chapter 8 the genetics of bacteria and their viruses © john wiley & sons, inc....
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Chapter 8The Genetics of Bacteria and
Their Viruses
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Multi-Drug-Resistant Bacteria (MDRB)
Chapter Outline
Viruses and Bacteria in GeneticsThe Genetics of Viruses and BacteriaMechanisms of Genetic Exchange in
BacteriaThe Evolutionary Significance of
Genetic Exchange in Bacteria
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Small size (~2 um)Rapid reproduction (~ hrs)Selective media (e.g., antibiotics)Relative Simple structures and
physiologyComplete genome sequences
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Bacteria and Viruses in Genetics
The Genetics of Viruses
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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.
Bacteriophage T4
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Double-stranded linear DNA genomeProtein headGenome contains 168,800 base pairs and 150
characterized genesLytic phage
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T4 DNAse:(Specific)5-hydroxymethylcytosine(HMC)
Lysosyme:(Specific)Cell wall
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-specificattP (virus) and attB (bacteria)
--- int gene (integrase)
--- GCTTTTTTATACTAA--- CGAAAAAATATGATT
--homologous recombination
The Genetics of Bacteria
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Bacteria contain genes that mutate to produce altered phenotypes.
Gene transfer in bacteria is unidirectional—
from donor cells to recipient cells.
Energy (Lac / lac); Synthesis Trp / trp; Resistant Amp / TetS r
Monopliod to monoploid?
BacteriaOne 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 BacteriaColony =one bacteria.Colony color and morphology.Nutritional mutants for energy sources.
(any sugars-galactose-Lac+ and Lac-)Prototrophs-produce any metabolites-
and 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
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--Small fragment to larger fragment (chromosome)
--Monoploid (to partial diploid)
-Donor to recipient cell
Mechanisms of Genetic Exchange in Bacteria
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Bacteria exchange genetic material through three different (para)sexual
processes.
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Bacteria exchange genetic material through three different (para)sexual processes.
The U-tube Experiment
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Transformation
Conjugation
Transduction
TRANSFORMATIONStreptococcus pneumoniae---Phenotypes
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Polysaccharide capsule (S, type I,II, III, IV, V)Type II, agglutination
Non- Polysaccharide capsule (R)
Transformation
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Type IIIS to type IIR
Transfer DNA
Hydrophilic
Non-membrane permeable
Transporter (competence-Com-protein)
Competent bacteria
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
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DNA is transferred from a donor to acceptor cell,
F: Fertility factor ; F pili
F+
The F Factor in E. coli
F: Fertility factor
F+ factor: --Autonomous and integrated states
Hfr: high frequency recombination
F’ factor: --Similar to F+ but with bacterial genes
Formation of Hfr Cells
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seq A -seq B
seq A -seq B
seq A -seq B
seq A -seq B
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Conjugation
MODELReplication: Rolling-cycle
strr
gal-
lac-
leu-
thr-
azir
tonr
strs
gal+
lac+
leu+
thr+
azis
tons
F-Hfr
Selected medium:-Thr, -Leu,+Streptomycin (str)
Interrupted Mating
Experiments
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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
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Homologous DNA recombination, but some bacterial DNA is translocated to the plasmid.
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 phage 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).
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Auxotrophic Salmonella typhimurium:
tyr, phe, trp and met, his.
Infected with P22 bacteriophage
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
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Genetic exchange is as important in bacteria as it is in other
organisms.
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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Bacteria genetically engineer plants with the Ti plasmid to control their differentiation (tumorigenic- Crown gall disease).