Download Chapter 8 The Genetics of Bacteria and Their Viruses

Chapter 8
The Genetics of Bacteria and
Their Viruses
Multi-Drug-Resistant Bacteria (MDRB)
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Chapter Outline
Viruses and Bacteria in Genetics
The Genetics of Viruses and Bacteria
Mechanisms of Genetic Exchange in
Bacteria
The Evolutionary Significance of
Genetic Exchange in Bacteria
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Bacteria and Viruses in
Genetics
Small size (~2 um)
Rapid reproduction (~ hrs)
Selective media (e.g., antibiotics)
Relative Simple structures and
physiology
Complete genome sequences
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The Genetics of Viruses
Viruses (DNA-RNA) can only reproduce by
infecting living host cells.
Bacteriophages are viruses that infect
bacteria.
Several important genetic concepts have
been discovered through studies of
bacteriophages. © John Wiley & Sons, Inc.
Bacteriophage T4
Double-stranded linear DNA genome
Protein head
Genome contains 168,800 base pairs and 150
characterized genes
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Lytic phage
T4 DNAse:
(Specific)
5-hydroxymethylcytosine
(HMC)
Lysosyme:
(Specific)
Cell wall
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Bacteriophage
 Double-stranded linear DNA genome
 Genome contains, 48,502 base pairs and about 50
genes
 May be lytic or lysogenic (inserted in the bacterial
chromosome)
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Differential genes expression
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Recombination process:
---site-specific
attP (virus) and attB (bacteria)
--- int gene (integrase)
--- GCTTTTTTATACTAA
--- CGAAAAAATATGATT
--homologous recombination
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The Genetics of Bacteria
Bacteria contain genes that mutate to
produce altered phenotypes.
S
Energy (Lac / lac); Synthesis Trp / trp; Resistant Amp / Tet
r
Gene transfer in bacteria is unidirectional—
from donor cells to recipient cells.
Monopliod to monoploid?
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Bacteria
One main chromosome with a few thousand
genes.
Variable number of plasmids and episomes.
Asexual reproduction by simple fission.
(Para) sexual transformation of DNA.
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Phenotypes in Bacteria
Colony =one bacteria.
Colony color and morphology.
Nutritional mutants for energy sources.
(any sugars-galactose-Lac+ and Lac-)
Prototrophs-produce any metabolitesand auxotrophs- do note produce
specific metabolites.
Antibiotic resistance (Ampr, Tetr, Purr).
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Bacterial colonies:
Serratia marcencens
P. aeruginosa.
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Recombination in Bacteria
--Small fragment to larger fragment (chromosome)
--Monoploid (to partial diploid)
-Donor to recipient cell
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Mechanisms of Genetic
Exchange in Bacteria
Bacteria exchange genetic material
through three different (para)sexual
processes.
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Bacteria exchange genetic material through three
different (para)sexual processes.
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The U-tube Experiment
Transformation
Conjugation
Transduction
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TRANSFORMATION
Streptococcus pneumoniae---Phenotypes
Polysaccharide capsule (S, type I,II, III, IV, V)
Type II, agglutination
Non- Polysaccharide capsule (R)
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Transformation
Type IIIS
to
type IIR
Transfer DNA
Hydrophilic
Non-membrane permeable
Transporter
(competence-Com-protein)
Competent bacteria
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Transformation in Bacillus subtilis
A heteroduplex is a double-stranded (duplex) molecule of DNA originated by recombination of
single complementary strands derived from exogenous sources.
Conjugation in E. coli
F+
DNA is transferred from a donor to acceptor cell,
F: Fertility factor ; F© pili
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The F Factor in E. coli
F+ factor: --Autonomous and integrated states
F’ factor: --Similar to F+ but with bacterial genes
Hfr: high frequency recombination
F: Fertility factor
Formation of Hfr Cells
seq A -seq B
seq A -seq B
seq A -seq B
seq A -seq B
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Conjugation
MODEL
Replication: Rolling-cycle
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Selected medium:
-Thr, -Leu,
+Streptomycin (str)
azi plus and minus
ton plus and minus
lac plus and minus
gal plus and minus
Interrupted
Mating
Experiments
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Azir and tonr
Azis and tons
Hfr
F-
strs
gal+
lac+
leu+
thr+
azir
tonr
strr
gallacleuthrazis
tons
Plasmids
A plasmid is a genetic element that can
replicate independently of the main
chromosome in an extrachromosomal state.
Most plasmids are not required for the
survival of the host cell.
Plasmids in E. coli
– F Factor (Fertility Factor)
– R Plasmids (Resistance Plasmids)
– Col Plasmids (synthesize compounds that kill
sensitive cells)
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Episomes
An episome is a genetic element that
is not essential to the host and that
can either replicate autonomously or
be integrated into the bacterial
chromosome.
Integration depends on the presence of
IS elements.
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Formation of an F’
Factor
Homologous DNA recombination,
but some bacterial DNA is
translocated to the plasmid.
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Transduction
 In transduction, a bacteriophage transfers DNA from
a donor cell to a recipient cell.
 In generalized transduction, a random fragment of
bacterial DNA is packaged in the page head in place
of the phage DNA (normal Excision).
 In specialized transduction, recombination between
the phage chromosome and the host chromosome
produces a phage chromosome containing a piece of
bacterial DNA (abnormal Excision).
Auxotrophic Salmonella typhimurium:
tyr, phe, trp and met, his.
Infected with P22 bacteriophage
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Normal Excision of  Prophage
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Anomalous Excision of  Prophage
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Specialized Transduction
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Three parasexual processes—transformation,
conjugation, and transduction—occur in
bacteria.
These processes can be distinguished by two
criteria: whether the gene transfer is inhibited
by deoxyribonuclease (DNAase) and
whether it requires cell contact.
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The Evolutionary Significance
of Genetic Exchange in
Bacteria
Genetic exchange is as important
in bacteria as it is in other
organisms.
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Genetic Exchange in Bacteria
Mutation is the source of new genetic variation.
(UV radiation)
Recombination produces new combinations of allele.
(normal and abnormal recombination)
Transformation, conjugation, and transduction
generate new combinations of genes in bacteria to
allow bacteria to adapt to new environments.
(soil, sewers and polluted waters)
(animal/plants)
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