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Viruses
Chapter 19
What is a Virus?
•Ultramicroscopic infectious agents that
replicates within cells of living hosts.
•Many are pathogenic;
•Composed of a piece of nucleic acid
wrapped in a thin coat of protein.
Virus Structures
1. Capsid – protein coat
2. Genetic material – DNA or RNA
Virus Shapes? No typical shape
Virus Size? Have to use an electron microscope to see!
Viral Shapes & Structures
How big is a virus?
Virus Genetic Material – Two types!
•
•
•
•
DNA Viruses
Fairly stable
Very few mutations
Vaccines work well
Ex: polio, small pox,
chicken pox
•
•
•
•
RNA Viruses
Retroviruses
Mutations are
common
No vaccines or new
ones have to be
made yearly.
Ex: influenza, HIV,
Ebola
Viruses are:
Not capable of reproduction
without a host
So, are viruses living things? Most
biologists say they are not…they are
on the border of non-living and living.
SPOOKY!
Lytic Cycle
• Infects cell with genetic
material
• Forces host cell to make
more virus.
• Uses host’s organelles
and materials
• Destroys host cell – “lyse”
https://www.youtube.com/watch?v=UjVcn5Mmz-U
Lysogenic Cycle
• Infects cell with genetic
material
• Prophage (viral DNA) is
inserted into host’s DNA
• Hides in host’s DNA until
activated.
• Once activated, continues
with Lytic cycle.
https://www.youtube.com/watch?v=_J9-xKitsd0
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???
– Because the drug is likely to be toxic to the host as well as the virus.
Expensive too.
Textbook Figure 19-15
Is there anything smaller than
a virus that is pathogenic?
Oh, YES!
VIROIDS and PRIONS
Prions
• Protein infectious
particles
• Diseases caused:
Mad Cow and others
• Changes good
structural proteins into
bad ones.
• Want to know more?
– Click here!