Restriction Enzymes & Sticky Ends — NEET Biology
📚 Practice ConceptIn 1966, Boyer took over assistant professorship at the University of California at San Francisco. By 1969, he performed studies on a couple of restriction enzymes of the E. coli bacterium with especially useful properties. Boyer observed that these enzymes have the capability of cutting DNA strands in a particular fashion, which left what has become known as 'sticky ends' on the strands. These clipped ends made pasting together pieces of DNA a precise exercise.
Which of the following statements about restriction enzymes and the DNA fragments they generate are NOT correct? S1: Restriction enzymes always cut DNA molecules at the exact center of palindromic recognition sequences. S2: Sticky ends are single-stranded overhangs that readily form hydrogen bonds with complementary sequences. S3: The formation of recombinant DNA molecules requires cutting both the vector and the alien DNA with different restriction enzymes to ensure specific ligation. S4: The stickiness of DNA ends facilitates the action of DNA ligase by directly promoting stable covalent bond formation between the sugar-phosphate backbones. S5: Exonucleases remove nucleotides from the ends of DNA, while endonucleases make cuts at specific positions within the DNA.
View solution & NCERT explanation
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.
What does NCERT say about 1966 Boyer took over?
Has this concept appeared in NEET?
Which chapter is this from?
Through deep analysis of NEET and NTA, 176 of the 180 ReNEET 2026 (June 21) questions were already in the MedicNEET question bank before the exam.
176/180MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.