After you have used ligase to make recombinant DNA, you will insert this DNA into E. coli cells which have no plasmids. The process of inserting the DNA is called transformation. Before they are transformed with plasmid DNA, these cells cannot survive in the presence of the antibiotics ampicillin or kanamycin.

After the transformation, any cell which takes up a plasmid containing the ampicillin resistance gene (and an origin for the plasmid to replicate) will be able to grow on a plate containing ampicillin. Similarly, cells containing the kanamycin resistance gene can grow on plates containing kanamycin.

Some cells will take up a plasmid which has both genes for resistance on it; these cells will be able to grow in the presence of both antibiotics. It is these cells we are looking for.

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