Stimulated DNA synthesis in frog nuclei by cytoplasmic extracts of temperature-sensitive mammalian cells

Cytoplasmic extracts of proliferating cells stimulate DNA synthesis in isolated nuclei of Xenopus laevis liver. When tested by the same assay, cytoplasmic extracts of resting cells are completely inactive. When cytoplasmic extracts are prepared from cell cycle specific temperature-sensitive mutants arrested in the G₁ phase of the cell cycle by the nonpermissive temperature, they also fail to stimulate DNA synthesis in frog nuclei. The results indicate that, to stimulate DNA synthesis in isolated frog nuclei, essentially all information of G₁ cells must be present.