10.3B: Regulation of the Cell Cycle at Internal Checkpoints
- Explain the effects of internal checkpoints on the regulation of the cell cycle
Regulation at Internal Checkpoints
It is essential that the daughter cells are exact duplicates of the parent cell. Mistakes in the duplication or distribution of the chromosomes lead to mutations that may be passed forward to every new cell produced from an abnormal cell. To prevent a compromised cell from continuing to divide, internal control mechanisms operate at three main cell cycle checkpoints. A checkpoint is one of 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 favorable (e.g. the DNA is repaired). These checkpoints occur near the end of G 1 , at the G 2 /M transition, and during metaphase.
The G 1 Checkpoint
The G 1 checkpoint determines whether all conditions are favorable 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.
Key Points
- A checkpoint is one of 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 favorable.
- Damage to DNA and other external factors are evaluated at the G1 checkpoint; if conditions are inadequate, the cell will not be allowed to continue to the S phase of interphase.
- The G2 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.
Key Terms
- restriction point : (G1 checkpoint) a point in the animal cell cycle at which the cell becomes “committed” to the cell cycle, which is determined by external factors and signals
- spindle checkpoint : (M checkpoint) prevents separation of the duplicated chromosomes until each chromosome is properly attached to the spindle apparatus
- cyclin : any of a group of proteins that regulates the cell cycle by forming a complex with kinases
- G2 checkpoint : ensures all of the chromosomes have been replicated and that the replicated DNA is not damaged