40. Jared finally succeeded in isolating the protein cyclin B, showing that adding purified cyclin B alone to (A) In the cells, cyclin B synthesis is on and off. When cyclin B synthesis is turned off, cells do not divide. (B) There is another factor that may inhibit cyclin B activity during the non-division phase. (C) Non-dividing cells have an intrinsic factor that complexes with cyclin B and triggers cell division. After cell division is complete, the concentration of this factor is diluted and becomes inadequate for binding to cyclin B. As a result, the cell does not begin to divide unless the level of this intrinsic factor builds up again. (D) Cyclin B has a limited half life and when cyclin B is degraded, cell division does not occur. (E) Cyclin B is sensitive to the copy number of chromosome and is active only when the cell contains replicated DNA materials.