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

The colonies of recombinant bacteria appear white because of insertional inactivation of lac z gene, the selectable marker in the plasmid vector. However, the colonies of non-recombinants appear blue as the lac z gene remains functional.

When pUC vector is used in a cloning experiment, the selectable marker is lack z gene (codes for ß- lactosidase) and blue-white colony screening is employed. Blue-white colony screening allows the elimination of non-transformants and also to distinguish the recombinant colonies from non-recombinants in a single step.

Insertion of a foreign gene within the ß-galactosidase gene of the vector inactivates the lac z gene. Therefore, no enzyme is formed in the recombinant bacteria. Thus, it cannot convert the colourless X-gal substrate in the medium into blue-coloured product. However, since there is no insert of foreign DNA within the non-recombinants, the selectable marker gene (lac z) is expressed, and the enzyme ß-galactosidase is synthesised. As a result, the colourless substrate turns to blue colour, imparting blue colour to E. coli colonies.