Question:
Blue-white colonies screening of transformed E. coli cells involved disruption of which of the following gene of cloning vector?
Blue-white colonies screening of transformed E. coli cells involved disruption of which of the following gene of cloning vector?
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Blue = Non-recombinant (gene intact).
White = Recombinant (gene disrupted).
Think: "White is Right" (for the researcher seeking recombinants).
White = Recombinant (gene disrupted).
Think: "White is Right" (for the researcher seeking recombinants).
Updated On: Aug 21, 2026
- X gal
- Kanamycin
- Ampicillin
- Lac Z’
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The Correct Option is D
Solution and Explanation
Step 1: Understanding the Concept:
Blue-white screening is a rapid technique for identifying recombinant bacterial colonies after transformation. It utilizes the principle of alpha-complementation of the enzyme \(\beta\)-galactosidase.
Step 2: Detailed Explanation:
The cloning vector (like pUC19) contains a gene called {lacZ' (an alpha-fragment of the lacZ gene). In an E. coli strain that lacks this fragment but has the rest of the lacZ gene, the two parts combine (alpha-complementation) to form a functional \(\beta\)-galactosidase enzyme. This enzyme cleaves the substrate X-gal added to the medium, producing a blue pigment.
In genetic engineering, the Multiple Cloning Site (MCS) is located inside the lacZ' gene. When a foreign DNA fragment is ligated into the MCS, it disrupts the lacZ' coding sequence (insertional inactivation). As a result, the lacZ' peptide is not produced, alpha-complementation fails, no functional enzyme is formed, and the colony remains white.
- X-gal is the substrate, not a gene.
- Ampicillin and Kanamycin are antibiotic resistance genes used for selection (surviving vs dying), but they do not distinguish between recombinant and non-recombinant survivors.
Step 3: Final Answer:
The lacZ' gene is disrupted by DNA insertion, leading to the white color in recombinant colonies. Option 4 is correct.
Blue-white screening is a rapid technique for identifying recombinant bacterial colonies after transformation. It utilizes the principle of alpha-complementation of the enzyme \(\beta\)-galactosidase.
Step 2: Detailed Explanation:
The cloning vector (like pUC19) contains a gene called {lacZ' (an alpha-fragment of the lacZ gene). In an E. coli strain that lacks this fragment but has the rest of the lacZ gene, the two parts combine (alpha-complementation) to form a functional \(\beta\)-galactosidase enzyme. This enzyme cleaves the substrate X-gal added to the medium, producing a blue pigment.
In genetic engineering, the Multiple Cloning Site (MCS) is located inside the lacZ' gene. When a foreign DNA fragment is ligated into the MCS, it disrupts the lacZ' coding sequence (insertional inactivation). As a result, the lacZ' peptide is not produced, alpha-complementation fails, no functional enzyme is formed, and the colony remains white.
- X-gal is the substrate, not a gene.
- Ampicillin and Kanamycin are antibiotic resistance genes used for selection (surviving vs dying), but they do not distinguish between recombinant and non-recombinant survivors.
Step 3: Final Answer:
The lacZ' gene is disrupted by DNA insertion, leading to the white color in recombinant colonies. Option 4 is correct.
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