Blue-white reaction is used as a method of selection of recombinants using the following substrate:

Options

A. X-gal
B. ethidium bromide
C. agarose gel
D. both (2) and (3)

Answer

**Step-by-Step Solution:**

1. **Understanding the Blue-White Reaction**: The blue-white reaction is a method used in molecular biology to differentiate between recombinant and non-recombinant bacteria. This method is particularly useful in cloning experiments.

2. **Identifying the Substrate**: The substrate used in the blue-white selection is X-gal. X-gal is a chromogenic substrate that is cleaved by the enzyme beta-galactosidase, which is produced by bacteria that do not have recombinant DNA inserted into the lacZ gene.

3. **Role of Beta-Galactosidase**: In the blue-white selection process, the lacZ gene encodes the enzyme beta-galactosidase. When the lacZ gene is intact (non-recombinant), beta-galactosidase is produced, which can then cleave X-gal to produce a blue product.

4. **Insertional Inactivation**: When recombinant DNA is inserted into the lacZ gene, it disrupts the gene's function, a phenomenon known as insertional inactivation. As a result, the bacteria containing the recombinant DNA do not produce beta-galactosidase.

5. **Observation of Colony Color**:
- Bacteria that are non-recombinant (with an intact lacZ gene) will produce blue colonies due to the activity of beta-galactosidase on X-gal.
- Bacteria that are recombinant (with the inserted DNA disrupting the lacZ gene) will produce white colonies because they do not produce beta-galactosidase and therefore do not cleave X-gal.

6. **Conclusion**: The blue-white selection method allows researchers to easily identify and select recombinant colonies (white colonies) from non-recombinant colonies (blue colonies) based on their color.

**Final Answer**: The blue-white reaction is used as a method of selection of recombinants using X-gal as a substrate.

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