NEET 2013 Biology question

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

(1)Non-recombinant bacteria containing beta-galactosidase
(2)Insertional inactivation of alpha-galactosidase in non-recombinant bacteria
(3)Insertional inactivation of alpha-galactosidase in recombinant bacteriaCorrect
(4)Inactivation of glycosidase enzyme in recombinant bacteria

Answer: (3)

Solution

In the blue-white screening system the cloning site lies within the lacZ gene (coding for the enzyme galactosidase) of the plasmid. When foreign DNA is inserted, lacZ is inactivated (insertional inactivation) - the recombinant cells cannot convert the chromogenic substrate (X-gal) to a blue product and so form white

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Question ID VK-091423