Q.38 Topoisomerase II introduces the :

1. Single stranded supercoils in DNA
2. Both positive and negative supercoils in DNA
3. Negative supercoils in DNA
4. Positive supercoils in DNA

   Topoisomerase II Introduces Both Positive and Negative Supercoils in DNA

   Topoisomerase II uses double-strand break and strand-passage mechanism to relieve both positive and negative supercoils, making option (2) correct. This type II enzyme is essential for DNA replication, transcription, and chromosome condensation.

   Introduction

   Topoisomerase II introduces both positive and negative supercoils through its double-strand breakage and religation mechanism, distinguishing it from type I topoisomerases. This bidirectional activity is crucial during DNA replication (positive supercoils ahead) and transcription (negative supercoils behind). This guide analyzes each option for molecular biology exam precision.

   Option Analysis

   Topoisomerase classification:

   • Single stranded supercoils in DNA: Incorrect. Type I topoisomerases (topo IA: gyrase, topo IB: topo I) create single-strand breaks. Topo II requires double-strand breaks.​

   • Both positive and negative supercoils in DNA: Correct. Topo II strand-passage mechanism relaxes both supercoil types via ATP-dependent double-strand break, T-segment passage, religation. Essential for replication fork progression.​

   • Negative supercoils in DNA: Incorrect (incomplete). While bacterial gyrase introduces negative supercoils, eukaryotic topo II primarily relaxes both types. Bidirectional capability defines topo II.

   • Positive supercoils in DNA: Incorrect. Topo II removes positive supercoils generated ahead of replication/transcription forks, doesn’t introduce them.

   Mechanism Comparison

   Enzyme	Break Type	ATP Required	Supercoil Activity	Key Role
   Topo I	Single strand	No	Negative only	Transcription
   Topo II	Double strand	Yes	Both +/-	Replication
   Gyrase	Double strand	Yes	Introduces negative	Bacteria initiation

   Topo II strand-passage cycle:

   text

   1. ATP binding → G-segment cleavage (double-strand break)
2. T-segment capture → strand passage through break
3. Religation → ATP hydrolysis → enzyme reset


   Replication context: +1 supercoil per 10 bp unwound → topo II removes ~10^6 supercoils/round.

   Biological Functions

   • Chromosome segregation: Decatenation (unlinks sister chromatids)

   • Condensation: Scaffold association during mitosis

   • Drugs: Etoposide stabilizes cleavage complex → apoptosis

   Exam distinction: Topo II = DSB + ATP + both supercoils vs Topo I = SSB + no ATP + negative only.

   Exam Relevance

   GATE Life Sciences: “TWO = Topo II (double-strand breaks, both supercoils).” Mnemonic: “DOUBLE duty – DOUBLE strand breaks, DOUBLE supercoils (+/-).” Classic trap: gyrase specificity vs eukaryotic topo II universality.