Gene Transfer Mechanisms: A Summary of Horizontal Gene Transfer

Gene Transfer Mechanisms

Horizontal Gene Transfer

• Horizontal gene transfer is the transfer of genetic material between two existing organisms, where the recipient is not the donor's offspring.

• Types:

  • Transduction

  • Conjugation

  • Transformation

Vertical Gene Transfer

• Genetic material is transmitted from parent to offspring.

• Occurs via sexual or asexual reproduction.

Transformation

• Definition: Uptake of free genetic material from the environment by a bacterium, plant, or non-animal eukaryotic cells (e.g., fungi).

• Focus: Examples primarily involve bacteria.

• Process:

  • A bacterial cell contains:

    • Circular bacterial chromosome

    • Extrachromosomal plasmid DNA (replicates autonomously)

  • Free-floating plasmid DNA in the environment is taken up.

  • This DNA may have been released by other bacteria.

• Competence:

  • Ability to take up free DNA.

  • Uptake and expression of new genes can provide a selective advantage.

  • Not all bacteria are naturally competent.

  • Natural competence is limited to certain bacterial species.

• Induced Competence:

  • Competence can be induced in the lab using chemical modifications.

  • Example: Heat shock transformation protocol.

    • Calcium chloride (CaCl_2) combined with a short heat shock depolarizes the cell membrane, increasing permeability.

    • Divalent cations, like calcium ions (Ca^{2+}), facilitate the uptake of negatively charged DNA.

  • The bacterial cell expresses genes from the plasmid, resulting in protein production.

Transduction

• Definition: Genetic material transmission between cells mediated by viruses.

• Process:

  • Bacteriophage infects a bacterial cell.

  • Viral nucleic acid enters and integrates into the host cell's DNA.

  • Upon excision, errors can occur, leading to accidental excision of bacterial DNA.

  • During virus assembly, bacterial host DNA can be mistakenly encapsulated.

  • Phages are released from the host cell.

  • The bacteriophage infects a recipient cell, injecting nucleic acid (containing bacterial genes).

  • This sequence integrates into the recipient's genome.

• Result: Genes from the host cell are transferred to the recipient cell via a viral vector.

• Application: In the lab, viral vectors (e.g., lentiviruses) are used to integrate genes into eukaryotic cells.

Conjugation

• Definition: Horizontal gene transfer between bacterial cells requiring cell-to-cell contact.

• Characteristics:

  • Unidirectional: one cell is the donor, and the other is the recipient.

  • Donor contains the fertility factor (F factor) on the F plasmid (extrachromosomal).

  • Donor is F positive (F^+), recipient is F negative (F^-).

• Mechanism:

  • F^+ cells form a sex pilus, an extending protein structure, to reach neighboring cells.

  • The sex pilus attaches to the recipient cell, bringing the cells together.

  • One strand of the plasmid DNA passes through a channel from donor to recipient.

  • Replication occurs, resulting in double-stranded DNA in both cells.

  • The recipient (originally F^-) becomes F^+ after transfer.