Microbial Models: Viruses and Bacteria AP Biology1/08/03

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Microbial Models Viruses and Bacteria AP
Biology 1/08/03
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Viruses and Bacteria
Discovery of viruses Science as a
process Tobacco Mosaic Virus Mayer
1883 Disease is contagious small maybe
bacteria Ivanowsky 1893 Filtered and found
pathogen to be very small bacteria or
toxin Beirjerinck 1897 Found it could reproduce
only in the host couldnt be cultivated not
inactivated by alcohol Stanley 1935 Crystalized
the infectious agent
Size comparison
size demonstration
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Basic structure of a virus
Bacterio- phages
Some form of nucleic acid (DNA or RNA) Enclosed
in a protein coat. (capsid)
Viral envelopes -membranes that cloak their
capsids. Often derived from host cell membrane.
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Viral life cycle
Viruses are obligate intracellular
parasites -they can reproduce only within a host
cell
Therefore, viruses are basically packages of
genetic material that move from host to host.
Something to ponder should viruses be
considered to be alive? (are computer viruses
or chain letters alive?)
Host range the potential hosts that a given
virus can infect. -Host specificity like a
lock-and-key system -depend on proteins on the
outside of the virus and the receptors on the
host cell
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The basics of viral reproduction
1) Entry into the host cell -injection -membrane
fusion 2) Replication and Translation of the
genetic material -using the host cells genetic
machinery 3) Assembly and release of the new
viral particles -lysis of host cell -budding
from the host cell
Symptoms from a viral infection -Host response
to the viral infection (immune response) -Prolifi
c cell death -Proteins produced by viral genetic
material (e.g. diptheria) -Cancer resulting from
disruption of cell growth control mechanisms
(oncogenes)
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A generalized viral reproduction cycle
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Lytic and Lysogenic viral cycles focusing on
phages
Lytic cycle reproductive cycle that results in
the death of the host cell as it breaks open
(lyses), releasing the new viral
particles. -lysis may be brought on by the
release of lysozyme, from the newly assemble
viral particles, that weakens the bacterial cell
wall.
Virulent viruses utilize this reproductive
cycle.
Lysogenic cycle replicates the viral genome
without destroying the host cell. Prophage
viral DNA that is incorporated into the genetic
material of the host cell.
Temperate viruses utilize both modes of
reproduction
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Lytic
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The lytic cycle

• Protection for the bacterial
• cells
• receptor variation
• restriction nucleases
• lysogenic evolution of
• viruses (?)

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Lytic and Lysogenic
Environmental trigger
Protein represses most of the other phage genome
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Lysogenic
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Animal viruses
Most vertebrate viruses have tissue
specificity - cold virus upper respiratory
tract - HIV lymphocytes -WNV brain tissue
Impact of the virus may depend on the type of
tissue and the possibility of cell renewal -
cold virus destroys epithelium that can be
repaired -poliovirus attacks nerve cells, and
therefore the damage is permanent.
Defenses
Vaccines Jenner and his faith in the milk
maid -Stimulate the immune system to set up
the defenses against the actual pathogen.
Disruption of the genetic translation
mechanisms - AZT for HIV
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Reproductive cycle of animal viruses
Viral envelopes Equipped with an outer membrane,
outside of the capsid. - lipid bilayer and
glycoproteins -often derived from the host
cell Helps the viral particle to enter the host
cell and also helps to disguise the viral
particles to limit recognition by the host
immune system.
Genetic material Animal viruses may contain DNA
or RNA Provirus DNA that is integrated into the
host cells DNA -Herpes RNA broad variety of
RNA genomes in animal viruses -mRNA
-retroviruses utilize reverse transcription
(e.g. HIV) RNA -gt DNA -gt Provirus -gt RNA
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Enveloped virus reproductive cycle
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HIV enveloped retrovirus
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Retrovirus
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Outbreaks Emerging viruses
Three main components to new viruses
appearing 1) Mutation of existing diseases -RNA
viruses have high mutation rates
lack proof-reading steps. -individuals may not
have immunity to new strains -flu virus 2)
Spread from a different host species -
hantavirus outbreak in 1993 spread from
rodents to humans -resulted from very high
populations of rodents 3) Dissemination of a
virus from an isolated population -international
travel and tourism -AIDS
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Bacteria
Short generation time facilitates evolutionary
adaptation. Binary fission -Asexual
reproduction -under optimal conditions E.
coli can reproduce every 20 minutes
Spontaneous mutation rate 1 107 per cell
division, therefore approx. 2000 mutations per
gene per day
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Genetic recombination in bacteria
Combining the DNA from two individuals into the
genome of a single individual.
After 24 hrs, the of cells that can synthesize
both Arg and Trp excedes the rate of
mutation. Must be recombination.
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Bacterial genomealteration
Transformation -alteration of bacterial DNA by
uptake of naked, foreign, DNA from the
surrounding environment.
Transduction -DNA transfer via
phages Generalized -random pieces of host DNA
gets transfered Specialized -prophage exits
chromosome and carried pieces of host DNA with it
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Plasmids
Small self-replicating DNA molecule
Can undergo reversible incorporation into the
cells Chromosomeplays an important role in
recombination
Episome genetic element that can replicate
either as a plasmid or as part of the bacterial
chromosome.
(lambda phage also an episomesimilarities
and differences between plasmids and episomes?)
F-plasmid R-plasmid fertility resistance
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Conjugation and plasmids
Hfr can mate with F- cell, sending DNA
fragment. Crossing over can then occur between
frament and the original DNA
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Transposons
-a piece of DNA that can move from one location
to another moves genes into new areas (target
sites)
Insertion sequence simplest transposon -one
gene that codes for transposase -transposase
recognizes the inverted repeates and cuts The DNA
at that site, and at the target site.
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Insertion sequence
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Composite transposons
Contain additional genes, such as those for
antibiotic resistance
May help bacteria adapt to new environments of
harsh conditions packaging these genes on an R
plasmid would be especially favorable.
(also found in eukaryotic cellschap 19)
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Control of gene expressionthe lac operon model