Bacterial Genetics (Ch18) Genetic diversity caused by Rapid

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Bacterial Genetics (Ch18)
Genetic diversity caused by Rapid
reproduction Mutation Recombination

• Bacteria
• One of the simplest genetic model systems to
  study the mechanisms of molecular genetics
• Escherichia coli (E. coli)
• Intestinal flora
• lab rat of molecular biology

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Replication of bacterial genome

• One circular DNA genome
• Single origin of replication (Ori)
• Bidirectional DNA replication
• May have plasmids
• smaller circular DNA molecules
• Autonomously replicated (contain ori)
• Bacteria divide by binary fission
• asexual reproduction
• Progeny are genetically identical to parent
  (clones)

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LE 18-14
Bacterial DNA replication
Replication fork
Origin of replication
Both genome plasmid replicate in this manner
Termination of replication
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Bacterial DNA Mutation

• Caused spontaneously (mistakes in DNA synthesis)
• Physicochemical forces (UV, X rays, chemical
  mutagens, etc.)

• Since reproduction is quick e.g.doubling time20
  min
• New mutations spread quickly

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Calculation of incidence of mutations
If doubling time 20 min, then 23cells/hr
Over 12 hr, 236 cells (1010)produced from a
single cell
If spontaneous mutation rate 1 x 10-7 / gene,
then in 12 hr (day) (1010) (10-7) 103
mutations/gene/day
If bacteria have 4000 genes then
(4x103genes)(103) 4 x 109 mutations/day
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Take home message bacterial mutations rare per
gene But due to rapid cell division, become
frequent
Major contribution to genetic diversity and
ability to adapt
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Another source of bacterial genetic
diversityGenetic Recombination

• Three processes bring bacterial DNA from
  different individuals together
• Transduction
• Transformation
• Conjugation

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Transduction

• Bacteriophages (bacterial viruses) transfer
  bacterial genes from one host cell to another

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LE 18-16
Phage DNA
A
B
A
B
Donor cell
A
Crossing over
A
A-
B
Recipient cell
A
B
Recombinant cell
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Transformation

• Alteration of a bacterial cells genotype and
  phenotype
• by the uptake of naked, foreign DNA from the
  surrounding environment

• For example, harmless Streptococcus pneumoniae
  bacteria can be transformed to pneumonia-causing
  cells
• Uptake of ampicillin resistant/ GFP-carrying
  plasmid (done in lab) (pGLO)

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Conjugation

• Direct transfer of DNA between live bacterial
  cells that are temporarily joined
• Transfer one-way
• Male donates DNA
• Female receives DNA

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• Maleness,
• Contains F (fertility) genes on plasmid or in
  genome
• Encode sex pilus
• Forms passage way for DNA from donor to recipient

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LE 18-17
Sex pilus
5 µm
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• Donor cells containing the F plasmid F
• Recipient cells F-
• Cells with F factor integrated into genome Hfr
  cell
• (high frequency of recombination)
• Hfr cells
• Transfer some genomic DNA to recipient cell

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LE 18-18_4
F plasmid
Bacterial chromosome
Fcell
F cell
Mating bridge
F cell
F cell
Bacterial chromosome
Conjugation and transfer of F plasmid from and
F donor to an F recipient
Formation of Hfr (high frequency of
recombination) cell
Hfr cell
F cell
F factor
Hfr cell
F cell
Temporary partial diploid
Recombinant F bacterium
Conjugation and transfer of part of the
bacterial chromosome from an Hfr donor to an F
recipient, resulting in recombiination
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Questions???