What is the purpose of a selectable marker in a vector?

To identify and eliminate non-transformants

The correct answer is Option (3): To identify and eliminate non-transformants

A selectable marker in a vector serves the purpose of allowing the identification and elimination of non-transformed host cells and selectively permitting the growth of transformed host cells. Transformation is the process of introducing a piece of foreign DNA into a host cell. After transformation, only a small fraction of host cells will successfully take up the recombinant DNA and become transformed (containing the foreign DNA).

Selectable markers are usually genes that confer resistance to specific antibiotics or other selection agents. The normal host cells do not carry resistance against these antibiotics, while the transformed cells, which have taken up the vector with the selectable marker, will be able to survive and grow in the presence of the antibiotic or selection agent.

Thus, when transformed cells are plated on a medium containing the antibiotic or selection agent, only the transformed cells (recombinants) will survive and form colonies, while the non-transformed cells (non-recombinants) will die. This allows researchers to easily identify and select the cells that have successfully taken up the vector and the desired foreign DNA.