Q.27 What are the function of topoisomerases?
1. Repair the chromosome ends
2.Fonn DNA isomers
3. Change the Linking Number of DNA
4. Fonn RNA isomers
Answer: Option 3 (Change the Linking Number of DNA).
Topoisomerases regulate DNA supercoiling by changing the linking number (Lk)—the number of times one DNA strand crosses the other. Type I topoisomerases change Lk by ±1 (single-strand break), Type II by ±2 (double-strand break).
Option Analysis
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1. Repair the chromosome ends: Wrong. Telomerase (reverse transcriptase) extends telomeres; topoisomerases don’t repair ends.
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2. Form DNA isomers: Wrong. Creates topoisomers (same sequence, different topology), not constitutional isomers (different connectivity).
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3. Change the Linking Number of DNA: Correct. Lk = Twist + Writhe. Topoisomerases alter Lk via transient phosphotyrosyl bonds.
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4. Form RNA isomers: Wrong. DNA enzymes only; RNA handled by different mechanisms.
Topoisomerase Mechanism Table
| Type | Break | ΔLk | Examples |
|---|---|---|---|
| Type I | 1 strand | ±1 | Topo I, III |
| Type II | 2 strands | ±2 | DNA gyrase, Topo IV |
Topoisomerase function change linking number DNA is essential GATE Life Sciences knowledge for DNA replication, transcription, chromosome segregation. Primary role: alter Lk via controlled strand breakage.
DNA Topology Fundamentals
Linking Number (Lk) = Twist (Tw) + Writhe (Wr)
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Tw: Helical turns (~10.5 bp/turn B-DNA)
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Wr: Superhelical coiling (negative in vivo)
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ΔLk = -1 (Type I), ΔLk = -2 (Type II)
Mechanism: Phosphotyrosyl Intermediate
1. Tyr attacks DNA phosphate → 5'-phosphotyrosine bond
2. Strand rotation/passage through break
3. Reverse transesterification → DNA resealed
Biological Roles Table
| Process | Topoisomerase Needed | Why? |
|---|---|---|
| Replication | Type II (Gyrase) | Unwind 2n turns ahead of fork |
| Transcription | Type I | Relieve RNA pol tension |
| Chromosome segregation | Type II | Decatenation |
GATE Life Sciences Pattern
“Function of topoisomerases” = linking number change. Traps test telomerase confusion (chromosome ends). Links to your nucleosome series (DNA packaging creates supercoiling → topoisomerases relieve).
Memory aid: “Topo = Topology = Lk change”. Jaipur lab: Etoposide (Topo II inhibitor) used in cancer research. Master topoisomerase function for guaranteed molecular biology scores.
