Rasmussen C D, Berger J D, Ching A S
Exp Cell Res. 1986 Jul;165(1):53-62. doi: 10.1016/0014-4827(86)90532-x.
In Paramecium, cell mass and macronuclear DNA content can vary substantially, and both variables affect the timing of initiation of macronuclear DNA synthesis. Cells normally begin macronuclear DNA synthesis at 0.25 in the cell cycle when the mean cell mass is about 119% of the initial value. Gene mutations were used to alter cell size by temporarily blocking cell cycle progression and to change DNA content by altering the segregation pattern of macronuclear material to daughter nuclei at fission. Changes in cell mass or macronuclear DNA content imposed at fission or in the subsequent G1 interval do not affect the timing of initiation of DNA synthesis in that cell cycle, but do affect the timing of initiation of DNA synthesis in the subsequent cell cycle. The progeny of cells with lower than average macronuclear DNA content tend to initiate DNA synthesis earlier than other cells. The G1 interval is proportionally shortened when initial cell mass is greater than normal, and no measurable G1 interval is present when initial cell mass equals or exceeds the normal cell mass present at initiation of DNA synthesis. These results suggest that the timing of initiation of DNA synthesis is established during the preceding cell cycle and that the 'timer' mechanism is not significantly affected by either drastic changes in gene dosage or gene concentration during the G1 interval.
在草履虫中，细胞质量和大核DNA含量会有很大差异，这两个变量都会影响大核DNA合成起始的时间。细胞通常在细胞周期的0.25时开始大核DNA合成，此时平均细胞质量约为初始值的119%。利用基因突变通过暂时阻断细胞周期进程来改变细胞大小，并通过改变大核物质在分裂时向子核的分离模式来改变DNA含量。在分裂时或随后的G1期施加的细胞质量或大核DNA含量的变化，不会影响该细胞周期中DNA合成起始的时间，但会影响随后细胞周期中DNA合成起始的时间。大核DNA含量低于平均水平的细胞的后代往往比其他细胞更早开始DNA合成。当初始细胞质量大于正常水平时，G1期会按比例缩短，当初始细胞质量等于或超过DNA合成起始时的正常细胞质量时，则不存在可测量的G1期。这些结果表明，DNA合成起始的时间是在前一个细胞周期中确定的，并且“定时器”机制在G1期不会受到基因剂量或基因浓度的剧烈变化的显著影响。
