Question

What are the different check points in cell division? Explain each of them in detail.

Answer 1

A checkpoint is one of the several points in the eukaryotic cell cycle at which the progression of a cell to the next stage in the cycle can be halted until conditions are favourable.

Damage to DNA and other external factors are evaluated at the $G_1$ checkpoint; if conditions are inadequate, the cell will not be allowed to continue to the S phase of interphase.

The $G_2$ checkpoint ensures all of the chromosomes have been replicated and that the replicated DNA is not damaged before cell enters mitosis.

The M checkpoint determines whether all the sister chromatids are correctly attached to the spindle microtubules before the cell enters the irreversible anaphase stage.

The $G_1$ Checkpoint

The $G_1$ checkpoint determines whether all conditions are favourable for cell division to proceed. The $G_1$ checkpoint, also called the restriction point (in yeast), is a point at which the cell irreversibly commits to the cell division process. External influences, such as growth factors, play a large role in carrying the cell past the $G_1$ checkpoint. The cell will only pass the checkpoint if it is an appropriate size and has adequate energy reserves. At this point, the cell also checks for DNA damage. A cell that does not meet all the requirements will not progress to the S phase. The cell can halt the cycle and attempt to remedy the problematic condition, or the cell can advance into $G_0$ (inactive) phase and await further signals when conditions improve.

If a cell meets the requirements for the $G_1$ checkpoint, the cell will enter S phase and begin DNA replication. This transition, as with all of the major checkpoint transitions in the cell cycle, is signaled by cyclins and cyclin dependent kinases (CDKs). Cyclins are cell-signaling molecules that regulate the cell cycle.

The $G_2$ Checkpoint

The $G_2$ checkpoint bars entry into the mitotic phase if certain conditions are not met. As with the $G_1$ checkpoint, cell size and protein reserves are assessed. However, the most important role of the $G_2$ checkpoint is to ensure that all of the chromosomes have been accurately replicated without mistakes or damage. If the checkpoint mechanisms detect problems with the DNA, the cell cycle is halted and the cell attempts to either complete DNA replication or repair the damaged DNA. If the DNA has been correctly replicated, cyclin-dependent kinases (CDKs) signal the beginning of mitotic cell division.

The M Checkpoint

The M checkpoint occurs near the end of the metaphase stage of mitosis. The M checkpoint is also known as the spindle checkpoint because it determines whether all the sister chromatids are correctly attached to the spindle microtubules. Because the separation of the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to at least two spindle fibers arising from opposite poles of the cell.