Which of the following gene helps in identifying transformed cells?

The correct answer is selectable marker.

Step-by-step Explanation:

1. Understanding DNA Transformation:
In recombinant DNA technology, transformation is the process by which a host cell takes up foreign DNA. However, this process is highly inefficient, meaning only a very small fraction of host cells successfully take up the recombinant plasmid (these are called transformants), while the majority remain untransformed (non-transformants).

2. Role of a Vector:
A cloning vector (like a plasmid) must have certain essential features to facilitate this process. These features include an origin of replication (ori), cloning sites, and a selectable marker.

3. Function of a Selectable Marker:
A selectable marker is a gene that helps in identifying and selecting transformants while eliminating non-transformants. It allows only the cells containing the vector to grow under specific selection conditions. For example, genes encoding resistance to antibiotics such as ampicillin, chloramphenicol, tetracycline, or kanamycin are commonly used as selectable markers in Escherichia coli, as normal E. coli cells do not carry resistance against these antibiotics.

4. Evaluating Other Options:
- Plasmid: A circular extrachromosomal DNA molecule used as a vector to transfer genetic material, but it is not the specific gene that helps in identification.
- Structural gene: A gene that codes for any RNA or protein product other than a regulatory factor (e.g., the actual gene of interest being cloned).
- Vector: The vehicle DNA molecule used to carry foreign genetic material into another cell.