Horizontal Gene Transfer: Uptake, Contact and Delivery
Bacteria can acquire DNA by taking up naked DNA, receiving bacterial DNA in a phage particle, or transferring DNA through direct cell contact. Transfer can occur between species, so a transferred gene's ancestry may differ from the ancestry of its host cells.
What a learner can do afterwards
- Distinguishes the three routes by what carries the DNA and what the recipient must do.
- Interprets controlled membrane and DNase experiments to identify which transfer routes the results exclude.
- Explains why transfer complicates a phylogeny built from a single gene.
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Horizontal transfer moves DNA between cells rather than only from parent to offspring. Transformation imports naked DNA from the surroundings into a competent cell, one able to take it up. Transduction uses a bacteriophage, a virus of bacteria, to deliver bacterial DNA. Conjugation transfers DNA from a living donor by direct contact with a recipient. A pilus helps establish contact in many systems.
To test routes, remove one requirement and check the controls. A membrane excludes conjugation only if cells cannot cross or contact each other. Check that free DNA and the relevant phage can pass before interpreting the other routes. DNase destroys exposed DNA. To test transformation, apply it before uptake and verify DNA removal, cell survival and continued function of the other routes. Continued transfer then excludes transformation, not both other routes.
A phylogeny is a tree of inferred ancestry. Vertical inheritance passes DNA from parent to offspring. A horizontally transferred gene may have a different history from its host cells, so one gene tree can misrepresent cell ancestry. Compare several independent genes with evidence of mainly vertical inheritance. Tree disagreement can suggest transfer, but errors in inference can also cause it.
DNA entry does not guarantee inheritance by descendants. Incoming DNA must be maintained, for example by joining the chromosome or replicating as a plasmid. DNA that is degraded without leaving a retained copy cannot be passed on. Distinguish evidence that DNA entered a recipient from evidence that the recipient's descendants inherited it.
Trace the DNA carrier, check the experimental controls, and distinguish gene history from host-cell ancestry.
Lesson explanations, questions and study cards were written for this dataset, with scientific background from Microbiology.
Readings and study cards were written for this dataset, with scientific background from Microbiology. The linked video is credited to Khan Academy.
More to read
Follow the DNA before entry
Suppose a recipient acquires DNA that was previously in another bacterium. Identify the route by asking where the DNA was immediately before entry. Naked DNA outside cells is the substrate for transformation. The recipient needs competence, the ability to take up that DNA. A phage particle carrying bacterial DNA indicates transduction. Direct transfer from a contacting living donor indicates conjugation.
A pilus can help establish conjugation contact, but describing all conjugation as a plasmid moving through a pilus is too restrictive. Some conjugation systems transfer chromosomal DNA as well.
Entry and inheritance are separate questions. To remain in descendants, incoming DNA must be maintained. It might join a chromosome or persist as a replicating plasmid. Detecting DNA briefly inside a recipient does not establish that its descendants will retain it.
Interpret a route test through its controls
A barrier experiment asks whether donor-recipient contact is necessary. First establish that cells cannot cross the barrier or contact each other across it. To leave transformation and transduction available for comparison, check that free DNA and the relevant phage can pass. Transfer across this tested barrier excludes direct-contact conjugation, but does not identify the remaining route.
A DNase experiment asks whether exposed DNA is necessary. Add the enzyme before uptake and verify that it removes the free DNA. Also check recipient survival and whether the treatment leaves contact transfer and phage delivery working. Transfer that continues under these conditions cannot be explained by transformation.
Loss of transfer needs equal care. A treatment may damage recipients or block more than one carrier. Compare treated and untreated conditions and verify the intended effect before assigning a route. One exclusion is not a complete identification.
Distinguish phage cargo from phage effects
During lysogeny, a bacterium maintains a phage genome as a prophage, often integrated into its chromosome. The bacterium carrying it is a lysogen. Expression of prophage genes can change the lysogen's traits. This trait change is lysogenic conversion.
Transduction concerns delivery of bacterial DNA. Generalized transduction can occur when a phage particle packages bacterial DNA by mistake. Specialized transduction can occur when an integrated prophage leaves the chromosome incorrectly and takes neighboring bacterial DNA with it. Delivery to a compatible recipient can then move that bacterial DNA. Specialized transduction and lysogenic conversion describe different processes: one transfers bacterial DNA, while the other concerns traits produced through prophage gene expression in a lysogen.
Phage-mediated delivery requires a compatible recipient, including the requirements for attachment and DNA entry. A phage therefore cannot deliver DNA to every bacterial species.
Transfer can separate a gene's history from its host cells' ancestry. A gene tree follows the gene, including movement between lineages. To investigate cell ancestry, compare independent genes with evidence of mainly parent-to-offspring inheritance. Different trees can motivate a transfer hypothesis, but errors in estimating trees can also produce disagreement.
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