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Nuclei act as independent and integrated units of replication in a Xenopus cell‐free DNA replication system.

We have used a novel approach to investigate the control of initiation of replication of sperm nuclei in a Xenopus cell‐free extract. Nascent DNA was labelled with biotin by supplementing the extract with biotin‐11‐dUTP, and isolated nuclei were then probed with fluorescein‐conjugated streptavidin. Flow cytometry was used to measure the biotin content of individual nuclei and their total DNA content. This showed that incorporation of the biotinylated precursor increases linearly with DNA content. Haploid sperm nuclei replicate fully to reach the diploid DNA content over 2‐6 h in the extract. Synthesis stops once the diploid DNA content is reached. Different nuclei enter S phase at different times over greater than 1.5 h, although they share the same cytoplasmic environment. Nuclei reach their maximum rates of synthesis soon after entry into S phase and some replicate fully in less than 0.5 h, resembling the rates of replication observed in the intact egg. These results indicate that initiations are coordinated within each nucleus such that the nucleus is the fundamental unit of replication in the cell‐free system.