Competence is a special physiological state of a bacterial cell in which a cell can take up DNA from the surrounding environment and is not easily decomposed by a restriction endonuclease inside the cell.
Using specific techniques in a laboratory setting, bacterial cells can be transformed into the state of competence for molecular biology applications. For example, the chemical method takes advantage of the low-permeability of bacterial cells in their rapidly-growing phase of their life cycle. By adding CaCl2 solution to bacteria at low temperatures, bacteria swells into a spherical shape and small holes are created in the cell wall. Under these conditions, foreign DNA molecules can bind to the cell surface of bacteria and form an anti-DNase hydroxy-calcium phosphate complex before being taken up by cells by heat shock.