Methods employed to introduce recombinant DNA into host cell are : 
(a) Chemical method - The cell is treated with a specific concentration of a divalent cation such as calcium to increase pore size in cell wall.
The cells are then incubated with recombinant DNA on ice, followed by placing them briefly at 42°C and then putting it back on ice. This is called heat shock treatment.
(b) Physical methods In this method, a recombinant DNA is directly injected into the nucleus of an animal cell by microinjection method.
• In plants, cells are bombarded with high velocity microparticles of gold or tungsten coated with DNA called as biolistics or gene gun method.