The colonies of recombinant bacteria appear white in contrast to blue colonies of non-recombinant bacteria because of:

Answers (1)

Blue-white screening allows detecting rapidly the recombinant bacteria in vector-based cloning experiments. The blue-white screening method works by disrupting the α-complementation process. The plasmid carries within the lacZ α sequence an internal multiple cloning site or MCS. 

This MCS can be cut by restriction enzymes so that the foreign DNA may be inserted, thereby disrupting the gene and production of α-peptide. Consequently, in cells containing the plasmid with an insert, no functional β-galactosidase may be formed. The presence of an active β-galactosidase can be detected by X-gal, a colourless analogue of lactose, that may be cleaved to form a bright blue insoluble pigment.

This results in a characteristic blue colour in cells containing the functional β-galactosidase. Blue colonies, therefore, show that they may contain a vector, while white colonies indicate the presence of an insert in lacZ α which disrupts the formation of an active β-galactosidase.

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