ch14ptB - Microbial Genetics Mechanisms of Genetic...

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Microbial Genetics: Mechanisms of Genetic Variation - Part B
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Acquisition of New Genes in  Bacteria Occurs through horizontal gene transfer (HGT) One mature independent organism to another Lack of sexual reproduction prevents new gene acquisition by vertical transfer Recombination: Re-arrangement of one or more nucleic acid sequences to produce a new sequence Usually accompanied by a phenotypic change
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Fates of Transferred DNA Stable recombinants Integrates into DNA Self-replication as plasmid Unstable recombinants No integration but DNA persists in cytoplasm Host restriction enzymes degrade DNA Remember: Exchange vs. Recombination!
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Mechanisms of  Recombination Homologous – depends on sequence similarity; exchange of similar DNA sequences carried out by RecA proteins Reciprocal (Double-stranded break Model) Non-reciprocal Site-specific – new gene insertion at specific points Viral genome integration Plasmid integration Transposition Some consider this also site-specific (addressed later this chapter)
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Homologous Recombination - Reciprocal Double DNA strand breaks in conjunciton with crossing over Can occur between sister chromatids in eukaryotes Can occur between donor DNA & recipient DNA in prokaryotes Additional constraint on fate of transferred DNA: Heteroduplex (recombinant section) can be seen as mutation and repaired Successful exchange, but unstable recombinant
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Homologous Recombination -  Nonreciprocal Single strand gets incorporated, creating heteroduplex DNA Additional constraint on fate of transferred DNA: Heteroduplex can be seen as mutation and repaired Successful exchange, but unstable recombinant
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Mechanisms of Genetic  Exchange Transposition (self-moving elements) Simple Composite Replicative Conjugation (plasmids; cell-mediated transfer) F+ x F- Hfr x F- F’ x F- Transformation (uptake of naked DNA) Transduction (virus-mediated transfer) Generalized Specialized
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Transposons A DNA segment that carries the gene required for excision from, and insertion into DNA Transposase Originally thought to insert randomly- e.g. ‘jumping genes’ and were discovered by Barbara McClintok Current knowledge – they require a marker and insert at specific DNA sequences (i.e. Site-specific recombination) Widespread in nature Found in Eukarya , Bacteria and Archaea Can move from one location to another: Within the chromosome From plasmid to chromosome From chromosome to plasmid From one plasmid to another
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Transposon Types Insertion sequences (IS)
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ch14ptB - Microbial Genetics Mechanisms of Genetic...

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