4 public questions tagged with this topic.
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
Among general Pol II factors, TFIIH alone possesses ATP-dependent DNA helicase activity essential for promoter opening. Complex composed of ten subunits includes XPB ERCC3 translocase with 3' to 5' helicase that pumps downstream DNA toward polymerase, generating torsional stress to melt eleven to fifteen base pairs around transcription start site. XPD ERCC2 helicase participates primarily in nucleotide excision repair. Helicase action requires ATP hydrolysis and is inhibited by triptolide anticancer drug. TFIIH entry after Pol II-TFIIF completes preinitiation complex assembly enabling open com
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: TFIIH helicase melts promoter DNA; Alberts 7th ed., General transcription factors TFIIH function
Bacterial RNA polymerase achieves strand separation without energy from ATP hydrolysis, contrasting eukaryotic Pol II requiring TFIIH helicase XPB ATPase to melt promoter. Binding free energy contributed by sigma-DNA interactions at -35 and -10 elements plus favorable unwinding of AT-rich Pribnow box and downstream DNA scattering provides sufficient energy to stabilize open complex spontaneously. Single-molecule FRET demonstrates closed to open isomerization occurs in absence nucleotide triphosphate hydrolysis. No helicase required, nor ATP dependent gyrase. Consequently bacterial promoter mel
Ref: Lodish Chapter 8: Promoter melting bacterial ATP independent vs eukaryotic ATP dependent; Alberts Chapter 6: Open complex formation energetics
Bidirectional unwinding of parental duplex at bacterial replication fork requires hexameric helicase DnaB encircling lagging strand template and moving 5' to 3' direction using ATP hydrolysis energy. DnaB disrupts base pairing ahead of polymerase generating single-stranded templates for both leading and lagging synthesis simultaneously. Activity stimulated by DnaC loader during initiation and by tau subunit and SSB during elongation. DnaA protein recognizes oriC boxes but does not unwind long stretches, while DnaG synthesizes RNA primers after unwinding provides single-stranded regions.
Ref: NCBI Bookshelf, Molecular Genetics: E. coli DnaB hexameric helicase mechanism and polarity
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