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Explain the following methods of selection of recombinant cell :

(i) Insertional inactivation 

(ii) Blue-white selection

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(i) Insertional inactivation: Harder problem to solve is to determine which of the transformed colonies comprise cells that contain recombinant DNA molecules, and which contain self-ligated vector molecules. Insertional inactivation is the inactivation of a gene by inserting a fragment of DNA into the middle of its coding sequence. Any future products from the inactivated gene will not work because of the extra codes added to it. Recombinants can therefore be identified because the characteristic coded by the inactivated gene is no longer visible.

pBR322 contains genes which code for ampicillin resistance and tetracycline resistance. BamHI cuts in the middle of the gene which codes for tetracycline resistance. If a gene is inserted here, the plasmid loses it ability to code for tetracycline resistance. Thus the plasmid containing the recombinant gene is resistant to ampicillin but sensitive to tetracycline. To screen, we use replica plates.

The pUC8. plasmid is ampicillin resistant and contains a gene lac Z’ which partially codes for β galactosidase. To make the plasmid capable of coding for the whole protein, we add the missing DNA along with the recombinant gene. The host which contains the plasmid pUC8 is resistant to ampicillin and is also capable of producing p galactosidase.

(ii) 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). In this method a reporter gene lac Z is inserted in the vector. The lac Z encodes for the enzyme b- galactosidase which breaks a synthetic substrate X-gal (5-bromo-4-chloro-indolyl-b-D-galacto-pyranoside) into insoluble blue colored product. The competent cells are grown in the presence of X-gal. If the ligation was successful, the bacterial colony
will be white because b- galactosidase is not synthesized due to the inactivation of lac Z; if not, the colony will be blue. This technique allows for the quick and easy detection of successful ligation, without the need to individually test each colony. An example of such a vector is the artificially reconstructed plasmid pUC19.

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