CH 10 Pt 7 Bacterial Genetics & Horizontal Gene Transfer

Review of Asexual Reproduction & the Need for Diversity

  • Bacteria primarily reproduce by binary fission (covered in Chapter 8):
    • One parent cell duplicates its chromosome and divides into two daughter cells.
    • Resulting daughters are genetically identical unless new DNA is introduced.
  • Despite clonal reproduction, bacteria display enormous genetic diversity → implies mechanisms for horizontal gene transfer (HGT).
  • Diversity allows bacteria to occupy new niches, evolve into different strains/species, and adapt to antibiotics or environmental stresses.

Three Major Mechanisms of Horizontal Gene Transfer

  • HGT = movement of genetic material between organisms, independent of vertical inheritance.
  • Mechanisms covered (virus-only slides omitted for this course except where they interact with bacteria):
    1. Transformation
    2. Transduction
    3. Conjugation

Transformation

  • Definition: Uptake of naked (free) DNA fragments directly from the environment into a competent bacterial cell.
  • Historical anchor: Griffith’s 1928 experiment—S strain (virulent) DNA transformed R strain (non-virulent) cells, making them lethal.
  • Process details:
    • Environmental DNA can originate from lysed bacteria, fecal matter, soil, creeks, gut flora, etc.
    • Fragment passes through cell wall & plasma membrane into cytoplasm.
    • Successful retention requires homologous recombination into the circular chromosome (≈ 10001000 genes in a typical bacterium).
    • Upon cell division, integrated DNA is replicated and inherited.
  • Significance/Real-world relevance:
    • Enables rapid acquisition of traits such as capsule production, toxin genes, or metabolic pathways.
    • Foundation for molecular techniques (e.g., lab-induced competence in E. coli).

Transduction

  • Definition: Transfer of bacterial genes mediated by a bacteriophage (virus that infects bacteria).
  • Mechanistic outline:
    • Phage injects its DNA → host transcribes/translates viral genes.
    • Occasionally, a fragment of phage (or even mis-packaged bacterial) DNA remains and integrates into host chromosome.
    • Integration confers new traits only if advantageous to the host (e.g., toxin genes in pathogenic Vibrio cholerae).
  • Scope for this course: basic concept; detailed viral genetics reserved for BIO 122.
  • Evolutionary significance:
    • Drives emergence of virulence factors.
    • Creates mosaic genomes observed in many pathogens.

Conjugation

  • Definition: Direct DNA transfer between two bacterial cells via a physical connection called a mating bridge (also termed conjugation pilus).
  • Steps & requirements:
    • Donor cell must possess sex pili encoded by the F (fertility) factor.
    • Pilus forms, cytoplasms connect → a copy of donor DNA (plasmid or chromosomal segment) replicates and moves into recipient.
    • Bridge disassembles; recipient now potentially becomes a donor if it received F factor genes.
  • Biological implications:
    • Rapid spread of multi-drug resistance among clinical isolates.
    • Can occur across species boundaries, contributing to speciation.

Plasmids: Accessory Replicons

  • Structure: Small, circular DNA molecules (much smaller than chromosome) with their own origin of replication (ori).
  • Typical gene content: only 11–22 essential genes, but occasionally more.
  • Example discussed:
    • F factor plasmid—carries genes for sex pilus formation enabling conjugation.
  • Functional advantages provided:
    • Antibiotic resistance (e.g., to ampicillin, neomycin).
    • Metabolic enzymes, heavy-metal detoxification, virulence determinants.
  • Energy economy:
    • Bacteria replicate plasmids only if advantageous; absence of selective pressure (e.g., no antibiotic) → plasmid loss.
  • Visualization:
    • Lysed cell photo: large tangled chromosome + multiple small plasmid rings.
  • Techniques & future link:
    • Chapter 12 will exploit plasmids as molecular biology vectors (cloning, recombinant protein expression).

Energetic Cost & Selective Retention of Foreign DNA

  • DNA replication demands ATP, dNTPs, and enzymatic resources.
  • Chromosome size baseline ≈ 10001000 genes; adding extra 500500 genes raises energetic load.
  • Therefore, only DNA conferring a clear survival advantage is maintained:
    • Drug resistance under antibiotic stress.
    • Virulence factors in host environments.
    • Novel metabolic capabilities in nutrient-limited niches.
  • Ecological & ethical implication:
    • Misuse of antibiotics selects for resistant plasmid-bearing strains.
    • Horizontal gene flow complicates containment of engineered genes in biotech applications.

Connections to Prior & Future Material

  • Binary fission from Chapter 8 sets baseline for clonal reproduction.
  • Viral structure/function (briefly touched on here) will be fully covered in BIO 122.
  • Plasmid cloning vectors form core of recombinant DNA techniques in Chapter 12—understanding natural plasmids provides conceptual foundation.

Key Takeaways

  • Bacteria circumvent clonal limitation via three HGT routes—transformation, transduction, conjugation.
  • Stable inheritance requires integration into chromosome or carriage on a self-replicating plasmid.
  • Retention driven by adaptive benefit; otherwise DNA is jettisoned to conserve energy.
  • HGT underpins rapid bacterial evolution, public-health challenges (antibiotic resistance), and modern genetic engineering tools.