Option (b) is the correct answer. Cloning vectors are the carriers of the desired gene in the host cell. The features desirable in a cloning vector are:-

• Presence of origin of replication
• Presence of marker genes
• Presence of very few, preferably single recognition site for the commonly used restriction enzymes

Not a Desirable Feature of a Cloning Vector: Presence of Two or More Recognition Sites

Cloning vectors are DNA molecules used to carry foreign DNA fragments into host cells during the process of molecular cloning. These vectors play a crucial role in genetic engineering experiments by allowing the replication and expression of foreign DNA fragments in host organisms.

There are several desirable features of a cloning vector that make it efficient and effective for cloning purposes. However, the presence of two or more recognition sites is not one of these desirable features.

Presence of Single Restriction Enzyme Site:
- A restriction enzyme site is a specific DNA sequence recognized and cleaved by a restriction enzyme.
- The presence of a single restriction enzyme site in a cloning vector is essential for the insertion of a foreign DNA fragment.
- The restriction enzyme recognizes and cuts the vector at the site, allowing the insertion of the desired DNA fragment.
- If a cloning vector lacks a restriction enzyme site, it cannot be used for cloning purposes.

Presence of Origin of Replication:
- The origin of replication is a DNA sequence that initiates DNA replication in a host cell.
- It allows the vector to replicate independently within the host cell, ensuring the production of multiple copies of the cloned DNA fragment.
- The presence of an origin of replication is crucial for the amplification and propagation of the cloned DNA.

Presence of a Marker Gene:
- A marker gene is a gene that confers a selectable phenotype to the host cell.
- It allows researchers to distinguish between cells that have taken up the vector with the foreign DNA and those that have not.
- The presence of a marker gene in a cloning vector enables the selection and identification of cells carrying the desired DNA fragment.

Presence of Two or More Recognition Sites (Not Desirable):
- The presence of two or more recognition sites in a cloning vector can lead to multiple insertions of foreign DNA fragments.
- Multiple insertions can result in the loss of the original DNA fragment or rearrangements within the vector.
- This can complicate the analysis and interpretation of experimental data.
- Therefore, it is not desirable to have two or more recognition sites in a cloning vector.

In conclusion, the presence of two or more recognition sites is not a desirable feature of a cloning vector. The presence of a single restriction enzyme site, an origin of replication, and a marker gene are essential for efficient cloning and propagation of the desired DNA fragment.