A good cloning vector should have all except :-

Options

A. Origin of replication to start replication
B. Many recognition sites for single restriction enzyme
C. High copy number
D. Selectable marker

Answer

To determine which characteristic is not essential for a good cloning vector, we can analyze the options provided:

1. **Origin of Replication**: A good cloning vector must have an origin of replication (ori) to ensure that it can replicate within the host cell. This is essential for the vector to propagate and produce multiple copies of the inserted DNA.

2. **Many Recognition Sites for a Single Restriction Enzyme**: This option is problematic. A good cloning vector should not have many recognition sites for a single restriction enzyme because this would lead to the vector being cut into multiple fragments, making it difficult to insert foreign DNA. Instead, a cloning vector should have multiple recognition sites for different restriction enzymes, allowing for more flexibility in cloning.

3. **High Copy Number**: A good cloning vector should have a high copy number, which means it can replicate many times within the host cell. This is important for producing a sufficient quantity of the cloned DNA.

4. **Selectable Marker**: A selectable marker is crucial for identifying which cells have successfully taken up the cloning vector. This usually involves antibiotic resistance genes that allow for the selection of transformed cells.

Based on this analysis, the correct answer to the question "A good cloning vector should have all except" is:

**Many recognition sites for a single restriction enzyme.**

### Step-by-Step Solution:
1. Identify the characteristics of a good cloning vector.
2. Analyze each option provided in the question.
3. Determine that an origin of replication, high copy number, and selectable markers are essential.
4. Conclude that having many recognition sites for a single restriction enzyme is not desirable as it complicates the cloning process.
5. State the answer clearly.