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Question

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


A
Non-recombinant bacteria containing β-galactosidase.
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B
Insertional inactivation of α-galactosidase in non-recombinant bacteria.
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C
Insertional inactivation of α-galactosidase in recombinant bacteria.
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D
Inactivation of galactase enzyme in recombinant bacteria.
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Solution

The correct option is A Insertional inactivation of $$\alpha$$-galactosidase in recombinant bacteria.
  • Blue-white screening allows rapid detecting of the recombinant bacteria in vector-based cloning experiments. 
  • The blue-white screening method works by disrupting the $$\alpha$$-complementation process. 
  • The plasmid carries within the lacZ $$\alpha$$ sequence an internal multiple cloning site or MCS. 
  • This MCS can be cut by restriction enzymes so that the foreign DNA may be inserted, thereby disrupting the gene and production of $$\alpha$$-peptide.
  • Consequently, in cells containing the plasmid with an insert, no functional $$\beta$$-galactosidase may be formed. The presence of an active $$\beta$$-galactosidase can be detected by X-gal, a colourless analogue of lactose, that may be cleaved to form a bright blue insoluble pigment.
  • This results in a characteristic blue colour in cells containing the functional $$\beta$$-galactosidase. 
  • Blue colonies, therefore, show that they may contain a vector, while white colonies indicate the presence of an insert in lacZ $$\alpha$$ which disrupts the formation of an active $$\beta$$-galactosidase.

Thus, the correct answer is option C. 

Biology

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