Describe the general conditions that must be met at each of the three main cell-cycle checkpoints.

The three checkpoints function as internal control mechanisms that identify if there is any mistakes in the duplication or distribution of the chromosomes. A checkpoint decides whether the cell should progress to the next stage in the cycle or not. It may also halt the cycle until conditions are favorable. The three checkpoints occur at the near end of G1, at the G2/M transition, and during the metaphase.

The G1 Checkpoint

The G1 checkpoint checks all conditions are favorable not for the cell cycle. This checkpoint is also called the restriction point. External factors like growth factors, help the cell to pass the G1 checkpoint. It assesses availability of energy reserves and checks the cell size. It also checks if there is any damaged DNA. A cell that has any faults will not be allowed to proceed into the next phase until it gets favourable condition. The cell can even enter into G0 to wait for favorable signals.

The G2 Checkpoint

This checkpoint prevents the cell from entering into the mitosis phase if there are some unfavorable conditions. Here also, the cell size and protein reserves are assessed. The most important role of the G2 checkpoint is to check all the chromosomes have been replicated or not and to ensure that DNA is not damaged. If the checkpoint detect any problems, the cell cycle can be halted.

The M Checkpoint

It occurs near the end of the metaphase of karyokinesis. This M checkpoint is also known as the spindle checkpoint. This checkpoint determines all sister chromatids are perfectly attached to the spindle fibres or not. As we know, during anaphase, the sister chromatid seperation is an irreversible step, the cycle will not proceed further until the kinetochores of sister chromatid pairs are correctly attached to the two spindle fibers originating from the two poles of the cell.