Microbial%20Genetics

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Microbial Genetics

• Genetic transfer and recombination

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Genetic recombination

• Genetic recombination through sexual reproduction
  is an important means of variation in eukaryotes
• Prokaryotes do not have an equivalent process of
  sexual reproduction
• However, prokaryotes do have mechanisms by which
  DNA can be transferred between strains of the
  same species, or even between different species
• Contributes to a populations genetic diversity

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Gene transfer

• Vertical gene transfer
• Occurs during reproduction. Transfer of genes
  from an organism to its offspring
• Horizontal gene transfer
• Transfer of genes from one organism to another
  within the same generation

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Horizontal gene transfer

• Involves one way transfer from a donor cell to a
  recipient cell
• A recipient cell that incorporates DNA from the
  donor is called a recombinant
• Genes are transferred naturally between bacteria
  by three mechanisms
• Transformation DNA is transferred as naked DNA
• Conjugation DNA is transferred between bacteria
  that are in contact with each other
• Transduction DNA is transferred by a bacterial
  virus (bacteriophage)

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Homologous recombination

• DNA introduced into bacteria usually does not
  have a mechanism to replicate itself
• It relies on integration into the genome of the
  host bacterium in order to survive and be passed
  on

Incoming DNA
gene A
gene C
gene Z
Host genome
gene A
gene C
gene B
Recombinant genome
gene A
gene C
gene Z
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Griffiths experiment, 1928Streptococcus
pneumoniae
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Naked DNA

• When a bacterial cell lyses, it releases its DNA
  into the environment

CENSORED!
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Transformation

• DNA is transferred as naked DNA
• DNA breaks into pieces on cell lysis
• DNA is taken up by the recipient cell
• A region of the recipient DNA is replaced by the
  donor DNA (recombination)
• Unrecombined DNA is degraded

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• Transformation occurs naturally in very few
  genera of bacteria
• Bacillus, Haemophilus, Neisseria, Acinetobacter,
  and some strains of Streptococcus and
  Staphylococcus.
• The recipient cell must be in a physiological
  state to take up DNA.
• Changes in the bacterial cell wall make it
  permeable to large DNA molecules
• Some bacteria, which are not normally can be made
  so in laboratory

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Conjugation

• DNA transfer between two bacteria that are in
  contact with one another
• Contact between donor and recipient cells is
  initiated by sex pili
• DNA is transfer through a conjugation bridge or
  open pore between donor and recipient cell
• Mediated by a plasmid, called an F-factor
  (fertility factor) or a conjugative plasmid

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Plasmids

• Small, circular molecules of DNA
• Replicate independently of the chromosome
• Usually dispensable for growth, but under some
  conditions provide a selective advantage such as
  antibiotic resistance or a unique metabolic
  pathway
• Conjugative plasmids carry genes for conjugation
  including sex pili

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Mechanism of conjugation

• Donor contacts recipient, attaches using sex
  pilus
• F-factor initiates transfer of a copy of itself
• Recipient is converted to a new donor cell

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Importance of plasmids

• Providing a selective advantage
• Some Pseudomonas sp. have plasmid encoded enzymes
  to degrade petroleum allowing them to live in
  fuel tanks or fuel spills
• Enhancing pathogenicity
• The E. coli strain causing infant or travelers
  diarrhea carries plasmids for toxin production
  and bacterial attachment.
• Antibiotic resistance
• Many antibiotic resistance genes are carried on
  plasmids which can be rapidly transferred to
  other bacteria, resulting in widespread
  resistance to antibiotics and strains that
  resistant to multiple antibiotics such as
  methicillin-resistant Staphylococcus aureus or
  Golden Staph

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Transduction

• Mediated by a bacterial virus (bacteriophage or
  phage)
• DNA from the donor is transferred to the
  recipient inside the phage particle
• Two types of transduction
• Generalized
• Specialized

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Generalized transduction

• Donor cell is infected with a phage
• Donor DNA is incorporated into the phage
  (transducing phage)
• The donor cell lyses
• The transducing phage infects the recipient cell
  and injects the donor DNA
• DNA integrated into the genome

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Transposons

• Segments of DNA that can move from one region of
  DNA to another and integrate through
  non-homologous recombination
• Contain information for their own transposition
• Transposase enzyme for cutting and resealing DNA
• Short terminal repeats which the transposase
  recognizes as recombination sites

• Insertion sequences are the simplest transposons
• Complex transposons carry other genes e.g.,
  antibiotic resistance genes

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Natural history of a transposon
chromosome
transposon
plasmid
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Genomes of bacteria are elastic

• Bacterial genomes are often receiving genetic
  information from other bacteria through genetic
  transfer and recombination
• In the same way that mutations can be beneficial,
  neutral or harmful, so is the recombination of
  incoming DNA
• New gene combinations are maintained if they
  provide the organism with a selective advantage