A selectable marker in a cloning vector, such as the ampicillin resis…

The correct answer is Option (1) → Non-transformants

When DNA is put into bacteria (transformation), only a few cells take up the plasmid. These are the transformants; the rest are non-transformants. A selectable marker is a gene on the vector that lets the scientist tell them apart. Normal E. coli cells are not resistant to antibiotics, so an antibiotic resistance gene such as amp^R works well as a marker.

Option (1) Non-transformants — Correct. On a medium with ampicillin, only cells that carry pBR322, with its amp^R gene, can grow. Cells without the plasmid die and are eliminated. So the marker identifies and removes the non-transformants.

Option (2) Transformants — Incorrect. Transformants are the cells the marker allows to grow, since they carry the resistance gene. The marker keeps them; it does not eliminate them.

Option (3) Recombinants — Incorrect. Recombinants carry the plasmid too, so they also survive on ampicillin. They are told apart from non-recombinants by a second marker, such as the loss of tetracycline resistance when the foreign DNA is inserted into tet^R.

Option (4) Cells with a high copy number of the plasmid — Incorrect. The number of copies of a plasmid in a cell is controlled by the origin of replication (ori), not by the selectable marker. Such cells also survive on ampicillin.

NCERT Extract:
Cloning Vectors – “… (ii) Selectable marker : In addition to ‘ori’, the vector requires a selectable marker, which helps in identifying and eliminating non-transformants 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. The normal E. coli cells do not carry resistance against any of these antibiotics.”

Figure 9.4 E. coli cloning vector pBR322 showing restriction sites (Hind III, EcoR I, BamH I, Sal I, Pvu II, Pst I, Cla I), ori and antibiotic resistance genes (amp^R and tet^R). rop codes for the proteins involved in the replication of the plasmid