Types of Horizontal Gene Transfer

• Natural transformation = uptake of DNA fragments from medium surrounding cell. Requires specific proteins in cell membrane, energy.
• Only found naturally in certain cells; e.g. Pneumococcus, Hemophilus, etc. Not found in E. coli.
• Why did this evolve? Maybe as way to scrounge DNA from dead cells, save trouble of having to make nucleotides from scratch.
• Cells that can take up DNA are said to be competent. Requires induction of several genes. Typically occurs during exponential growth, shuts off in stationary phase.
• Artificial transformation = laboratory technique to enhance DNA uptake of cells that do not have genetic machinery for transformation.
• Example: treat E. coli with high Ca⁺⁺ concentrations, then chill. DNA uptake now occurs (though not as well as with naturally competent bacteria).
• Electroporation: pulsed electric fields produce short-lived membrane pores, allows DNA movement (either in or out of cell). Useful way to move small pieces of DNA and plasmids between cells.
• See natural transformation and artificial transformation ("protected" image.)