The most important feature in a plasmid to be used as a vector is :

Options

A. origin of replication (ori)
B. presence of a selectable marker
C. presence of sites for restriction endo- nuclease
D. its size

Answer

To determine the most important feature in a plasmid to be used as a vector, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Plasmids**:
- Plasmids are small, circular pieces of DNA found in bacteria, separate from the chromosomal DNA. They can replicate independently and are often used in genetic engineering as vectors to introduce foreign genes into host cells.

2. **Identifying Key Features of Plasmids**:
- Plasmids have several important features:
- **Origin of Replication (ori)**: This is the site where replication begins, allowing the plasmid to be copied within the host cell.
- **Selectable Markers**: These are genes that confer resistance to antibiotics, allowing for the selection of cells that have taken up the plasmid.
- **Restriction Sites**: These are specific sequences where restriction enzymes can cut the plasmid, allowing for the insertion of foreign DNA.
- **Size**: The size of the plasmid can affect its stability and ease of manipulation.

3. **Evaluating the Importance of Each Feature**:
- While all features are important for different reasons, the **Origin of Replication (ori)** is crucial because:
- Without the ori, the plasmid cannot replicate. If the plasmid does not replicate, it cannot be passed on to daughter cells during cell division, which is essential for the propagation of the desired genetic material.
- The ability to replicate is fundamental for any plasmid to function as a vector in genetic engineering.

4. **Conclusion**:
- Therefore, the most important feature in a plasmid to be used as a vector is the **Origin of Replication (ori)**.

### Final Answer:
The most important feature in a plasmid to be used as a vector is the **Origin of Replication (ori)**.

---