Bacterial Horizontal DNA Transfer (Ditki Notes)

Bacterial Horizontal DNA Transfer

• Horizontal, also known as lateral, DNA transfer involves the transfer of genetic information between cells.
• Horizontal DNA transfer enables bacteria to adapt to changes in their environment within a single generation.
• The three mechanisms of DNA transfer between cells are conjugation, transduction, and transformation.

Conjugation

• Conjugation typically involves the unidirectional transfer of plasmid DNA.
• A donor cell has a circular F plasmid (fertility factor), while the recipient cell lacks an F plasmid.
• The F+ cell extends its sex pilus (conjugative pilus) to attach to the F- cell, bringing them into close physical contact.
• One strand of the F plasmid unwraps and enters the recipient cell.
• Each cell synthesizes a complementary strand of plasmid DNA, resulting in both cells becoming F+.
• High-frequency recombination (Hfr) occurs when the plasmid integrates into the bacterial DNA; transfer to a new recipient cell carries some of the donor cell's DNA.
• Resistance plasmids carry genes that confer antibiotic resistance to bacterial cells; conjugation allows quick dissemination of this trait throughout a colony, ensuring survival despite antibiotic drugs.

Transduction

• Transduction occurs when bacteriophages transfer DNA between cells.
• Generalized transduction involves the transfer of non-specific genes; specialized transduction involves the transfer of specific genes.
• A bacterial virus (bacteriophage) injects its DNA into a bacterial cell, leading to phage replication and fragmentation of bacterial DNA.
• As phages replicate and assemble, some incorporate bacterial DNA fragments.
• Bacterial cell lysis releases these phages.
• Phages with bacterial DNA can infect new bacterial cells and inject the donor bacterium's DNA into the recipient.
• The donor DNA combines with the recipient's DNA within the new cell, forming recombinant DNA.

Transformation

• Transformation occurs when bacteria take up foreign DNA and incorporate it into their own.
• A donor cell with bacterial DNA lyses, releasing DNA fragments.
• These fragments can then enter recipient cells and integrate into their DNA.
• Some bacteria like Neisseria, have surface receptors to facilitate transformation.
• Frequent transformation may facilitate immune system and/or antibiotic evasion.
• Scientists use transformation to study gene expression or inhibition by artificially injecting foreign DNA into a eukaryotic cell nucleus in a process called transfection; scientists can also use viruses in a form of artificial transduction.

DNA Transfer to Eukaryotes

• Agrobacterium tumefaciens causes crown gall disease in plants by transferring the Ti (tumor-inducing) plasmid DNA, leading to tumor development.
• Harnessing this natural phenomenon, scientists study gene effects by inserting additional genes into the plasmid to produce transgenic (genetically modified) plants.