Question

How is the gene z (for $\beta$ galactosidase) used as a marker?

Answer 1

A recombinant DNA is inserted within coding sequence of $\beta$ galactosidase; this results into activation of the enzyme (called insertional inactivation). It is treated with a chromogenic substance. If the plasmid in the bacteria have an insert blue colour appears, if they do not produce blue colour, they are recombinants.