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A second level of control is at the level of the joint molecule, either extending the length of the heteroduplex joint by threestrand or fourstrand DNA branch migration, or by disrupting the joint molecule to reverse the paired DNA intermediate.The control of lament formation has essentially a yesno decision with regards to recombination.However, control exerted at the level of joint molecules has several possible complex outcomes: disruption of a joint molecule that had not been replicated aborts recombination; disruption of a joint molecule that had been replicated can lead to recombination DNA repair by SSA or SDSA; and extension of a joint molecule can stabilize the nascent joint molecule and lead to HJ or dHJ formation.The role of motor proteins in determining the fate of nascent joint molecules will be summarized. Rad will also drive fourstrand DNA heteroduplex extension due it high afnity for a <a href="http://www.targetmol.com/compound/Diclofenac"></a> fourway junction having one ssDNA arm. These proteins branch migrate various three and fourway junctions.In addition to these dsDNA translocases, several helicases can branch migrate fourway junctions.BLM can specically bind a HJ and migrate it several kbp.   RECQ is another helicase that has the capacity to migrate both three and fourstranded intermediates for at least several hundred base pairs. When the two joined DNA duplexes are fully homologous, this fourway junction can branch migrate, resulting in the genetically veried tracts of heteroduplex DNA.Branch migration can random, driven by thermal energy, or it can be directional, catalyzed by a motor protein.To separate the two DNA molecules, at least transient endonucleolytic cleavage is required.Separation was initially was proposed to involve symmetric cleavage and was envisioned to occur by a nuclease that was smart enough to cut across the junction and, ideally, to not cutoff one arm.However, it was subsequently recognized that parental duplexes joined by a dHJ possessed a unique attribute that was absent in duplexes joined by a single HJ: namely, the intermediate with a dHJ represents two topologically linked duplex DNA molecules. This realization meant that, in principle, the two chromosomes could be separated by transiently passing single strands of DNA through each of the intertwined parental duplexes to unlink them and with the concomitant movement of one junction toward the other. This process was envisioned to require a type IA topoisomerase, perhaps assisted by a motor protein to move the HJ andor to produce ssDNA for the topoisomerase.These processes are comprehensively addressed elsewhere. The interesting feature of these two very different reactions is that symmetric cleavage by a nuclease produces a: mixture of crossover and noncrossover recombinants, whereas dissolution leads to only noncrossovers.For bacteria, a single crossover between circular chromosomes produces a dimeric circular chromosome that cannot partition into daughter cells on cell division.Topo III is a type IA topoisomerase that passes one strand of DNA through another. In addition, unless a nicked HJ was used, the cleavage products were not perfectly symmetrical, because the cleavage products could not be religated.In general, the products of these reactions are not ligatable, implying asymmetric cutting, further emphasizing the distinction between the prokaryotic and eukaryotic nucleases.

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