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Bacterial Genetics

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Topic updated on 03/12/15 10:00am

Overview
  • Transformation
    • the uptake of naked DNA from the environment
      • any DNA can be used
    • only bacteria that are "competent" are able to undergo transformation
      • competence factor released by stressed bacteria induces this unique ability
    • a feature of many bacteria
      • especially S. pneumoniae, H. influenzae type B, Neisseria ("SHiN")
    • "having naked SHiNs was a transforming experience"
  • Conjugation
    • "bacterial sex"
    • the major mechanism by which bacteria exchange plasmids
      • allows the transmission of resistance to drugs
    • F plasmid contains the genes required for conjugation
      • F+ bacteria have the plasmid
      • F- bacteria do not
      • Hfr ("high frequency") bacteria have incorporated the F plasmid into their chromosomal DNA
    • F+ x F-
      • plasmid is replicated inside F+ cell
      • plasmid is transferred through pilus from F+ cell to F- cell
        • only plasmid itself is transferred
        • no transfer of chromosomal genes
    • Hfr x F-
      • plasmid and some flanking chromosomal DNA is replicated inside Hfr cell
      • plasmid and some chromsomal DNA is transferred through pilus from F+ cell to F- cell
        • in this case, both plasmid itself and chromosomal genes are transferred
    • "a conjugal visit"
  • Transduction
    • transfer of bacterial DNA using virus (phage) as a carrier
    • generalized transduction ("packaging")
      • lytic phage
      • phage cleaves bacterial DNA
      • parts of bacterial chromosomal DNA may become packaged in viral capsid
      • DNA then travels in phage to infect another bacterium, transferring genes
    • specialized ("excision")
      • lysogenic phage
      • phage incorporates viral DNA into bacterial chromosome
      • when phage DNA is excised from the bacterial chromosome, flanking bacterial genes may be excised with it
      • DNA then travels in phage to infect another bacterium, transferring genes
      • toxin genes encoded in a lysogenic phage
        • shiga-like
        • botulinum
        • cholera
        • diphtheria
        • erythrogenic toxin of S. pyogenes
  • Transposition
    • transposons (also known as insertion elements or transposable elements) are segments of DNA that can move from one location to another
    • allows transfer of genes from plasmid to chromosome and vice-versa
      • removal of a plasmid from the chromosomal DNA may include some flanking chromosomal DNA that may be transferred to another bacterium along with the plasmid


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