The nuclear membrane prevents replication of human G2 nuclei but not G1 nuclei in Xenopus egg extract.

We have used synchronized HeLa cells to investigate the role of the nuclear membrane in preventing rereplication in a single cell cycle. Nuclei were prepared with intact nuclear membranes using streptolysin-O or digitonin and assayed for replication in Xenopus egg extracts. Intact G1 nuclei replicate semiconservatively, but intact G2 nuclei do not replicate in egg extract. However, permeabilizing the nuclear membranes of G2 nuclei by treatment with NP-40 allows them all to replicate in egg extract under cell cycle control, suggesting that integrity of the nuclear membrane is required to distinguish G2 from G1 human nuclei and to prevent rereplication within a single cell cycle. The results are discussed in terms of the previously proposed licensing factor model.