6 public questions tagged with this topic.
Topoisomerase prevents excessive supercoiling of DNA ahead of the replication fork by cutting and rejoining the DNA strands.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance DNA Replication - Semiconservative and Machinery
Topoisomerase prevents the overwinding of DNA ahead of the replication fork by relieving torsional strain.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance DNA Replication - Semiconservative and Machinery
Topoisomerase relieves supercoiling tension in DNA by cutting and rejoining strands.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance DNA Replication - Semiconservative and Machinery
After circular chromosome duplication, two daughter molecules become interlinked as catenanes resembling linked rings generated naturally during replication termination and intertwining. Topoisomerase IV, type II topoisomerase of ParC and ParE subunits as C2E2 tetramer, performs decatenation by introducing transient double-strand breaks in one duplex, passing other duplex through opening, and resealing using ATP hydrolysis energy. DNA gyrase primarily introduces supercoils, topoisomerase I relaxes negative supercoils but cannot decatenate due to single-strand mechanism. Topo IV activity essential for chromosome segregation before cell division.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: Topoisomerase IV decatenation of sister chromosomes after replication
Topoisomerases classified mechanistically by whether cleavage of one versus two DNA strands occurs during catalytic cycle. Type I cuts single strand allowing rotation to change linking number in steps of one. Type II cuts both strands, passes intact duplex through break, alters linking number by two. DNA gyrase and topoisomerase IV are bacterial type II enzymes requiring ATP and Mg2+ forming transient covalent phosphotyrosine intermediates. Gyrase uniquely introduces negative supercoils and removes positive supercoils ahead of replication, while topo IV primarily decatenates chromosomes. Double-strand mechanism distinguishes gyrase from type I enzymes.
Ref: Berg et al., Biochemistry, 9th ed., Chapter 27: Type II topoisomerases, DNA gyrase mechanism and classification
Unwinding helicase generates positive supercoils ahead of fork increasing helical twist and resisting further strand separation requiring topoisomerase relief. DNA gyrase, type II topoisomerase composed of GyrA and GyrB subunits as A2B2 tetramer, introduces negative supercoils using ATP hydrolysis to counterbalance positive supercoiling essentially removing overwinding stress. It makes transient double-strand cuts, passes another segment through break, and religates. SSB binds single strands but does not resolve topology, ligase seals nicks only. Gyrase thus prevents fork stall and facilitates rapid fork progression.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: DNA gyrase relieving positive supercoils ahead of fork
