FIGURE 1.

Cell-cycle checkpoints. The cell-cycle checkpoints serve at critical points of the cell cycle when the conditions of the cell are assessed and defects are repaired before the cell progresses to the next phase of the cell cycle. Classic cell-cycle checkpoints include the G₂ checkpoint that monitors DNA damage and the metaphase checkpoint that surveils spindle formation. Another cell-cycle checkpoint is the restriction point at the G₁ phase of the cell cycle. Recent advances in hematopoietic stem cell (HSC) quiescence support the existence of a mitochondrial metabolic checkpoint in HSCs at the transition between G₀ and G₁ phases of the cell cycle (restriction point 1). Studies in human mammary epithelial cells identified a novel exit from mitosis to G₀ (restriction point 2), which may also monitor mitochondrial stresses.