Competent cells are the cells that can take up foreign DNA easily since they have altered cell walls. Most of the cells cannot take up unless they have been exposed to certain physical or chemical treatments. The treatment using Calcium chloride (CaCl2) is one such method of preparation of competent cells.
The exposure of a cell to ice-cold CaCl2 (0 - 5°C) and a subsequent heat shock (37 - 45°C for 85 - 90 seconds) creates pores in the bacterial cell thereby allowing the uptake of plasmid DNA easily into the cell. This is because the Ca ions being positively charged attack both the negatively charged DNA and also the lipopolysaccharide membrane. Thus, the DNA can then pass through the cell on subsequent heat shock treatment.