Recall that one approach to the study of cell cycle regulation has been to fuse cultured cells that are at different stages of the cell cycle and observe the effect of the fusion on the nuclei of the fused cells (heterokaryons). When cells in Gl are fused with cells in S, the nuclei from the G1 cells begin DNA replication earlier than they would have if they had not been fused. In fusions of cells in G2 and S, however, nuclei continue their previous activities, apparently uninfluenced by the fusion. Fusions between mitotic cells and interphase cells always lead to chromatin condensation in the nonmitotic nuclei. Based on these results, identify each of the following statements about cell cycle regulation as probably true (T), probably false (F), or not possible to conclude from the data (NP).

a. The activation of DNA synthesis may result from the stimulatory activity of one or more cytoplasmic factors.
b. The transition from S to G2 may result from the presence of a cytoplasmic factor that inhibits DNA synthesis.
c. The transition from G2 to mitosis may result from the presence in the G2 cytoplasm of one or more factors that induce chromatin condensation.
d. G1 is not an obligatory phase of all cell cycles.
e. The transition from mitosis to G1 appears to result from the disappearance or inactivation of a cytoplasmic factor present during M phase.