Final answer:
To make E. coli cells competent to take in exogenous DNA, they must be chemically treated with calcium chloride and given a heat shock. Only a small fraction of cells are transformed, requiring the use of ampicillin and tetracycline to select those that have successfully taken up the DNA.
Step-by-step explanation:
Exogenous DNA does not passively enter E. coli cells unless they are competent. To make E. coli cells competent, they require specific treatments that increase the permeability of the cell membrane. These methods include chemical treatment with a solution of calcium chloride and the use of heat shock or electroporation.
For chemical treatment, E. coli cells are incubated with an ice-cold solution of 50 mM calcium chloride at 4°C for 30 minutes. Subsequently, the cells are exposed to a heat shock at 42°C for 2 minutes, allowing the DNA molecules to enter the cell. However, even with these treatments, typically only 0.01% of the bacteria are transformed, so it's necessary to select the transformed cells from those that remain non-transformed.
Selection is done by growing the cells on media containing the antibiotics ampicillin and tetracycline. Only the cells that have taken up a plasmid such as pBR322, which provides resistance to these antibiotics, can survive. This approach ensures that only transformed cells, those containing the recombinant DNA, can grow.