In insertional inactivation of `beta`-galactosidase gene, the bacteria in white colonies have

Options

A. (a) Nonrecombinant plasmid
B. (b) Recombinant plasmid
C. (c) No plasmid
D. (d) Linear foreign DNA

Answer

### Step-by-Step Solution:

1. **Understanding the Concept**: The question pertains to the process of insertional inactivation of the beta-galactosidase gene, which is a key part of the blue-white screening method used in molecular biology to differentiate between recombinant and non-recombinant bacteria.

2. **Identifying the Gene**: The beta-galactosidase gene (also referred to as the lacZ gene) encodes an enzyme that allows bacteria to metabolize lactose. In the context of blue-white screening, this gene serves as a selectable marker.

3. **Mechanism of Insertional Inactivation**: When a foreign DNA is inserted into the lacZ gene, it disrupts the gene's function. This process is known as insertional inactivation. If the lacZ gene is inactivated, the bacteria will not produce the beta-galactosidase enzyme.

4. **Culturing with X-gal and IPTG**: The transformed bacteria are cultured on a medium containing X-gal and IPTG. X-gal is a substrate that, when cleaved by beta-galactosidase, produces a blue pigment. IPTG induces the expression of the lacZ gene.

5. **Observing Colony Color**:
- **White Colonies**: If the foreign DNA successfully inserted into the lacZ gene, the gene is inactivated, and the bacteria will not produce the blue pigment, resulting in white colonies. These white colonies indicate the presence of recombinant plasmids.
- **Blue Colonies**: If there is no insertion or the insertion is outside the lacZ gene, the gene remains functional, and the bacteria produce the blue pigment, resulting in blue colonies. These indicate non-recombinant plasmids.

6. **Conclusion**: Therefore, the bacteria in the white colonies represent those that have successfully taken up the recombinant plasmid containing the foreign DNA inserted into the lacZ gene, leading to its inactivation.

### Final Answer:
The bacteria in white colonies have recombinant plasmids.

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