ON THE PROPOSAL OF A G0 PHASE AND THE RESTRICTION POINT

Zetterberg and Larsson proposed that the restriction point divides the G1 phase into two parts. Cells before this point are able to leave th e division cycle and enter a GO phase; cells past this point are unaff ected by a short period of low serum. Additional results of Zetterberg and Larsson-1) cycloheximide treatment affects cells in the same way as low serum, and 2) a delay in the second division after serum starva tion in the cells not initially affected in their first division-indic ate that their experiments are consistent with serum removal affecting cells in all phases of the cell cycle equally. Their experiments are consistent with the continuum model of the mammalian division cycle. T here is no need to postulate a restriction point or a GO phase to expl ain the serum starvation results.