THE MOLECULAR BASIS OF INHERITANCE
RECOMBINANT DNA
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Question
[CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
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cells that have not taken up the plasmid vector
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cells with recombinant plasmids containing a new insert
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cells containing empty plasmid vectors
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cells with a non-functional lacZ gene
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Detailed explanation-1: -Blue colonies therefore show that they may contain a vector with an uninterrupted lacZ (therefore no insert), while white colonies, where X-gal is not hydrolyzed, indicate the presence of an insert in lacZ which disrupts the formation of an active -galactosidase .
Detailed explanation-2: -Any colony containing the plasmid (and therefore the functioning -galactosidase gene) will turn blue, a result of the -galactosidase activity. This is called -complementation.
Detailed explanation-3: -Plasmids used in cloning contain an antibiotic resistance gene. Thus, all of the bacteria are placed on an antibiotic plate to select for ones that took up a plasmid. Bacteria without a plasmid die.
Detailed explanation-4: -The presence of a promoter is necessary when screening techniques such as blue-white selection are used. Cloning vectors without promoter and RBS for the cloned DNA sequence are sometimes used, for example when cloning genes whose products are toxic to E. coli cells.
Detailed explanation-5: -However, insertion of a fragment of foreign DNA into the polycloning site of the plasmid almost invariably results in production of an amino-terminal fragment that is no longer capable of -complementation. Bacteria carrying recombinant plasmids therefore form white colonies.