Download Gene exchange

Sharing of genetic information
Bacteria are successful because
1. They carefully regulate their use of energy in metabolic
processes by shutting down unneeded pathways at the
biochemical and genetic levels.
2. They share genetic information with other bacteria,
increasing their ability to adapt to their environment.
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Genetic Recombination
• Bacteria are asexual
– With sexual reproduction, multiplication and gene
recombination are linked.
– In bacteria, they are separate
– Bacteria acquire new DNA from mutation, phage
infection, and transfer from other bacteria
• Bacterial genotypes are somewhat fluid
– Due to the ease of gene transfer, many genes can be
widely distributed among many bacteria
– One multicomponent organism?
2
Gene transfer
• Ways that bacteria can acquire new genetic info
– Transformation
• Taking up of “naked DNA” from solution
– Transduction
• Transfer of DNA one to cell to another by a virus
– Conjugation
• “Mating”: transfer of DNA from one bacterium to
another by direct contact.
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Transformation
Both G+ and Gram –
bacteria can take up DNA.
Cells in a state in which
they can take up DNA are
referred to as competent.
DNA may be actively
released by some cells, http://openwetware.org/images/0/0c/Competence2.jpg
suggesting that DNA
exchange is “intentional”.
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Transformation requires homologous
recombination
New DNA must be similar.
Successful transformation requires that donor
and recipient be related.
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Transduction: the vector is a virus
• New DNA brought by a bacteriophage
– Requires donor and recipient be related
• Generalized transduction
– Chopped up bacterial DNA is incorporated into a capsid
– Phage binds to new host bacterium, but injects donor
bacterial DNA instead
• Specialized transduction
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–
–
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In lysogeny, virus DNA inserts.
If DNA excises, can take adjacent bacterial DNA with it
Upon infection of new host, new bacterial DNA added
Important in bacterial evolution
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Gene transfer between bacteria-2
• Transduction: transfer of
DNA via a virus.
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Conjugation
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• A plasmid that can be
spread by conjugation
codes for a sex pilus.
• Pilus attaches donor to
recipient cell.
– Shortening of pilus draws
bacteria into contact
– Channel opens thru cell
wall of both bacteria
• Copy of plasmid sent.
http://parts.mit.edu/igem07/images/1/16/BU_conjugation.jpg
Conjugation and gene transfer
• Sex pili bind specifically to surface
molecules on bacteria
– But many bacteria have similar
molecules and can participate.
• Homologous recombination is
not needed
– Plasmids remain in cytoplasm
– Once the plasmid has been copied and sent, both donor
and recipient bacteria have the genes. Recipient can now
be a donor.
– Several types of plasmids, incl R, can be widely spread
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