Download Viruses

Chapter 18.2
Pages 525 - 531
How were Viruses Discovered?
• Late 1800’s - bacteria known to causes disease
• Scientists found tobacco plants were becoming
diseased.
• Scientists collected samples of the diseased
plant and filtered it through pores small
enough to catch bacteria.
• Observed that the filtered fluid still caused
disease in tobacco plants
• Scientists infer the disease is not caused by
bacteria, but by something small enough to
get through the filter.
• This pathogen later named the “Tobacco
Mosaic Virus” (TMV)
A tobacco plant suffering from
“Tobacco Mosaic Virus”
Is a Virus Alive?
• Viruses are NOT considered living
because they do not display the 8
characteristics of life:
– They do not grow
– They cannot reproduce on their own
– They don’t carry out metabolic activities
– They do not maintain homeostasis
• Though not alive, viruses have a major
impact on the world.
Viral Structure
• Nucleic Acids
– Can be DNA or RNA
– Code for components of the virus
• Capsid
– Protein coat that houses genetic material
• Envelope
– Gives a virus its overall spherical shape which can
help a virus enter a cell
• Tail Fibers
– Act like a syringe which inject viral DNA into host
• All viruses have nucleic acid and a capsid. Only
some viruses have an envelope and tail fibers.
Bacteriophage
• A virus that infects bacteria
Bacterial Cell
Protein
Coat
(Capsid)
Tail
Fibers
Viral
DNA
Viral Replication
• Viral infection begins when a virus’ genetic material
enters a host cell.
• Then it can either enter the lytic or lysogenic cycle
• Lytic Cycle
– Viral infection, reproduction, and cell destruction
– Virus uses host cells “stuff” to make more of itself
– Cell breaks open, and new viruses are released to infect
more cells.
• Lysogenic Cycle (“gentle”)
– When viral DNA becomes part of its host
cell’s DNA (prophage)
– Viral DNA replicates with host cell’s DNA
– Under stressful situations, virus may enter lytic cycle
Animation for Lytic and
Lysogenic Cycles
• http://highered.mcgrawhill.com/sites/0072995246/student_view0/
chapter8/lambda_phage_replication_cycle
.html
Viruses and Humans
• Viruses cause most common respiratory illnesses,
such as colds and coughs.
• They can cause a variety of infections from diarrhea
to meningitis.
• It may take up to 2-3 weeks for your body to resolve a
viral infection completely.
• Antibiotics DO NOT work to
fight viruses!
• Viral infection can be
prevented by vaccination.
How was Vaccination Discovered?
• Edward Jenner noticed that the milk-maids who
contracted cow pox (not so serious) didn’t contract
small pox (very serious!)
• He inoculated a healthy boy with cow pox (don’t try
this at home!) to test his hypothesis that getting
cow pox can prevent getting small pox.
• HE WAS RIGHT! (phew!)
How does Vaccination Work?
• Vaccination is the process of using one virus to
immunize against another virus.
• Vaccines are usually a very mild dose of the
disease-causing virus which
has been weakened
or killed.
• These vaccines
stimulate the creation
of antibodies in the
body that recognize
and attack a particular
infection.