In the light of the above statements, choose the correct answer from the options given below:

1. Both Statement I and Statement II are correct
2. Both Statement I and Statement II are incorrect
3. Statement I is correct but Statement II is incorrect
4. Statement I is incorrect but Statement II is correct

   Statement I is correct but Statement II is incorrect.

   Question Breakdown

   This question evaluates understanding of DNA topology. Statement I defines supercoiling accurately, while Statement II misstates DNA gyrase function—it’s the opposite of what’s claimed.

   Statement Analysis

   Statement I (Correct): DNA supercoiling occurs when the DNA double helix experiences torsional stress from over- or under-winding beyond its relaxed B-form state (10.4 bp/turn). This creates compensatory writhe (interwinding) to relieve twist strain, forming plectonemic supercoils. Negative supercoiling unwinds DNA (facilitates transcription); positive supercoiling overwinds it.​

   Statement II (Incorrect): DNA gyrase (topoisomerase II) introduces negative supercoils into relaxed/overwound DNA using ATP hydrolysis, not relaxes positive supercoils. It creates a transient double-strand break, passes DNA segments through, and reseals—actively underwinding DNA (ΔLk = -2 per cycle). Topoisomerase I relaxes both positive/negative supercoils without ATP.​

   Why Other Options Are Wrong

   • Both correct: Statement II factually wrong about gyrase function.

   • Both incorrect: Statement I accurately defines supercoiling via twisting forces.

   • Statement I incorrect, II correct: Reverses both truths; gyrase doesn’t relax positive supercoils.

   DNA supercoiling formation due to twisting forces drives bacterial chromosome compaction and gene regulation. This analysis clarifies Statement I (correct): “Formation of additional coils due to twisting forces is referred to as DNA supercoiling” vs Statement II (incorrect): “DNA gyrase relaxes positive supercoiled DNA”—critical for molecular biology competitive exams.​

   DNA Topology Fundamentals

   Lk = Tw + Wr (Linking number = Twist + Writhe). Relaxed DNA: Lk₀ = N/10.4 (N = bp). Supercoiling: σ = (Lk – Lk₀)/Lk₀.

   • Negative supercoiling (σ < 0): Underwinds DNA (-7000 supercoils in E. coli)

   • Positive supercoiling (σ > 0): Overwinds DNA

   DNA Gyrase Mechanism (Statement II Error)

   text

   Relaxed DNA → [ATP] → DNA Gyrase → Negatively Supercoiled DNA
(not: Positive → Relaxed)


   Gyrase cycle:

   1. DNA wraps around gyrase (ATP binding)

   2. Double-strand break (tyrosine nucleases)

   3. T-segment passes through G-segment

   4. Resealing (ΔLk = -2)​

   Topoisomerase I: Relaxes both (+/-) supercoils, no ATP, single-strand breaks.

   Enzyme	Function	ATP?	Supercoiling Change
   DNA Gyrase (Topo II)	Introduces negative	Yes	ΔLk = -2
   Topoisomerase I	Relaxes (+/-)	No	ΔLk → 0
   Topoisomerase IV	Decatenation	Yes	Relaxes positive​

   DNA gyrase maintains -6% superhelical density in bacteria, essential for replication/transcription initiation.​