Restriction Enzymes & DNA Cutting Sites — NEET Biology

Correct answer: B — S1, S3, S4 and S5

The core concept here is the mechanism of blue-white screening, an alternative selectable marker system used to differentiate recombinant from non-recombinant clones. S1: Correct. Blue-white screening uses insertional inactivation. When foreign DNA is inserted into the coding sequence of the lacZ gene (which codes for β-galactosidase), it inactivates the gene. (NCERT Page 170, (iii) Cloning sites) S2: Incorrect. Non-recombinant colonies contain plasmids without an insert, thus their lacZ gene is functional. They produce β-galactosidase which cleaves the chromogenic substrate (X-gal) to give blue colonies, not white. White colonies are typically recombinant. (NCERT Page 170, (iii) Cloning sites) S3: Correct. The presence of a chromogenic substrate (like X-gal) allows β-galactosidase to cleave it, resulting in a blue-coloured product. (NCERT Page 170, (iii) Cloning sites) S4: Correct. When a foreign DNA is inserted into the lacZ gene, it causes insertional inactivation, meaning no functional β-galactosidase is produced. Therefore, the chromogenic substrate is not cleaved, and the colonies remain white, identifying them as recombinant. (NCERT Page 170, (iii) Cloning sites) S5: Correct. The NCERT states that selection of recombinants due to inactivation of antibiotics is a cumbersome procedure requiring simultaneous plating on two plates, and thus alternative selectable markers like blue-white screening (based on colour production) were developed as they differentiate recombinants from non-recombinants based on colour. (NCERT Page 170, (iii) Cloning sites) Therefore, statements S1, S3, S4, and S5 are correct. The correct option is B as it includes all the correct statements (S1, S3, S4, S5). Options A, C, and D are incorrect because they either include the incorrect statement (S2) or miss correct statements.