Knot what we thought before: the twisted story of replication

DNA replication requires the unwinding of the parental duplex, which genera tes (+) supercoiling ahead of the replication fork. It has been thought tha t removal of these (+) supercoils was the only method of unlinking the pare ntal strands. Recent evidence implies that supercoils can diffuse across th e replication fork, resulting in interwound replicated strands called preca tenanes. Topoisomerases can then act both in front of and behind the replic ation fork. A new study by Sogo et al. [J Mot Biol 1999;286:637-643 (Ref. 1 )], using a topological analysis, provides the best evidence that precatena nes exist in negatively supercoiled, partially replicated molecules in vivo . BioEssays 21:805-808, 1999. (C) 1999 John Wiley & Sons, Inc.