The blue and white selectable markers have been developed which differentiate recombinant colonies from non-recombinant colonies on the basis of their ability to produce colour in the presence of a chromogenic substrate. Given below are two statements about this method: Statement I : The blue coloured colonies have DNA insert in the plasmid and they are identified as recombinant colonies. Statement II : The colonies without blue colour have DNA insert in the plasmid and are identified as recombinant colonies. In the light of the above statements, choose the most appropriate answer from the options given below :

Solution & Explanation

Answer: (Option 2) Statement I is incorrect but Statement II is correct Solution: In the blue-white selection method, a foreign DNA insert disrupts (insertionally inactivates) the beta-galactosidase gene, so recombinant colonies cannot make the enzyme and produce NO colour (white), while non-recombinant colonies retain the gene and turn blue on the chromogenic substrate. Therefore Statement I (blue colonies are recombinant) is incorrect, and Statement II (colonies WITHOUT blue colour carry the insert and are recombinant) is correct. NCERT Reference: Ch 9, p.170, line(s) 16-18 — "Presence of insert results into insertional inactivation of the β-galactosidase gene and the colonies do not produce any colour, these are identified as recombinant colonies."