DNA replication in Xenopus

Cell-free extracts of eggs of the South African clawed toad Xenopus faevis support complete chromosome replication under normal cell cycle control (Blow and Laskey 1986). Apart from a similar system derived from embryos of the fruit fly Drosophifa mefanogaster (Crevel and Cot-terill 1991), it is currently the only eukaryotic cell-free system that sup-ports efficient chromosome replication in vitro. Since the Xenopus cell-free system progresses through the complete cell cycle in vitro (Hutchison et al. 1987; Murray and Kirschner 1989), it offers a unique opportunity to study the way that DNA replication is coordinated with other cell-cycle events. This subject has recently been reviewed in detail (Blow 1996); the major conclusions are summarized here. When Xenopus eggs are crushed by low-speed centrifugation, the resultant low-speed supernatants, which contain abundant particulate material (including nuclear envelope precursors), can support all the major activities of the early embryonic cell cycle. During each in vitro cell cycle there is an ordered sequence of events as follows