Restriction Endonucleases: DNA Cutting — NEET Biology

Correct answer: C — S1, S3 and S4 only

The core concept here relates to the function of restriction enzymes and the nature of sticky ends. Let's evaluate each statement: S1: Incorrect. NCERT states, "Restriction enzymes cut the strand of DNA a little away from the centre of the palindrome sites, but between the same two bases on the opposite strands." Therefore, they do not always cut at the exact center. S2: Correct. NCERT mentions, "There are overhanging stretches called sticky ends on each strand...These are named so because they form hydrogen bonds with their complementary cut counterparts." These are indeed single-stranded overhangs. S3: Incorrect. NCERT states, "normally, unless one cuts the vector and the source DNA with the same restriction enzyme, the recombinant vector molecule cannot be created." Cutting with different enzymes would prevent complementary sticky ends from forming. S4: Incorrect. Sticky ends form hydrogen bonds, which bring the DNA fragments together in the correct orientation. This 'stickiness' facilitates the action of DNA ligase, which then forms the stable phosphodiester (covalent) bonds. Sticky ends themselves do not directly form covalent bonds, they enable the conditions for ligase to act. The statement misrepresents the direct action. S5: Correct. NCERT clarifies, "Exonucleases remove nucleotides from the ends of the DNA whereas, endonucleases make cuts at specific positions within the DNA." Based on the analysis, S1, S3, and S4 are not correct. Option A is incorrect because S4 is also not correct. Option B is incorrect because S2 and S5 are correct, and S4 is not correct. Option C is correct as S1, S3, and S4 are the incorrect statements. Option D is incorrect because S2 is correct. The tested concept is the precise mechanism of restriction enzyme action, the characteristics of sticky ends, and the process of ligation in recombinant DNA technology.