In the technique of selection of transformed recombinant host cells by insertional inactivation method, identify the correct statement.

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The enzyme $β$ -galactosidase is inactivated by the presence of desired alien gene in the medium

Cells of blue coloured colonies turn colourless due to insertional inactivation of $β$ –galctosidase gene.

$β$ -galactosidase gene is inactivated due to insertion of desired gene in the amino acid sequence of the enzyme $β$ -galactosidase

Colonies of normal host cells which do not contain an insert produce blue colour in the presence of suitable chromogenic substrate in the medium

answer is D.

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Detailed Solution

We can differentiate recombinants from non-recombinants on the basis of their ability to produce colour in the presence of a chromogenic substrate. In this, a recombinant DNA is inserted within the coding sequence of an enzyme, β-galactosidase. This results into inactivation of the gene for synthesis of this enzyme, which is referred to as insertional inactivation. The presence of a chromogenic substrate gives blue coloured colonies if the plasmid in the bacteria does not have an insert. 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.

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