Blue white screen

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A LB agar plate showing the result of a blue white screen.

The blue-white screen is a molecular technique that allows for the detection of successful ligations in vector-based gene cloning. DNA of interest is ligated into a vector. The vector is then transformed into competent cell (bacteria). The competent cells are grown in the presence of X-gal. If the ligation was successful, the bacterial colony will be white; if not, the colony will be blue. This technique allows for the quick and easy detection of successful ligations, without the need to individually test each colony.

[edit] Background

Cloning, alongside PCR, is one of the most common techniques in molecular biology. Blue white screening makes this procedure less time and labour intensive by allowing for the screening of successful cloning reactions through the color of the bacteria colony.

The molecular mechanism for blue white screening is based on the Lac operon. The vector (e.g. pBluescript) contains the Lac Z gene with an internal Multiple cloning site(MCS). The MCS can be cleaved by different restriction enzymes so that the foreign DNA can be inserted within Lac Z gene, thus disrupting the activity of the β-galactosidase when the protein is expressed. The chemical required for this screen is X-gal, which functions as indicator, and Isopropyl β-D-1-thiogalactopyranoside(IPTG), which functions as the inducer of the Lac operon. The hydrolysis of colourless X-gal by the β-galactosidase causes the characteristic blue color in the colonies; it shows that the colonies contain unligated vector. White colonies indicate insertion of foreign DNA and loss of the cells' ability to hydrolyse the marker.

The correct type of vector and competent cells are important considerations when planning a blue white screen.

[edit] Error

Some white colonies may not contain the recombinant vector. The LacZ, in the vector transformed into the cells of colonies, may be misfunctioned and can't produce the beta-galactosidase. As a result, these cells can't convert X-gal to be the blue substances.