Xenopus egg extracts: a model system for chromatin replication

A cell-free system derived from Xenopus eggs enables in vitro reproduction of the steps occurring during eukaryotic DNA replication. With a circular single-stranded DNA template, extracts obtained from high-speed centrifugation perform complementary DNA strand synthesis coupled to chromatin assembly. Nucleosomes are formed on the newly replicated DNA and the overall reaction mimics the events occurring during chromosomal replication on the lagging strand at the replication fork. ATP is necessary at all steps examined individually, including RNA priming, elongation of DNA strands and chromatin assembly. Although not required for nucleosome formation, ATP is involved in the correct spacing of nucleosomes and the stability of the assembled chromatin. Replication of double-stranded DNA was observed only with extracts obtained from low-speed centrifugation using demembraned sperm nuclei as substrate. Nuclei are reconstituted around the DNA and then undergo a series of events characteristic of a cell cycle. In contrast, neither DNA elongation or chromatin assembly require formation of the nucleus, and both are independent of the cell cycle.