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Competent cell
From Wikipedia, the free encyclopedia
Competent cells are bacteria which can accept extra-chromosomal DNA or plasmids. Cells can be made competent in several ways. One such way is to "shock" it, which involves cooling the bacteria in a Calcium Chloride bath at 0 degrees Celsius, then quickly heating it to around 47 degrees Celsius for approximately 90 seconds (too long will denature the cell membrane, killing the bacteria). The calcium chloride ions neutralize the repulsion between the negatively charged phospholipid heads of the cell membrane and the negatively charged phosphate groups on the DNA. The quick heat shock creates a thermal gradient which, in turn, creates a draft leading into the cell, allowing extra-chromosomal DNA (such as plasmids) to enter the cell, allowing the bacterium to be genetically modifed.
Refer to: Mandel M, Higa A. Calcium-dependent bacteriophage DNA infection. J Mol Biol. 1970 Oct 14;53(1):159–162.
In addition cells are most competent during their mid-log phase. In an inoculum of E. coli cells, competent cells are found at an absorbancy of 0.25 using a standard spectrophotmeter at 660 nm. The time required to reach mid-log, thus competent cells, depends on the size of the initial inoculum.
See also: Transformation (genetics)