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Viruses
pp. 104 to 107
Viruses vs. Bacteria: What’s the
difference?
• Bacteria  unicellular organisms
• Virus  protein-covered genetic material
(DNA) that has NO CELLS, but invades
other living cells
• True or false? “Viruses are living things.”
FALSE!!!
What are Viruses?
• Not a living organism!
• Complicated assembly of molecules:
–
–
–
–
Proteins (protective)
DNA or RNA
Lipids
Carbohydrates
• No cells or organelles
• Dormant outside a living cell
(can’t do anything, including
reproduce)
• Once it invades a living cell, it
can reproduce
Viral Diversity
• Viral specificity: viruses can only invade certain
host cells (based on DNA, proteins, shape)
– Example of viruses: tobacco mosaic virus (tobacco
plant), rabies (mammals), swine flu (pigs &
humans), HIV (immune system)
– Ex. bacteriophage viruses can only infect bacteria
Viral Diversity:
Hosts: Plants
Animals
Animals
Bacteria
Viral Reproduction
• All viruses enter host
cells, but some are not
pathogenic (infectious) –
they do not kill their host
or make them “sick”
• There are two processes
viruses can use to make
more copies of
themselves:
1. Lytic Cycle
2. Lysogenic Cycle
Lytic Cycle
1. Attachment (entrance)  virus
chemically recognizes host cell and
attaches to it. Whole virus or only
DNA enters cell cytoplasm. It
integrates itself into host cell DNA.
2. Synthesis  virus DNA (or RNA)
hijacks host cell and programs it to
replicate viral components (nucleic
acids, enzymes, capsid proteins,
etc)
3. Assembly  all components of
virus (nucleic acids, enzymes,
proteins) come together to create
new virus particles
4. Release  new virus particles
released from the infected cell to
find other cells. Host cell dies
(lysis).
Lysogenic Cyle
1. Attachment (entrance)  virus chemically
recognizes host cell and attaches to it. Whole virus
or only DNA enters cell cytoplasm. It integrates
itself into host cell DNA.
2. Reproduction  Viral DNA is replicated along with
host DNA during normal mitosis processes
(*Remember: mitosis is how cells reproduce*)
3. Cytokinesis daughter cells
of mitosis have virus DNA
integrated with their own
Virus will often be in a dormant state
(lysogeny) until it becomes activated by a
stimulant (eg. Change in pH, temperature,
food supply, etc) –” triggers the lytic cycle.
Lytic Vs. Lysogenic Cycle
• Lytic cycle:
– take as little as 25 -40 mins
– produces up to 300 new virus
particles, but kills host cell
• Lysogenic cycle:
– host is not killed
– many generations of virus
can be produced without
harm to the host
– Virus is dormant until
stimulant triggers activation
of virus. It will then switch
over to the lytic cycle.
Examples of Viruses
DNA viruses
• Chicken pox
• Mono
• Small pox
RNA viruses
• Rabies
• Measles
• Mumps
• The Flu
• AIDS
• Common cold
• Some cancers
Vaccines:
Vaccines “train” our bodies to defend against
viruses. By injecting a small amount of virus (dead or
weakened) into our bodies, our immune system
learns how to fight it off by building antibodies
against it. It builds up an army of WBC’s & antibodies
to kill the living virus immediately before it can
reproduce. In the future, exposure to that same
virus won’t wreak havoc on our bodies and make us
sick because antibodies will already be available to
fight it off.
This is also why some people feel a little sick after
getting their annual flu shot.
• Symptoms appear as cells
are destroyed (lytic cycle)
• Most viruses hard to treat since antibiotics (inhibits growth of
microorganisms) cannot kill them.