Characterization of the Xenopus replication licensing system.

In eukaryotes the entire genome is replicated precisely once during each S phase of the cell cycle. Cell-free extracts of Xenopus eggs support chromosomal DNA replication in vitro, apparently under the same cell cycle controls that exist in vivo.1,2 When DNA is added to the cell-free system it is assembled into chromatin, and then into structures resembling normal interphase nuclei,3 processes that are necessary before the initiation of DNA replication can occur.2,4-6 Once nuclear assembly is complete, the DNA is replicated precisely once.