The Cloning Process

The recombinant vector molecule must be introduced into its ‘matching’ host cell in order to replicate and produce multiple copies. The process by which DNA is introduced into the host cell is known as bacterial transformation. Since ‘naked’ vector DNA is hydrophilic and the bacterial cell wall is normally impermeable to such molecules, the host cell must be made ‘competent’ by treatment with calcium chloride in the early log phase of growth. This causes the cell to become permeable to chloride ions. When competent cells are mixed with DNA and heat shocked at 42 1C, the swollen cells are able to take up the naked DNA molecules. It is believed that only a single DNA molecule is permitted to enter any single cell. Thus individual colonies of transformed bacteria grow single recombinant vector molecules. The bacterial cells are usually grown on selective media so that only transformants survive to form colonies. Thus, for example, if the vector contains an ampicillin resistance gene and the cells are grown on ampicillin-containing media, only the cells containing the vector will form  colonies.