In the above represented plasmid an alien piece of DNA is inserted at EcoRI site. Which of the following strategies will be chosen to select the recombinant colonies?
Answer: (A) White color colonies will be selected. Answer: (A) White color colonies will be selected Solution: When alien DNA is inserted at a restriction site within the β-galactosidase coding sequence, insertional inactivation occurs — recombinant colonies cannot cleave the chromogenic substrate and so remain colourless/white, while non-recombinants (intact β-galactosidase) turn blue.
- A.White color colonies will be selected✓
- B.Blue color colonies grown on ampicillin
- C.Using ampicillin & tetracyclin containing medium plate.
- D.Blue color colonies will be selected.
Correct Answer
(A) White color colonies will be selected
Solution & Explanation
Answer: (A) White color colonies will be selected Solution: When alien DNA is inserted at a restriction site within the β-galactosidase coding sequence, insertional inactivation occurs — recombinant colonies cannot cleave the chromogenic substrate and so remain colourless/white, while non-recombinants (intact β-galactosidase) turn blue. Hence recombinants are identified as the white colonies, not blue. Antibiotic double-plating options describe the older pBR322 method, not this blue-white screen. NCERT Reference: Ch 9, p.170, line(s) 16-18 — "β-galactosidase gene and the colonies do not produce any colour,"