaccines or new
ones have to be
made yearly.
Ex: influenza, HIV,
Ebola
Viral Replication
What do they need in order to
reproduce?
A HOST!
Viruses are:
“obligatory intracellular parasites”
•Not capable of self-replication
without a host
So, are viruses living things?
They are at the borderline of nonliving and living.
Lytic Cycle
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Lytic Cycle
Quick in process
Takes over host cell
Forces host to make more
virus.
Uses host’s materials
Destroys host cell – “lyse”
Lytic Cycle Animation
Lysogenic Cycle
• Slower process
• Prophage inserted
into host’s DNA
• Hides in host’s DNA
until activated.
• Once activated,
continues with Lytic
cycle.
• Ex: HIV
How can a viral infection
be cured?
• There is NO cure for a viral infection.
• Vaccines must be taken before you are
infected.
• Once infected, body must fight off the
infection.
• Antiviral drugs are available to treat only a
few viral diseases. Why is this so???
The reason for this is: The drug is likely to be toxic to
the host as well as the virus. Expensive too.
Textbook Figure 19-15
Is there anything pathogenic
smaller than a virus?
Oh, YES!
PRIONS
What is a Prion?
• Protein infectious
particles
• No genetic material
• Diseases caused:
Mad Cow, scrapie
and Creutzfeldt-Jacob
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