http://www.cellsalive.com/howbig.htm
Ch 18
Microbial
Models:
Genetics of
Viruses and
Bacteria
Virus-
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*biological particle (not a cell)
*composed of nucleic acid
(RNA or DNA)
Virus-
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*surrounded by capsid
(protein coat)
and sometimes a
envelope
(membrane surrounding)
viral
The head of a dress-maker's
pin can provide seating
accommodation for five
hundred million rhinoviruses
(cause of the common cold)
!
Why not alive?
•Lacks: nucleus, cytoplasm,
organelles, cell membrane
•Why not alive?
•No reproduction by mitosis
No meiosis 
(require a host cell
in order to
reproduce)
obligate intracellular
parasites
•Why not alive?
•Resistant to alcohol 
resistant to dehydration
by osmosis
obligate intracellular
parasites
•Why not alive?
•virus crystallization 
cells cannot form
crystals
obligate intracellular
parasites
Lytic Cycle
Lytic Cycle
“lock & key”
Lytic Cycle
phage
injects DNA
(hopefully, for the virus, it
is resistant to the
restriction nucleases!)
Lytic Cycle
phage DNA
hydrolysises
bacterial
DNA via.
enzymes
Lytic Cycle
phage
proteins are
produced,
phage DNA
reproduced
Lytic Cycle
host cell
provides:
nucleotides,
ribosomes,
tRNA etc.
Lytic Cycle
Lysozyme
produced
and
bacterial
wall is
digested
Lytic Cycle
Lytic Cycle
Lysogenic
Cycle
Lysogenic
Cycle
temperate
viruses
Lysogenic
Cycle
“phage”
inserts DNA;
via. “lock &
key”
Lysogenic
Cycle

encorporate
into bacterial
DNA via.
crossing over
Lysogenic
Cycle
cellular
division
w/ a
“friend” replicates
along with
Lysogenic
Cycle
at some point,
 exits the
chromosome,
& initiates a
LYTIC cycle
Lysogenic
Cycle
Phages
manufactured
virulent
latent/
temperate
Why haven’t phages
exterminated all bacteria?
• Restriction Enzymes
in bacteria
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– Methylation of
bacterial DNA prevents
the restriction enzyme
from attacking its own
self
Why haven’t phages
exterminated all bacteria?
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• Natural Selection
favors the mutant
bacteria w/ protein
receptors that have an
altered shape
– no longer recognizable
by phage
Why haven’t phages
exterminated all bacteria?
• LYSOGENY
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Viruses are Classified
as:
1. DNA viruses
2. RNA viruses
Viruses are Classified
as:
1. membranous
envelope present
2. membranous
envelope absent
ex. of enveloped = herpes virus
Reproduce in nucleus
Envelope derived from
nuclear membrane
Typically, leave some DNA
behind in nerve cells
ex. Herpes Simplex Type 1
(cold sore)
Herpes Viruses
• Use nuclear membrane to
derive new envelope.
• Viral DNA integrated into
Host DNA as a provirus.
• Shows both lytic and
lysogenic life cycles.





 






RETROVIRUS ex. HIV
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human immunodificiency




`
host is a WBC called
a T-cell
http://www.youtube.com/watch?v=B7ITZgag6w0&feature=
related
http://www.youtube.com/watch?v=dyxujBKHdWc
http://www.youtube.com/watch?v=9leO28ydyfU&NR=1
HIV needs to connect to TWO protein types
on the outside of the Immune System’s
Helper T Cells
HIV needs to connect to TWO protein types
on the outside of the Immune System’s
Helper T Cells (WBC)
LAST KIND
• DNA
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DNA Viruses
1.Insert their DNA into host/ the host
produces mRNA compliment 
2.Production of new viral proteins, or
join with the DNA of the host cell 
3.Directs production of new viruses

Viral
Diseases
Measles
Polio
 Smallpox
 Influenza
 S.A.R.S.
 Bird flu

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“The single biggest threat to man's
continued dominance on the planet is
the virus!”
-Joshua Lederberg, Ph.D, Nobel laureate
1969
(won prize with Beadle and Tatum)
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Actually, he won the prize for discovering
that bacteria can exchange genes… but what
a great quote!
EMERGING VIRUSES:
HOW? HOW?HOW? HOW?
Mutations of existing virus
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Mutated form
spread from
animals
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How to combate
Virus?
VACCINES - *variants of pathogen
*stimulates the immune
system to “arm” and
“defend”
Note: Can prevent
some viral illnesses,
Cannot cure the viral
infection
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Some viruses
cause
Human papilloma virus
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Viral nucleic acid
becomes
integrated into the
hosts DNA
Proto-oncogenes
code for proteins
that affect the cell
cycle.
•Viroid“naked” RNA
*disrupt
metabolism of
plants
stunts
growth
•ViroidBIG NEWS b/c
*molecules* can
be an infectious
agent and spread
disease!
•Prions- infectious
*present in brain cells
*misfolded proteins
*converts “regular” proteins to
prion version
Mad cow
disease
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