Mitosis is the process of cell division that results in two genetically identical daughter cells. It is divided into four stages: prophase, metaphase, anaphase, and telophase.
Prophase:
- The chromatin condenses into visible chromosomes.
- The nuclear envelope breaks down, allowing the spindle fibers to interact with the chromosomes.
- The spindle fibers, consisting of microtubules, form and extend from two centrosomes located at opposite ends of the cell.
Metaphase:
- The spindle fibers attach to the centromeres of the chromosomes and align them along the equator of the spindle, known as the metaphase plate.
- This alignment ensures that each daughter cell will receive one copy of each chromosome during cell division.
Anaphase:
- The spindle fibers begin to contract, pulling the sister chromatids apart.
- The separated chromatids move towards opposite poles of the spindle.
Telophase:
- The chromatids reach their respective poles and begin to decondense into chromatin.
- A new nuclear envelope forms around each set of chromosomes, completing the formation of two nuclei.
- The spindle fibers disassemble and the cell begins to undergo cytokinesis, the process of physically dividing the cytoplasm and organelles between the two daughter cells.
The regulation of mitosis is critical to ensure that the process occurs accurately and efficiently. Checkpoints throughout the cell cycle monitor the progress of mitosis and ensure that errors are corrected before the cell proceeds to the next stage.
The checkpoints include:
- G1 checkpoint: checks for DNA damage and the presence of necessary cellular resources.
- G2 checkpoint: checks for DNA damage and completion of DNA replication.
- Spindle assembly checkpoint: ensures that all chromosomes are properly aligned on the spindle before proceeding to anaphase.
The regulation of mitosis is achieved through the activity of various protein complexes, including cyclin-dependent kinases (CDKs) and cyclins. These proteins control the progression of the cell cycle by phosphorylating and activating key regulatory proteins. Dysregulation of mitosis can lead to chromosome missegregation and aneuploidy, which are associated with developmental defects and cancer.