Competent Host (For Transformation with Recombinant DNA)

• DNA being a hydrophilic molecule, cannot pass through cell membranes.
• Therefore, the bacteria should be made competent to  accept the DNA molecules.
• Competency is the ability of a cell to take up foreign DNA.
• The cell is made competent by the following methods:

1. Chemical method
2. Physical method

1. Chemical method

• The cell is treated with specific concentration of a divalent cation such as calcium to increase pore size in cell wall.
• The cells are incubated with recombinant DNA on ice, followed by placing them briefly at 42^°Cand then putting it back on ice. This is called heat shock treatment.
• The bacteria now take up the recombinant DNA.

2. Physical methods

The physical methods include

• Micro-injection method: Recombinant DNA is directly injected into the nucleus of an animal cell.
• Biolistics or gene gum method:Cells are bombarded with high velocity micro-particles of gold or tungsten coated with DNA in plants.
• Disarmed pathogen vectors are also used to transfer Rdna