Download 3-3 Viruses

Name ____________________________ Date ______________ Period ________
Biology
Unit 3 – Infectious Disease, Background Paper 3-3
Viruses
From the common
cold to AIDS and Ebola,
viruses affect every living
thing on earth. Even plants are
not immune to viral infections.
The ironic thing is that viruses
themselves are not alive. They
do not belong to any of the six
kingdoms of living organisms
(Eubacteria, Archaebacteria,
Protists, Fungi, Plants, &
Virus Capsid
Animals). They fall short of
the ‘living’ designation since they do not metabolize energy, they
Viruses attacking bacteria
cannot reproduce on their own, and they do not grow. They consist
simply of a strand of genetic information – either DNA or RNA – surrounded by a shell of protein called the
capsid1.
Although they can do few of the things we usually use to typify living organisms they are masters at
one thing. They can hijack the mechanisms of living organisms to make viral copies of themselves. Viruses
have evolved two different systems of cell invasion and hijacking.
The first and fastest method is called the lytic cycle. You catch a cold and the virus injects its
own DNA into your cell where the instructions encoded on that strand of DNA tell your cell to stop
functioning normally and divert all future energy to the production and assembly of new viruses.
Once your cell is packed with these viral copies it splits open and spills millions of viruses out to
infect other cells. These cells then become mini factories for more viruses. The cycle continues until
either your body’s defense systems identify and hunt down the viruses or you die.
The second method, the lysogenic cycle, is slower but very effective. The virus injects its
genetic code into your cell. Rather than take over your cells machinery the DNA splices itself into
your DNA and remains inactive. Every time your infected cell makes a copy of itself it also makes a
copy of the viral DNA. At some point in the future the viral DNA becomes active and ‘breaks out’
into the lytic cycle of infection described above. Instead of just a few cells producing viruses your
body now has millions of cells with copies of the virus DNA ready to become virus factories.
On the back of this reading draw the steps of infection of both the lyic and lysogenic cycles. Your best source
will be a search for images on the internet. When the reading is returned cut out and paste the drawings into
your journal. I found a good one at http://www.bio.miami.edu/~cmallery/150/gene/18x5.jpg
1
Images CC BY Graham Colm
Background 3-3,Disease2012
Text 2012
Greg Ballog
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Name ____________________________ Date ______________ Period ________
Title of Reading: 3-3 Viruses
Points Earned
1 2 3 4 5 6 7
Virus Life Cycles (3 points)
A) What one question do I have about this article?
1 Point
A&B
B) Show or describe the reading to a parent or guardian (Name ___________________).
What comments or questions did they have concerning the reading?
(Biology Homework Time __________)
Background 3-3,Disease2012
Text 2012
Greg Ballog
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Answer
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