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Bacterial Genetics

  • In this section we discuss how the complement of DNA within a bacteria can be modified, allowing bacteria to synthesize novel proteins.
  • Mutation involves changes in the identity of individual nucleotides or additions of nucleotides within the bacterial genome. Mutations generate tremendous diversity even within a single population of bacteria and form the raw genetic canvas upon which evolutionary selection can operate.
  • Transformation involves molecules of DNA crossing the bacterial cell wall and entering the bacterial cytoplasm. Transformation likely happens frequently in nature and allows bacteria to potentially pick up useful sequences of DNA. However, it is not thought to be of clinical importance and is primarily of significance in biomedical research where it is a key tool in generating recombinant DNA for experimental purposes.
  • Overview
    • Transduction involves the exchange of genetic material between bacteria via "Phages" or viruses that infect bacteria. Like mammalian viruses, phages are composed of their nucleic acid genome packaged within a proteinacious capsid. The capsid binds bacteria and injects the genome along with accessory proteins which then hijack the bacterial metabolic machinery to replicate and then lyse the cell to release their progeny. In some cases, termed lysogenic conversion, the phage genome stably integrates within an infected bacteria and does not replicate or lyse the host organism until certain environmental conditions are met, at which point the phage genome is clipped out and begins the replication program.
  • Generalized Transduction
    • In a small fraction of cases, pieces of the infected bacterial genome, rather than the phage genome, can be packaged within an infectious phage capsid and released. When the mis-packaged capsid injects its internal nucleic acid into a fresh victim, genomic fragments of the previous bacteria are injected and in some cases integrate into the new organism's genome.
  • Specialized Transduction:
    • In certain instances, small fragments of the host bacterial genome are clipped out along with a lysogenically integrated phage genome
    • Consequently, when a new bacterium is infected, genomic fragments of the previous bacteria which are tagging along can become integrated into the new victim. Specialized transduction is an important mechanism by which several important bacterial exotoxins are exchanged.
  • Conjugation involves the transfer of genetic material between plasmids via a specialized "Sex Pilus". The proteins of the sex pilus and conjugation machinery are encoded on a specialized plasmid termed the "F Plasmid" (F is for Fertility). Recall that Plasmids are small, circular pieces of double stranded DNA that can replicate independent of the bacterial genome within the bacterial cytoplasm. Bacteria which contain the F Plasmid (termed F+) can synthesize a sex pilus which attaches to bacteria which do not possess the F Plasmid (termed F-). The F Plasmid in F+ cells then makes a single-stranded copy of itself which is transfered to the F- bacteria through the "Sex Pilus". In a small fraction of cases, the F Plasmid becomes integrated into the bacterial host genome and is subsequently clipped out prior to F Plasmid transfer. Fragments of the bacterial host genome can sometimes be accidentally clipped out along with the F Plasmid and can thus be transfered to other bacteria along with the F Plasmid.
  • Transposons are elements of DNA integrated within the bacterial genome which can move to other locations within the same genome. Transposons do this by coding for proteins which clip out the transposon from the bacterial genome and then re-integrate it into another location within the genome. Certain antibiotic resistance genes are coded on transposons and can spread from bacteria to bacteria via processes of transduction and conjugation by moving onto lysogenic phage genomes or onto F Plasmids.