Explain the method of introduction of alien DNA into bacterial cells.

Explanation

Bacterial cells are treated priorly with a specific concentration of a divalent cation such as calcium, which increases the efficiency with which DNA enters the bacterium through pores in its cell wall. Recombinant DNA can then be forced into such cells by incubating the cells with recombinant DNA on ice, followed by placing them at `42^(@)C` (heat shock), and then putting them back on ice.