Mitosis is a process of cell division in which a single eukaryotic cell (the parent cell) divides into two identical daughter cells. This process ensures that each daughter cell receives a complete and accurate copy of the genetic material contained in the parent cell’s nucleus. Mitosis is essential for growth, repair, and maintenance of tissues in multicellular organisms, as well as for asexual reproduction in some single-celled organisms.
Mitosis can be divided into several stages:
- Prophase: During prophase, chromatin condenses into distinct chromosomes, each composed of two identical sister chromatids connected at a region called the centromere. The nucleolus disappears, and the nuclear envelope starts to break down. The mitotic spindle, composed of microtubules and associated proteins, begins to form between the two centrosomes, which have migrated to opposite poles of the cell.
- Prometaphase: The nuclear envelope disintegrates completely, allowing spindle microtubules to access and interact with the chromosomes. The microtubules attach to the chromosomes at their kinetochores, specialized protein structures located at the centromeres.
- Metaphase: The chromosomes, now attached to spindle microtubules, align at the cell’s equator, forming a structure called the metaphase plate. This ensures that each daughter cell will receive an equal and complete set of chromosomes during cell division.
- Anaphase: Sister chromatids separate and are pulled toward opposite poles of the cell by the shortening of spindle microtubules. Each chromatid is now considered a separate chromosome.
- Telophase: Chromosomes reach the poles of the cell and begin to decondense, returning to a more extended chromatin state. The nuclear envelope re-forms around each set of chromosomes, creating two separate nuclei, and the nucleolus reappears.
- Cytokinesis: The final stage of cell division, cytokinesis, involves the physical separation of the cytoplasm and the organelles. In animal cells, this is achieved through the formation of a contractile ring composed of actin and myosin filaments, which constricts and eventually pinches the cell into two. In plant cells, a cell plate forms between the two nuclei and eventually develops into a new cell wall, separating the daughter cells.
At the end of mitosis and cytokinesis, two genetically identical daughter cells are produced, each containing the same number of chromosomes as the original parent cell. Mitosis is a tightly regulated process, and errors during mitosis can lead to chromosomal abnormalities and various diseases, including cancer.