Q.33 Which of the following statement(s) about eukaryotic DNA topoisomerase
is/are correct?
(A) Topoisomerase I creates transient single–strand breaks
(B) Topoisomerase I creates transient double–strand breaks
(C) Topoisomerase II creates transient single–strand breaks
(D) Topoisomerase II creates transient double–strand breaks
Correct answer: (A) and (D)
Eukaryotic DNA topoisomerases manage DNA supercoiling by creating transient breaks, with Type I handling single-strand breaks and Type II managing double-strand breaks.
Option Analysis
Option A: Correct. Topoisomerase I creates transient single-strand breaks in DNA, forming a covalent bond with the 3′-phosphate end to allow strand rotation and relaxation without ATP.
Option B: Incorrect. Topoisomerase I does not create double-strand breaks; it specifically targets one strand to relieve torsional stress.
Option C: Incorrect. Topoisomerase II creates double-strand breaks, not single-strand ones, enabling strand passage between duplexes.
Option D: Correct. Topoisomerase II introduces transient double-strand breaks, requiring ATP hydrolysis to pass one DNA segment through another before resealing.
Eukaryotic DNA topoisomerase enzymes are crucial for resolving supercoiling during replication and transcription in CSIR NET Life Sciences topics. These enzymes prevent DNA tangling by creating controlled, transient breaks—single-strand for Topoisomerase I and double-strand for Topoisomerase II—making them high-yield for competitive exams.
Key Mechanisms
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Topoisomerase I (Type IB in eukaryotes): Binds duplex DNA, cleaves one strand via transesterification with a tyrosine residue, allows swiveling, then religates. No ATP needed; changes linking number by 1.
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Topoisomerase II (Type IIA): Forms a dimeric complex, cleaves both strands (ATP-dependent), passes an intact duplex through the gate, and reseals. Essential for decatenation; changes linking number by 2.
Exam Relevance
In CSIR NET questions like Q.33, identifying that Topoisomerase I creates transient single-strand breaks (A) and Topoisomerase II creates double-strand breaks (D) scores full marks. Options B and C confuse the types, testing precise knowledge of eukaryotic specifics over prokaryotic (e.g., gyrase).
