How does the presence of an antibiotic-resistance selectable marker g…

The correct answer is Option (2) → It allows researchers to grow only the bacteria that have successfully taken up the foreign DNA.

A cloning vector needs a selectable marker. During transformation only a few bacterial cells take up the recombinant DNA; these are the transformants. Genes for resistance to antibiotics such as ampicillin or tetracycline are used as markers, because normal E. coli cells are not resistant to them. When the cells are grown on a medium with the antibiotic, the non-transformants die and only the transformants grow.

Option (1) It speeds up the replication of the foreign DNA. — Incorrect. The number of copies of the linked DNA is controlled by the origin of replication (ori), not by the marker gene.

Option (2) It allows researchers to grow only the bacteria that have successfully taken up the foreign DNA. — Correct. The antibiotic kills the cells that did not take up the vector, so only the transformants grow. This is how the marker helps the researcher pick them out.

Option (3) It enhances the expression of the foreign DNA. — Incorrect. A selectable marker only helps in selection. It does not make the foreign gene work any harder.

Option (4) It helps in the degradation of the foreign DNA. — Incorrect. The purpose of cloning is to keep and multiply the foreign DNA. The marker gene does not break it down.

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.”