Biology Biotechnology PMDC Conceptual Practice
PMDC Verified Question 6 of 65
In recombinant DNA selection, non-recombinant colonies appear blue on X-gal media due to expression of:
A
Ampicillin resistance
B
DNA ligase
C
\( \beta \)-galactosidase
D
Reverse transcriptase
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: \( \beta \)-galactosidase
Concept:

Blue-white screening relies on \( \alpha \)-complementation of the lacZ gene product.

Solution:

Non-recombinant plasmids maintain an intact lacZ gene, which expresses functional \( \beta \)-galactosidase. This enzyme hydrolyzes the chromogenic substrate X-gal, turning non-recombinant colonies blue.

Why other options are incorrect:

  • Ampicillin resistance: Conferred by the ampR gene, allowing cells to survive on antibiotic agar, but does not impart a blue color.


  • DNA ligase: Catalyzes phosphodiester bond formation in ligation reactions; does not cleave X-gal.


  • Reverse transcriptase: Synthesizes cDNA from RNA templates and plays no role in bacterial blue-white selection.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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