Bacterial Horizontal DNA Transfer
Three mechanisms of DNA transfer between cells:
Conjugation, which typically involves the unidirectional transfer of plasmid DNA.
Transduction, which occurs when bacteriophages transfer DNA between cells.
Transformation, which occurs when bacteria take up foreign DNA and incorporate it into their own.
Review
Bacterial Chromosome Replication
First Drawing:
Donor cell has circular F plasmid (F stands for "fertility factor").
Recipient cell, which does not have an F plasmid.
F+ cell extends its sex pilus, aka, conjugative pilus, and attaches to the F- cell.
This brings the two cells in close physical contact.
Second Drawing:
One strand of the F plasmid unwraps and enters the recipient cell.
Third Drawing:
Each cell now has a single strand of the original F plasmid.
Fourth Drawing:
Each cell synthesizes a complementary strand of plasmid DNA, so that both cells are now F+.
High-frequency recombination (Hfr)
Occurs when the plasmid is integrated into the bacterial DNA; when the plasmid is transferred to a new recipient cell, it takes with it 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 its survival despite antibiotic drugs.
There are two types:
Generalized, which involves the transfer of non-specific genes.
Specialized, which involves the transfer of specific genes.
We show an example of generalized transduction.
First Drawing:
Bacterial cell and with its DNA.
A bacteriophage, aka, bacterial virus, injects its DNA into the bacterial cell.
Second Drawing:
Within the bacterial cell, the phage replicates, and the bacterial DNA is fragmented.
Third Drawing:
As the phage replicates and assembles, some phages will incorporate bacterial DNA fragments.
Fourth Drawing:
Ultimately, bacterial cell lysis releases these phages.
Fifth Drawing:
Phages with the bacterial DNA can inject the donor bacterium's DNA into a new recipient.
Within the new cell, the donor DNA combines with the recipient's DNA, forming
recombinant DNA.
First Drawing:
The donor cell with its bacterial DNA.
Second Drawing:
When the bacterium lyses, it releases DNA fragments.
These fragments can then enter recipient cells.
Third Drawing:
Fragments are integrated into recipient DNA.
Some bacteria, such as
Neisseria, have surface receptors to facilitate transformation. Frequent transformation may facilitate immune system and/or antibiotic evasion.
Transfection
Scientists use transformation to study gene expression or inhibition by artificially injecting foreign DNA into a eukaryotic cell nucleus, a process called transfection; scientists can also use viruses in a form of artificial transduction.
DNA transfer to Eukaryotes
Agrobacterium tumefaciens is a bacterium that causes crown gall disease in plants.
It does so by transferring the Ti (tumor-inducing) plasmid DNA, which, as its name suggests, leads to tumor development in the infected plant.
Scientists harness this natural phenomenon to study gene effects in the lab; they can insert additional genes into the plasmid to produce transgenic, aka, genetically modified, plants.
