Notes on Horizontal Gene Transfer and Transduction

Horizontal Gene Transfer (HGT) in Prokaryotes

  • Definition: HGT refers to the transfer of genes between organisms in a manner other than traditional reproduction.
  • Key Components:
  • Exogenote: DNA that is transferred to the recipient.
  • Endogenote: Genome of the recipient organism.
  • Merozygote: Recipient cell that is partially diploid due to the transfer process.

Mechanisms of Horizontal Gene Transfer

  • Transformation: Uptake of naked DNA from the environment by a competent cell.
  • Transduction: Transfer of DNA from one bacterium to another via a virus (bacteriophage).
  • Conjugation: Direct transfer of DNA between two bacterial cells via a pilus or mating bridge.

Transformation

  • Involves receptor-mediated uptake of freed DNA from the environment.

Transduction

  • Lytic Cycle: Phage infects a bacterium, replicates, and lyses the cell, releasing new phages.
  • Lysogenic Cycle: Phage integrates its genome into the bacterial chromosome, becoming a prophage.
Types of Transduction

  • Generalized Transduction:

  • Occurs when fragments of the host bacterium's DNA are packaged into phage particles during the lytic cycle.

  • Transducing phages can inject this bacterial DNA into a new host, potentially leading to recombination.

  • Specialized Transduction:

  • Only specific genes are transferred adjacent to the integration site of the prophage during incorrect excision from the genome.

Phage Life Cycles

  • Lytic Cycle Steps:
  1. Phage injects its DNA.
  2. Bacterial DNA is hydrolyzed.
  3. New phages are assembled.
  4. The cell lyses, releasing new phages.
  • Lysogenic Cycle Steps:
  1. Phage DNA integrates into bacterial chromosome as a prophage.
  2. Prophage replicates with the bacterial DNA during cell division.
  3. Exposure to stress can trigger the prophage to excise and enter the lytic cycle.

Importance of Transduction

  • Transduction is a major mechanism for introducing genetic variation in microbial populations.
  • Used in molecular biology for cloning and genetic mapping due to its ability to transfer genes between bacteria.

Mapping with Generalized Transduction

  • Generalized transduction can map genes based on cotransduction frequency.
  • Closely linked genes are more likely to be packaged together in a single transducing particle, aiding in gene mapping studies.

Key Experiments

  • U-tube Experiment (Lederberg & Zinder):
  • Demonstrated that transformation of prototrophic Salmonella could occur via transduction despite barriers preventing direct contact, indicating the role of phages.

Summary of Transduction Processes

  • Generalized Transduction: Involves the accident of packaging random bacterial DNA.
  • Specialized Transduction: Involves specific regions of the bacterial genome near the prophage integration site.

Conclusion

  • Horizontal gene transfer through transformation, transduction, and conjugation enhances genetic diversity in prokaryotes, impacting evolution and adaptation processes.