Understanding:
This question asks about the functional role of selectable markers in recombinant DNA cloning experiments.
Step 1: The challenge of transformation
When recombinant plasmids are introduced into competent host cells (transformation), the uptake efficiency is low — only a small fraction of cells actually take up the plasmid. Selectable markers provide a way to distinguish these successful transformants from the vast majority of untransformed cells.
Step 2: How selectable markers work
A commonly used selectable marker is an antibiotic resistance gene (e.g., ampicillin resistance — bla gene encoding beta-lactamase). After transformation, all cells are plated on growth medium containing the antibiotic. Only cells harbouring the plasmid — and thus expressing the resistance gene — survive and form colonies. Cells without the plasmid are killed.
Step 3: Insertional inactivation as a refinement
In vectors like pUC19, the lacZ-alpha gene serves as a second marker. If the foreign insert disrupts the lacZ gene, colonies appear white (no functional beta-galactosidase) rather than blue (functional enzyme), allowing blue-white screening to distinguish recombinant from non-recombinant plasmids.
Step 4: Why other options are incorrect
The origin of replication is a separate functional element. Restriction enzymes are supplied externally in the experiment, not encoded by the vector marker. Promoter sequences for gene expression are separate elements from selectable markers.
Answer:
A selectable marker allows identification and selection of host cells that have successfully taken up the recombinant plasmid.
Selection of host cells that have taken up the recombinant plasmid
Quick Tip:
Insertional inactivation into the ampicillin resistance gene can also be used — cells with intact plasmid (no insert) are AmpR, while those with insert-disrupted AmpR gene are AmpS. However, blue-white screening via lacZ is now more common in modern vectors.
