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Moving genes into bacteria
Back in 1970, researchers found that they could make the bacterium Escherichia coli (E. coli) take up DNA and become transformed. Their methods were used and refined many times, until E. coli became the workhorse of recombinant DNA.
How do researchers transform bacteria?
Mix the DNA and the cells that can take up DNA.
Treat the mixture to allow DNA to enter the host. (For E. coli, one treatment is adding ice cold calcium chloride to the bacteria. This allows 1% of the bacteria to become transformed.)
Grow the host cells. The DNA added to the host should include a "marker" gene that makes the changed cells easy to recognize and pick out from the others. With E. coli and other bacteria, the marker gene is a gene for resistance to an antibiotic such as ampicillin or tetracycline.
Identify the transformed cells. (Only the transformed cells can grow on a medium that contains the antibiotic. You can be pretty sure that any living colonies are transformed.)