Question

In insertional inactivation of Beta galactosidase gene, the bacteria in white colonies have
A. Non- recombinant plasmid
B. Recombinant plasmid
C. No plasmid
D. Linear foreign DNA

Answer 1

Hint: Insertional inactivation is a technique in which a gene is inactivated because of insertion of foreign gene.

Complete Answer:
Insertional inactivation is a technique that is used in recombinant DNA technology. In this technique, a bacterium carrying recombinant plasmids or a fragment of foreign DNA is inserted into a restriction site inside a gene to resist antibiotics. This disables the function of genes.

- Option a: In insertional inactivation, bacteria carrying recombinant plasmid is inserted at the restriction site in the gene. Blue colored colonies will appear in the case of non-recombinant cells. Therefore, this option is incorrect.

- Option b: Enzyme beta galactosidase encoded by the lacZ gene which comprises a few recognition sites for restriction enzymes. This enzyme breaks a synthetic substrate called X-gal into an insoluble product that is blue in color.
However, when this lac Z is introduced into a foreign gene, i.e., the recombinant plasmid, it gets deactivated. Hence no blue color colonies develop as beta galactosidase is not produced due to deactivation of the lacZ gene.
Therefore, white-colored colonies appear in the case of recombinant DNA and blue colored colonies will appear in the case of non-recombinant cells. Therefore, this option is correct.

- Option c: The absence of plasmid means that the cell is non-recombinant. Hence, blue colored colonies will develop. Therefore, this option is incorrect.

- Option d: Linear DNA cannot be introduced as it will not replicate. It will also not survive exonuclease activities inside the bacterial cell. Therefore, this option is incorrect.

Hence the correct answer is option ‘B’.

Note: Insertional inactivation is used in bacterial DNA technology to identify transformant cells which took up recombinant plasmids. It is also used to test antibiotic resistance.