This engine must satisfy at least three requirements in order to drive a successful cell cycle: 1/ It must trigger the S phase first and mitosis later, so that the S and M phases always alternate during the normal mitotic, 2/ The engine must be subject to checkpoint controls: If one cell-cycle event cannot happen for some reason, then further progress through the cycle must be blocked, and 3/ Balanced growth and division. A cell must double its cytoplasmic mass between two successive divisions. This requirement is satisfied if the cell-cycle engine is sensitive to the cytoplasmic mass per DNA ratio, because in this case the engine will trigger cell-cycle events with the periodicity of the mass-doubling time.