Insertional inactivation explain.

The selection of recombinants due to inactivation of antibiotic gene is a cumbersome procedure, because it requires simultaneous plaring on two plates having different antibiotics. Therefore, the alternative selectable markers have been developed, which differentiate recombinants from non-recombinants. It is based on their ability to produce colour in the presence of chromogenic substrate. In this process, a recombinant DNA is inserted within the coding sequence of an enzyme, beta-galactosidase. This results into inactivation of the enzyme, which os referred to as insertional inactivation. Therefore, the bacterial colonies having inserted plasmid, shows no colouration (recombinant colonies) while, those without plasmid will show blue colour. Here beta-galactosidase works as a reporter enzyme (produced by reporter gene) whose inactivity helps to identify transformed and non-transformed bacterial colonies.