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Chapter 20- Viruses
Big Idea: Are all microbes that make us
sick made of living cells?
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20.1 Viruses
How do viruses reproduce?
Viruses are not considered “alive”,
and are made up of proteins, nucleic
acids, and sometimes lipids.
Viruses can reproduce only by
infecting living cells
Viruses vary widely in structure and
size.
All viruses have surrounding coat
made of protein called a capsid.
-Viruses can have a few to
hundreds of genes depending
on their complexity
Viruses attach to host and insert
their genetic material
Most viruses only infect a specific type of
cell (plant viruses infect plant cells, animal
viruses infect certain animal cells.
Bacteriophages- viruses that infect
bacterial cells
Inside living cells, viruses use their
genetic information to make multiple
copies of themselves. Some do this
immediately, while others wait.
Lytic infection- is when a virus enters a
bacterial cell and causes it to rupture, or
lyse. This releases hundreds of virus
particles.
Lysogenic infection is when a host cell is
not immediately taken over, but all
daughter cells produced will have the
virus.
A prophage is when a bacteriophage’s
DNA becomes imbedded in the bacterial
host’s DNA.
About 70% of viruses infect RNA instead of
DNA. These viruses range from the
common cold to HIV.
HIV is a sample of a retrovirus- and RNA
virus copies RNA into DNA
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Viruses vs. Cells
Viruses
 Need a host to survive
 Do
not grow/develop
 Do
not respond to
environment
 Do
not obtain/use
energy
 DO
evolve over time
Cells
 Independent cell
division
 Grow/
develop
 Obtain/use
 Respond
 Change
energy
to environment
over time
+ 20.3 Diseases Caused by Bacteria
and Viruses
Pathogens are disease-causing agents.
Bacteria can cause disease by
destroying living cells or by releasing
chemicals that upset homeostasis
Ex that damages host- tuberculosis
Ex of releasing toxins- diptheria
Physical removal
Disinfectants
Food Storage
Food processing
Sterilization by heat
Vaccines are weakened or killed preparations
of pathogens or inactivated toxins.
Your body has it’s own natural anti-bodies to
fight off diseases and infections, but antibodies
are also sold as a drug to attack bacteria.
- Antibodies block the growth and
reproduction of bacteria
Viruses cause disease by directly
destroying living cells or by
affecting cellular processes in
ways that upset homeostasis
Virus and bacterial infection
prevention= good hygiene, vaccination,
limiting contact with sick individuals
Unlike bacterial infections, viruses
cannot be treated with antibiotics.
Some anti-viral medications are out
there and they help speed up recovery
Emerging diseases are diseases that appear
in the population for the first time, or are old
diseases that suddenly becomes harder to
control
Emerging diseases are particularly
threatening b/c human populations have
little to no resistance to them yet.
Over time viruses mutate and evolve, which
means new antibiotics and vaccines must be
developed
Prions are small proteinparticles that can cause disease.
Prions can mis-fold proteins, which
can damage cell function