In order to insert alien DNA at the site of tetracycline resistance gene in pBR322 the restriction endonuclease used can be :

Options

A. Pvu I
B. Cla I
C. SaI I
D. Pst I

Answer

To solve the question regarding the restriction endonuclease that can be used to insert alien DNA at the site of the tetracycline resistance gene in the plasmid pBR322, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding pBR322**: pBR322 is a plasmid that contains genes for antibiotic resistance, specifically for tetracycline (Tet) and ampicillin (Amp). These genes serve as selectable markers.

2. **Identifying the Target Site**: The tetracycline resistance gene is located in a specific region of the pBR322 plasmid. To insert alien DNA, we need to target this specific site.

3. **Role of Restriction Endonucleases**: Restriction endonucleases (or restriction enzymes) are used to cut DNA at specific sequences. By using a restriction enzyme that cuts within the tetracycline resistance gene, we can create a site for the insertion of foreign DNA.

4. **Insertional Inactivation**: When the foreign DNA is inserted into the tetracycline resistance gene, it disrupts the gene's function, leading to the loss of tetracycline resistance. This process is known as insertional inactivation.

5. **Selecting Recombinant Plasmids**: After the insertion, we can differentiate between recombinant and non-recombinant plasmids. Non-recombinant plasmids will grow in both tetracycline and ampicillin, while recombinant plasmids will only grow in ampicillin, as they have lost tetracycline resistance.

6. **Choosing the Correct Restriction Enzyme**: The restriction enzyme that can cut at the tetracycline resistance gene site in pBR322 is SalI, which is known to recognize and cut specific sequences within that region.

7. **Final Answer**: Therefore, the correct restriction endonuclease to use for inserting alien DNA at the site of the tetracycline resistance gene in pBR322 is **SalI**.