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 to differentiate between recombinant and non-recombinant bacteria. This is crucial in molecular cloning to identify which bacteria have successfully taken up a plasmid containing foreign DNA.

2. **Role of Beta-Galactosidase:**
The blue-white selection involves the enzyme beta-galactosidase. When a plasmid is successfully taken up by a bacterium, it may contain a gene that encodes for this enzyme.

3. **Substrate X-gal:**
X-gal is the substrate for beta-galactosidase. When beta-galactosidase is active (produced by non-recombinant bacteria), it hydrolyzes X-gal, resulting in a blue color. This indicates that the bacteria are non-recombinant.

4. **Insertional Inactivation:**
If foreign DNA is inserted into the plasmid at the site where the beta-galactosidase gene is located, it disrupts the gene, leading to a phenomenon known as insertional inactivation. This means that the enzyme is not produced.

5. **Color Indication:**
In bacteria that have taken up the recombinant plasmid (with foreign DNA inserted), beta-galactosidase is not produced, and thus, X-gal is not hydrolyzed. As a result, these bacteria will remain white, indicating that they are recombinant.

6. **Conclusion:**
Therefore, the substrate used in the blue-white reaction for the selection of recombinants is X-gal. Other options like ethidium bromide and agarose gel are not relevant to the blue-white selection process.

**Final Answer:**
The substrate used in the blue-white reaction for the selection of recombinants is **X-gal**.

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