Microbial Models: The Genetics of Viruses and Bacteria

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Microbial Models The Genetics of Viruses and
Bacteria

• A virus is an infectious particle and
• the simplest forms of life are bacteria.

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Outline

• Outline
• 1. Key Concepts
• 2. The Structure of Viruses
• 3. Viral Reproduction
• 4. Infectious Agents Tinier Than Viruses
• 5. Key terms
• 6. Conclusions
• _____________________________________
• 7. Genetic recombination of Bacteria
• 8. Key Terms
• 9. Conclusions

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Genetic recombination of Bacteria

• The short generation span of bacteria facilitates
  their evolutionary adaptation to changing
  environment
• Circular DNA, Plasmid, one replication origin,
  binary fission (produce clones), mutations
• 2. Three natural processes of genetic
  recombination in bacteria
• a. Transformation
• b. Transduction
• c. Conjugation and plasmids

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How Small Are Bacteria?
Magnified 14,000 times
Bacillus cells on the tip of a pin
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Where is the bacterium?
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Replication of the bacterial chromosome
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E. Coli Dividing
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Bacteria are very adaptable

• The major component of the bacterial genome is
  one double-stranded, circular DNA molecule.
• For E. coli, 100 times more DNA than in a typical
  virus and 1,000 times less than in a typical
  eukaryote cell.
• In addition, many bacteria have plasmids, much
  smaller circles of DNA.
• Each plasmid has only a small number of genes,
  from just a few to several dozen.

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Bacteria are very adaptable

• Bacteria proliferate very rapidly in a favorable
  natural or laboratory environment.
• In the human colon, E. coli reproduces about 2 x
  1010 bacteria lost each day in feces.
• Through binary fission, most of the bacteria in a
  colony are genetically identical to the parent
  cell.
• However, the spontaneous mutation rate of E.
  coliis 1 x 10-7 mutations per gene per cell
  division.
• This will produce about 2,000 bacteria in the
  human colon that have a mutation in that gene per
  day.

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Genetic recombination produces new bacterial
strains

• In addition to mutations, genetic recombination
  generates diversity within bacterial populations.
• Here, recombination is defined as the combining
  of DNA from two individuals into a single genome.
• Recombination occurs through three processes
  transformation transduction
  conjugation

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Detecting genetic recombination in bacteria
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Transformation

• Transformation Process of genetic transfer
  during which a bacterial cell assimilates foreign
  DNA from the surroundings
• Take up naked DNA from the surroundings
• Assimilate foreign DNA
• Progeny will carry a new combination of genes
• E. coli can be artificially induced to take up
  foreign DNA by incubating
• This technique is used by Biotechnology industry
  to introduce foreign genes into bacterial genomes

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Transduction

• Transduction Gene transfer from one bacterium
  to another by a bacteriophage
• Generalized transduction occurs when random
  pieces of host cell DNA are packaged within a
  phage capsid during the lytic cycle of a phage
• This process can transfer almost any host gene
• When the phage infects a new cell, the donor cell
  DNA can recombine with the recipient cell DNA
• Specialized transduction (restricted
  transduction)
• Transduced bacterial genes are restricted to
  specific genes adjacent to the prophage insertion
  site

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Transductions
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Conjugation and plasmids

• Conjugation The direct transfer of genes between
  two cells that are temporarily jointed
• Plasmid A small, circular, double-stranded,
  self-replicating molecule ring of DNA in some
  bacteria
• Transposon (Transposable Genetic Element) DNA
  sequence that can move from one chromosomal site
  to another

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Bacterial mating
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Plasmids
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Conjugation and recombination in E. coli
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 Conjugation and recombination in E. coli
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Insertion sequences, the simplest transposons
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Insertion of a transposon and creation of direct
repeats
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Key Terms
Transformation Transduction Conjugation Plasmid Tr
ansposon Generalized transduction Specialized
transduction
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In Conclusion

• 1. When bacteria divide, they form clones of
  identical cells.
• 2. A bacterium can transfer its genes to another
  bacterium by transformation, transduction, or
  conjugation.
• 3. Plasmids are small bacterial DNA that are
  independent of the main chromosome.
• 4. Transposons are movable stretches of DNA that
  can jump from one place to another.