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A person obtains transformants by inserting the desired gene within the coding sequence of enzyme 𝛃-galactosidase. Which of the following observations will help him separate out recombinants from non-recombinants?

A
Non-recombinant colonies do not produce any colour while recombinants give blue coloured colonies
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B
Recombinant colonies do not produce any colour while non-recombinants give blue coloured colonies
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C
Both recombinants and non-recombinants produce blue-coloured colonies
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D
No colonies are formed due to insertional inactivation
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Solution

The correct option is B Recombinant colonies do not produce any colour while non-recombinants give blue coloured colonies
The phenomenon of inactivation of a functional gene due to the insertion of a foreign DNA within its coding sequence is known as insertional inactivation.

This phenomenon of insertional inactivation helps in the selection of recombinants in recombinant DNA technology.

If the desired gene (foreign DNA) is inserted into the plasmid within the gene that encodes the enzyme 𝛃-galactosidase, it will cause the insertional inactivation of the 𝛃-galactosidase gene. These plasmids are then introduced into the host cells, here, bacterial cells.

The bacterial cells with recombinant DNA (plasmid DNA along with desired gene) are called recombinants. The recombinants can be selected and non-recombinants can be eliminated by culturing these bacterial cells in chromogenic substance (X-Gal).

The enzyme 𝛃-galactosidase is capable of converting a chromogenic substance (X-Gal) present in the culture medium into galactose and a blue coloured byproduct. Thus, the bacterial cells which have a functional 𝛃-galactosidase gene, produce blue coloured colonies when grown in a medium containing X-Gal.

If the transformants (bacterial cells) have the foreign DNA (desired gene) incorporated within the coding sequence of the enzyme 𝛃-galactosidase, then it causes insertional inactivation of the said gene. This does not allow the transformants to produce a functional 𝛃-galactosidase enzyme. Hence, they are unable to metabolise X-Gal to produce the blue coloured byproduct. Thus, non-coloured colonies are formed when grown in X-Gal containing medium.

Thus, the selection of recombinants over non-recombinants becomes very easy as recombinants do not produce coloured colonies whereas non-recombinants give blue-coloured colonies.

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