Q.31 Pick the correct statement(s) with respect to the inter-conversion of the topoisomers of a circularly closed double stranded DNA.
- (A) Only one strand needs to be cut
- (B) Both strands have to be cut
- (C) No strand needs to be cut
- (D) ATP is required for inter-conversion
Correct answer: (A) and (D)
Type I topoisomerases enable inter-conversion of topoisomers in circularly closed double-stranded DNA by cutting only one strand, while type II enzymes require cutting both strands; ATP is needed specifically for type II activities like those of DNA gyrase.
Option Analysis
Option (A): Only one strand needs to be cut
Type I topoisomerases (IA and IB) create a transient single-strand break, allowing rotation to relax supercoils and interconvert topoisomers without altering linking number by 2. This mechanism supports inter-conversion, making (A) correct.Option (B): Both strands have to be cut
Type II topoisomerases cut both strands to pass another double helix through, changing linking number by 2; however, type I enzymes achieve inter-conversion without this, so (B) does not apply universally.Option (C): No strand needs to be cut
All topoisomerases require strand breakage (single or double) for strand passage or swivel mechanisms; no enzyme inter-converts topoisomers of closed circular DNA without cutting.Option (D): ATP is required for inter-conversion
Type I topoisomerases work without ATP, but type II (including gyrase) hydrolyze ATP to drive strand passage and supercoiling; since the question allows multiple correct statements, (D) holds for type II-mediated inter-conversions.Introduction to DNA Topoisomers
Circularly closed double-stranded DNA forms topoisomers differing in linking number (Lk), twist (Tw), and writhe (Wr), where Lk = Tw + Wr remains constant without topoisomerases. Inter-conversion between relaxed, positively, or negatively supercoiled forms occurs via enzymatic strand manipulation during replication and transcription.
Type I Topoisomerases: Single-Strand Mechanism
These enzymes cut one DNA strand, forming a phosphotyrosyl intermediate, allowing swivel or passage to relax supercoils by ±1 Lk units without ATP. Eukaryotic Topo IB and bacterial Topo IA exemplify this for efficient topoisomer inter-conversion.
Type II Topoisomerases: Double-Strand and ATP-Dependent
Type II enzymes cleave both strands (G-segment), pass another duplex (T-segment) through, and religate, altering Lk by ±2; ATP hydrolysis closes the N-gate for strand passage. DNA gyrase introduces negative supercoils, essential for bacterial topology.
CSIR NET Exam Insights
For questions like “inter-conversion of the topoisomers of a circularly closed double stranded DNA,” select (A) for Type I action and (D) for Type II ATP needs; (B) and (C) are mechanism-specific. Practice distinguishes these for competitive exams.
