Learning Objective
Describe the major mechanisms of horizontal gene transfer in bacteria (transformation, conjugation, transduction) and explain how incoming DNA becomes stabilized through plasmid reformation or recombination.
Bacterial Reproduction
- Bacteria reproduce asexually by binary fission.
- Daughter cells are genetically identical unless a de novo mutation occurs.
- Despite this, bacteria show genetic diversity due to horizontal gene transfer.
Mechanisms of Gene Transfer
Bacteria acquire novel genetic material through three high-yield processes:
Transformation
- Uptake of free/naked DNA from the environment.
- Requires a competent bacterium (e.g., S. pneumoniae, H. influenzae, Neisseria).
Conjugation
- Direct transfer of DNA between bacteria via a sex pilus (F pilus).
- F+ cell transfers plasmid to F− cell.
- Hfr strains (episome integrated into the chromosome) can transfer chromosomal genes.
Transduction
- Bacteriophage-mediated gene transfer.
- Generalized transduction = transfer of random DNA.
- Specialized transduction = transfer of adjacent genes with an excised prophage.
Stabilization of Incoming DNA
After gene transfer, new DNA must be stabilized by one of two methods:
Plasmid reformation
- Incoming circular DNA can reform as a plasmid independent of the chromosome.
Homologous recombination
- Linear DNA can only be stabilized by homologous recombination into the chromosome.
- Without stabilization, → DNA is degraded.
Episomes
- Some plasmids can integrate into the chromosome through site-specific recombination.
- These integrated plasmids are called episomes.
- Classic example: F plasmid → Hfr cell.
