Cell division or mitosis is the process whereby one cell produces two identical daughter cells. This process allows multicellular organisms to repair damaged tissue and to grow. Although interphase, when the cell is preparing for mitosis by creating a nucleolus or dark spot and centrioles (organizational sites for microtubules), is sometimes included in the mitosis discussion, it is actually part of the cell cycle and not part of the cell division process. The six stages of actual cell division are called phases.
In prophase, the first phase of mitosis, chromatin (a precise, compact, dense string-like fiber containing DNA and proteins) condenses into chromosomes, each of which has been replicated internally into two daughter chromosomes that are connected at the centromeres. Also during prophase, the dark spot or nucleolus disappears (as does the nucleus membrane) as the centrioles (organelles found only in animal cells composed of nine bundles of microtubules) move to the opposite sides of the cell. The mitotic spindle or spindle apparatus is formed by some of these fibers emerging from the centromeres, the central part of the chromosome.
Before prometaphase, the nuclear membrane dissolves; during prometaphase, kinetochores (proteins that attach to the centromeres of the chromosomes) develop. The chromosomes start moving toward the center of the cell after the microtubules (hollow strands of protein) attach themselves at the kinetochores.
Metaphase occurs when the metaplate is established by the spindle fibers of the spindle apparatus which align the chromosomes along the center of the cell nucleus. This alignment ensures that when the chromosomes are separated in the anaphase, each new nucleus will receive one partner of each pair of chromosomes. In the last part of the metaphase stage, the chromosomes separate into the daughter chromosomes called chromatids, which double the number of chromosomes.
During anaphase, actual cell division begins as the newly formed chromosomes head toward the opposite poles of the spindle. This movement comes from the polar microtubules’ physical interaction and the movement of the kinetochore along the spindle microtubules.
During telophase, chromosomes reach the opposite poles of the cell and return to chromatin; at each spindle pole, new nuclear membranes and the nucleoli (structures that produce ribosomal subunits containing RNA and proteins), reappear, creating the two new nuclei. The spindle apparatus dissolves, and the partitioning of the cell may begin.
Cytokinesis, or the separating of the two daughter cells, in animal cells happens when a fiber ring containing a protein called actin, located in the cell’s center, contracts, pulling the cell membrane to it, and subsequently squeezing the two nuclei apart. Because cell walls are so rigid in plant cells, cytokinesis in plant cells occurs when a cell plate takes the place of the metaplate and grows between the two new cells. This plate becomes a membrane that inculcates itself into the cell wall in each of the daughter cells.