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Topoisomerase-II rolls better
All processes of gene activity are accompanied by alterations in DNA helical tension. Consequently, the double helix twists on itself, or supercoiled to remain stable throughout its length. This tension may be very high during the processes of transcription and DNA replication, since the two strands, always linked in the double helix, are forced to separate, in part or total. Due to the enormous length and the large condensation of DNA molecules in chromosomes, the coil voltage can not dissipate spontaneously.
The natural solution to the problem is offered by the so-called topoisomerases, resulting in transient cuts the strands of DNA to relax the tension coil. A team from the Institute of Molecular Biology of Barcelona, CSIC has described the mechanism by which stress relaxes when helical DNA is condensed into chromosomes. The results of work directed by Joaquim Roca and published in EMBO Journal contradict the general view of the role of topoisomerases. So far, it was believed that the topoisomerase-I was the main relaxing the DNA, while topoisomerase-II was the only function and trigger untangle DNA before dividing. This study demonstrates that topoisomerase-II is also the main relaxing of tension when the DNA is folded into chromosomes.
The key finding has been to discover the precise type of cut made by each enzyme DNA: features of the two strands of double helix-II topoisomerase cuts both, allowing another DNA chain nearby temporary pass through this gate that just opened. Thus, it relieves tension and knots are removed. By contrast, the topoisomerase-I just cut one strand, and therefore is not effective in reducing tension in the whole molecule.