THE SYNTHESIS OF PHOSPHOLIPIDS IN THE NUCLEUS AND NUCLEAR MEMBRANE OF SYNCHRONIZED HeLa CELLS

HeLa cells were synchronized by a double thymidine block and pulse labeled at different stages of the cell cycle with ³H-choline. The specific activity of phospholipids extracted from the cell, the nucleus and the nuclear membrane showed a progressive increase from S to G₁; the incorporation of choline into phospholipids of asynchronous cells showed a specific activity intermediate between the values of S and G₁ cells. Similar results were obtained when ³²phosphorus was used as a precursor instead of choline. Thin layer chromatographic analysis of phospholipids extracted from cells in S and from cells in G₁ failed to show any difference in the distribution of radioactivity among the various phospholipid classes. Choline uptake by HeLa cells in different phases of the cell cycle did not show significant variations. However, during the synchronization process, shortly after the addition of excess thymidine, an increased uptake of choline by cells and an increased incorporation of choline into phospholipids were found. The results indicate that some of the changes occurring in phospholipids synthesis may not be cell cycle dependent, but may be the effect of the synchronizing process.