The following diagram showing restriction sites in E. coli cloning vector pBR322. Find the role of ‘X’ and ‘Y’ genes :

Correct Option: The gene ‘X’ is responsible for controlling the copy number of the linked DNA and ‘Y’ for protein involved in the replication of Plasmid.

To understand the roles of the regions labeled ‘X’ and ‘Y’ in the cloning vector pBR322, let us analyze the provided diagram and the standard structure of this plasmid:
1. Identifying the Vector: The diagram shows the circular double-stranded DNA plasmid pBR322, which is a widely used E. coli cloning vector. The diagram explicitly labels several restriction enzyme recognition sites, including:

• EcoR I, Cla I, and Hind III near the top region.
• BamH I and Sal I on the right side.
• Pvu II at the bottom.
• Pst I and Pvu I on the left side.

2. Analyzing Label ‘X’ (Origin of Replication - ori):
The label ‘X’ points to the segment of the plasmid known as the origin of replication (ori).
- The ori is a specific sequence where replication initiates.
- When a foreign DNA fragment is ligated to this vector, the host machinery recognizes the ori sequence to begin replicating the vector along with the linked foreign DNA.
- Importantly, the origin of replication is also the genetic element responsible for controlling and regulating the copy number of the linked cloning vector (and consequently, the foreign target gene) inside the host cell.

3. Analyzing Label ‘Y’ (rop Gene):
The label ‘Y’ points to the region labeled as the rop gene (Repressor of Primer) situated near the Pvu II site at the bottom.
- The rop gene encodes a small protein (Rop protein) that acts to regulate plasmid replication.
- Specifically, the Rop protein is involved in maintaining and stabilizing the interaction between RNA I and RNA II, which regulates the initiation of plasmid replication, thus helping in controlling the copy number of the plasmid.

4. Conclusion:
Based on the functions of the vector components:
- ‘X’ (ori) is responsible for controlling the copy number of the linked DNA.
- ‘Y’ (rop) codes for the protein involved in the replication of the plasmid.
Therefore, the correct choice aligns perfectly with these definitions.