Q.The selection of recombinants due to inactivation of antibiotics is a cumbersome procedure. Therefore alternative selectable markers have been developed which differentiate on the basis of colour in presence of chromogenic substrate.

The blue-white colony screening method uses colour to distinguish recombinant bacteria (which fail to make functional β-galactosidase) from non-recombinant ones.

This question refers to the blue-white selection method used to screen for recombinant bacterial colonies. The vector used carries the lacZ gene coding for the enzyme β-galactosidase, and a chromogenic substrate is included in the growth medium. If a foreign DNA fragment gets successfully inserted within the lacZ gene, it disrupts (inactivates) the gene, so the bacterium cannot produce functional β-galactosidase; such recombinant colonies therefore fail to break down the chromogenic substrate and remain colourless (white). Conversely, if the recombinant DNA has failed to get inserted into this gene, the lacZ gene remains intact and functional, and the result …

Blue-white screening works by insertional inactivation of the lacZ (β-galactosidase) gene — recombinants (white colonies) have the gene disrupted by the inserted DNA.

In this selectable marker system, the recombinant DNA (foreign gene) is inserted within the coding sequence of the β-galactosidase gene (lacZ) present on the vector. This insertion inactivates the gene — a phenomenon called insertional inactivation. When plated on a medium containing the chromogenic substrate X-gal:

• Bacteria carrying non-recombinant plasmids (lacZ gene intact) express active β-galactosidase, which hydrolyses X-gal to give a blue colour — blue colonies. …