The cell cycle describes the ordered series of events a cell undergoes as it grows and divides into two daughter cells. This process involves precise duplication of cellular components and genetic material. Within this cycle, control mechanisms known as cell cycle checkpoints operate to ensure the accuracy and integrity of cell division. These checkpoints act as surveillance systems, monitoring the cell’s internal and external conditions before allowing progression to the next phase.
Why Cells Need Checkpoints
Cell cycle checkpoints maintain the stability of a cell’s genetic information. They prevent errors during DNA replication and chromosome segregation, steps in cell division. Without these controls, cells could divide prematurely or with damaged genetic material. Such improper division can lead to mutations and the accumulation of genetic abnormalities in daughter cells.
Major Cell Cycle Checkpoints
The cell cycle is broadly divided into interphase (G1, S, G2 phases) and the M phase (mitosis and cytokinesis). Cells utilize specific checkpoints to monitor their progression. Three main checkpoints regulate the eukaryotic cell cycle: the G1 checkpoint (near the end of G1), the G2 checkpoint (at the transition between G2 and M phase), and the M checkpoint (also known as the spindle assembly checkpoint, operating during metaphase).
How Each Checkpoint Works
The G1 checkpoint
The G1 checkpoint, also referred to as the restriction point, determines whether conditions are suitable for cell division. It assesses the cell’s size and verifies the availability of sufficient nutrients and growth factors. Most importantly, this checkpoint inspects the integrity of the cell’s DNA to ensure there is no damage before DNA replication begins. If any issues are detected, the cell cycle can be halted, allowing time for repairs or for the cell to enter a resting state.
The G2 checkpoint
The G2 checkpoint functions as a control point before the cell commits to mitosis. Its role is to confirm that DNA replication has been completed accurately during the S phase. This checkpoint also screens for any DNA damage that might have occurred. If DNA damage or incomplete replication is identified, the cell cycle is paused, providing an opportunity for repair.
The M checkpoint
The M checkpoint, also known as the spindle assembly checkpoint, operates during metaphase. This checkpoint ensures all chromosomes are properly attached to the spindle fibers, which pull chromosomes apart. It verifies each chromosome’s kinetochore, a protein structure, is correctly connected to spindle microtubules from opposite poles. This attachment ensures each new daughter cell receives an accurate and complete set of chromosomes. The cell will not proceed to chromosome separation until this attachment is confirmed.
When Checkpoints Fail
A malfunction in cell cycle checkpoints can have considerable consequences. If these controls fail, cells may proceed through division with errors in their genetic material. This can lead to genetic instability and an increased rate of mutations. Such unchecked division of cells with genetic errors can contribute to the development of various conditions. For instance, the loss of checkpoint function is a factor in the uncontrolled cell growth observed in cancer.