Which statement about the cell-cycle control system is not true?

The statement regarding the cell-cycle control system ensuring that M phase is completed before cytokinesis begins is not true because cytokinesis is actually part of the M phase itself. The M phase encompasses both mitosis, where the chromosomes are separated, and cytokinesis, where the cell divides into two daughter cells. Therefore, cytokinesis occurs concurrently with the events of M phase; it is not something that comes after M phase has been completed. Each phase of the cell cycle is tightly regulated to ensure proper division and function, but cytokinesis follows directly after the completion of mitosis, rather than being a separate sequential event.

In contrast, the other statements outline critical checkpoints and functions of the cell-cycle control system, such as ensuring DNA replication is completed before entering M phase and confirming that all chromosomes are properly attached to the spindle apparatus prior to segregation, which are essential for maintaining genomic integrity. Additionally, the control system assesses whether the environment is conducive for DNA replication, thereby ensuring optimal conditions for cell division.