INITIATION OF DNA-SYNTHESIS BY NUCLEI FROM SCRAPE-RUPTURED QUIESCENT MAMMALIAN-CELLS IN HIGH-SPEED SUPERNATANTS OF XENOPUS EGG EXTRACTS

Demembranated sperm heads, detergent-isolated somatic nuclei and even naked DNA are efficiently replicated in cytoplasmic extracts of activa ted amphibian eggs, but only after nuclear assembly and the formation of an intact nuclear envelope. DNA synthesis has not previously been s hown to be initiated in high-speed (200,000 g) supernatants of egg cyt oplasm because they are depleted of the vesicular material required to support nuclear envelope formation. Here we show that mammalian nucle i prepared by scrape-rupture are able to initiate DNA replication in s uch high-speed supernatants. These nuclei begin DNA synthesis asynchro nously, This asynchrony cannot be attributed to differences in the tim e taken for nuclear assembly, Instead, we suggest that the asynchrony reflects intrinsic differences between nuclei and that these differenc es are a major cause of cell cycle variability. Our demonstration of i nitiation in high-speed supernatants now enables the initiation of euk aryotic DNA synthesis to be studied independently of nuclear assembly.