DNA supercoiling helps to unlink sister duplexes after replication.

DNA supercoiling is one of the mechanisms that can help unlinking of newly replicated DNA molecules. Although DNA topoisomerases, which catalyze the strand passing of DNA segments through one another, make the unlinking problem solvable in principle, it remains difficult to complete the process that enables the separation of the sister duplexes. A few different mechanisms were developed by nature to solve the problem. Some of the mechanisms are very intuitive while the others, like topology simplification by type II DNA topoisomerases and DNA supercoiling, are not so evident. A computer simulation and analysis of linked sister plasmids formed in Escherichia coli cells with suppressed topoisomerase IV suggests an insight into the latter mechanism.