For a DNA to funtion as a cloning vectors the most essential requirement is

Options

A. multiple restriction sites
B. several selectable markers
C. circular nature
D. ori' sequence

Answer

To determine the most essential requirement for a DNA to function as a cloning vector, we can break down the necessary components and functions of a cloning vector step by step.

### Step-by-Step Solution:

1. **Understanding Cloning Vectors**: Cloning vectors are DNA molecules used to transport foreign genetic material into a host cell. They are essential tools in biotechnology for gene cloning.

2. **Requirement for Replication**: For a cloning vector to be effective, it must be able to replicate within the host organism. This means that once the vector is introduced into the host, it should be able to multiply and produce multiple copies of itself along with the inserted foreign DNA.

3. **Origin of Replication (ORI)**: The key feature that allows a cloning vector to replicate is the presence of an "Origin of Replication" (ORI). The ORI is a specific sequence of DNA that signals the cellular machinery to start the replication process.

4. **Plasmids as Cloning Vectors**: Plasmids are commonly used as cloning vectors because they are small, circular pieces of DNA that naturally exist in bacteria. They typically contain an ORI, allowing them to replicate independently of the chromosomal DNA of the host.

5. **Conclusion**: Therefore, the most essential requirement for a DNA to function as a cloning vector is the presence of an Origin of Replication (ORI). Without this, the vector cannot replicate within the host, and the foreign DNA cannot be cloned.

### Final Answer:
The most essential requirement for a DNA to function as a cloning vector is the presence of an Origin of Replication (ORI).