Alternative selectable markers can differentiate recombinants from non-recombinants due to their capacity to form colour in presence of a chromogenic substance. In coding sequence of enzyme -galactosidase, recombinant DNA is inserted. Due to this enzyme gets inactivated and step is called as insertional inactivation. If in plasmid of bacteria, insert is not present, blue coloured colonies appear due to presence of chromogenic substrate. In -galactosidase, due to the presence of insert, process of insertional inactivation takes place and no colour is produced. Such colonels are identified as recombinant colonies.
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