How does blue white screening work in selecting E coli that have taken up a plasmid vector?
The traditional blue/white screening is set up so that blue colonies are considered negative for the insert, and white colonies are positive for recombinant DNA. The gene responsible is the lacZ gene, or beta-galactosidase. This enzyme converts a synthetic substrate, X-gal, into an insoluble blue compound.

The pCR2.1 TOPO plasmid is a blue/white selection plasmid in which white colonies are considered positive for the insert. The way this works is by inserting the DNA sequence to clone inside the coding region for lacZ. By having any inserted DNA interrupting the sequences of lacZ, so that the enzyme has lost its function (because it is no longer being translated correctly in the bacteria). On the other hand, no insert present means that lacZ is ligated back together, and the entire enzyme is correctly made, and therefore can metabolize the X-gal substrate to form an insoluble blue dye.