In transformation, a bacterium takes in DNA from its environment, often DNA that’s been shed by other bacteria. In a laboratory, the DNA may be introduced by scientists (see biotechnology article). If the DNA is in the form of a circular DNA called a plasmid, it can be copied in the receiving cell and passed on to its descendants.
Left: plasmid taken up by transformation. Right: linear DNA fragment taken up by transformation and swapped into the bacterial chromosome by homologous recombination. Image modified from “Conjugation,” by Adenosine (CC BY-SA 3.0). The modified image is licensed under a CC BY-SA 3.0 license.Why would this be important? Imagine that a harmless bacterium takes up DNA for a toxin gene from a pathogenic (disease-causing) species of bacterium. If the receiving cell incorporates the new DNA into its own chromosome (which can happen by a process called homologous recombination), it too may become pathogenic.