What is the primary purpose of a selectable marker in genetic engineering?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → To identify and distinguish transformed cells from non-transformed cells
When foreign DNA is put into bacteria, only a few cells actually take it up. These are called transformants; the rest are non-transformants. A cloning vector therefore carries a selectable marker, usually a gene for resistance to an antibiotic such as ampicillin or tetracycline. On a medium with that antibiotic, only the transformed cells survive, so they can be identified and separated from the others.
Option (1) To enhance the replication of foreign DNA — Incorrect. Replication of the linked DNA is controlled by the origin of replication (ori), which also decides the copy number. The marker does not do this.
Option (2) To identify and distinguish transformed cells from non-transformed cells — Correct. The selectable marker helps in identifying and eliminating non-transformants and lets only the transformants grow.
Option (3) To increase the size of the foreign DNA fragment — Incorrect. No part of the vector is meant to make the foreign DNA bigger. The marker only helps to pick out the cells that received the vector.
Option (4) To decrease the growth rate of transformed cells — Incorrect. The marker does the reverse. It permits the growth of transformants on the selective medium, while the non-transformants fail to grow.
NCERT Extract:
Cloning Vectors – “… (ii) Selectable marker : In addition to ‘ori’, the vector requires a selectable marker, which helps in identifying and eliminating nontransformants and selectively permitting the growth of the transformants. Transformation is a procedure through which a piece of DNA is introduced in a host bacterium (you will study the process in subsequent section). Normally, the genes encoding resistance to antibiotics such as ampicillin, chloramphenicol, tetracycline or kanamycin, etc., are considered useful selectable markers for E. coli.”