The human body has trillions of cells and many of them, such as skin cells, are constantly dividing to replace the cells that die. The outermost layer of skin is made of dead skin cells that slough off, and cells underneath this layer must replicate to replenish the dead cells above. Mitosis is the process by which somatic cells divide or replicate themselves. When mitosis goes wrong, cells can grow uncontrollably. Cells have ways of ensuring that mitosis does not go wrong, but when these safeguards fail, faulty mitosis produces mutant cells.
Safeguards for Faulty Mitosis
Mitosis is a multi-step process with checkpoints to make sure things are going properly. If a cell detects that something has gone wrong during mitosis, the first thing it does is to stop the process. The three major checkpoints during mitosis -- the G1, G2 and mitotic spindle checkpoints -- all provide the cell with an opportunity to intervene if something goes wrong so that it can fix the problem. Each checkpoint is policed by specific proteins that ensure that everything is working properly.
Time for Self-Repair
During mitosis, the DNA within the cell duplicates. The process involves many changes and the movement of chromosomes -- which carry the DNA -- and organelles within the cell. This activity can sometimes cause DNA to break. If the cell is able to detect broken DNA, mitosis ceases and an elaborate DNA repair process is activated. A protein called p53, otherwise known as the "guardian of the genome," is a key sensor of DNA damage in animal cells. After detecting damage, it causes the cell cycle to stop, which allows time for repairs.
Sometimes a cell cannot fix the problem, however. When this happens, sometimes the cell commits a special type of suicide called apoptosis. Apoptosis, otherwise known as programmed cell death, ensures that if mitosis goes wrong and cannot be fixed, the cell does not continue to divide, as this would lead to the production of mutant cells that might cause problems. But sometimes the cell does not "catch" a flawed mitotic process, and thus the process is allowed to proceed. One example of this is cancer.
If an error occurs during mitosis but the cell still divides, it produces two mutant cells. These mutants can contain the wrong number of chromosomes or broken chromosomes. Since chromosomes carry the cell's DNA -- the genetic information that instructs the cell on how to live and grow -- mutant cells will not function normally. This means that mutant cells may die sooner than they should, or they may grow uncontrollably, which results in cancer.
- Boundless: Cell cycles have checkpoints which regulate Growth
- Cell Signaling Technology: Cell Cycle Control: G2/M DNA Damage Checkpoint
- Genentech: Apoptosis: A Critical Process in Homeostasis
- Genentech: Resisting Apoptosis: A Hallmark of Cancer
- Proceedings of the National Academy of Science: p53 controls both the G2/M and the G1 cell cycle checkpoints and mediates reversible growth arrest in human fibroblasts.
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