How do you explain the ability of the S-phase cells in experiment I to induce premature entry of the G1 nuclei into S? (Hint: What protein(s) are present and active in S-phase cells but not G1 cells?) Briefly explain your reasoning. Also, how do you explain the different outcome observed in experiment II? In both cases the nuclei from the G1 cells are being exposed to cellular contents of cells further along in the cell cycle, but the G2 cells seem to have lost the “power” to drive premature entry into S.
One approach to studying the regulation of cell cycle progression (particularly in an era when genetic and molecular biology manipulations were less readily accomplished in mammalian cells) was to use treatments that induced cells to fuse and then monitor the behavior of the two nuclei in the resulting cell. The figure below depicts data from one such study. The investigators did preliminary work to produce populations of cells that were synchronized in various stages of the cell cycle (G1, S, or G2 in the examples shown below). They then fused the cells in different combinations and monitored subsequent events in each of the nuclei. For purposes of this question, we will pay particular attention to what occurred in the nucleus that came from the cell in G1. In one experiment (I), cells in the G1 and S phases were fused. That event caused the nucleus from the G1 cell to very quickly enter the S phase (sooner than it would otherwise have done so). In contrast, in a second experiment (II), in which cells in the G1 and G2 phases were fused, the nucleus from the G1 cell remained on its original “timetable.” That is, it did eventually enter S, but not any sooner than was the case for nuclei in the un-fused cells in G1.
How do you explain the ability of the S-phase cells in experiment I to induce premature entry of the G1 nuclei into S? (Hint: What protein(s) are present and active in S-phase cells but not G1 cells?) Briefly explain your reasoning. Also, how do you explain the different outcome observed in experiment II? In both cases the nuclei from the G1 cells are being exposed to cellular contents of cells further along in the cell cycle, but the G2 cells seem to have lost the “power” to drive premature entry into S.
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