Hosts are selected to carry the recombinant DNA. In this case E. coli bacteria act as host. Recombinant DNA is introduced into host cells. But under normal conditions bacteria do not accept other plasmids. If the culture medium in which the bacterium is grown is heated and a suitable environment is created by adding calcium, the bacterium takes up another plasmid. The process of introducing recombinant DNA into bacterial cells is called transformation. Besides, recombinant DNA is introduced into host cells through conjugation, microinjection, liposomes, electroporation etc.