Mitosis, also called karyokinesis, is division of the nucleus and its chromosomes that allows each daughter nucleus to receive one copy of each chromosome. It is typically followed by division of the cytoplasm known as cytokinesis. When an organism needs new cells to repair damage, grow, develop, or reproduce asexually, cells undergo mitosis. Both mitosis and cytokinesis are parts of the life of a cell called the cell cycle. Most of the life of a cell is spent in a non-dividing phase called Interphase. Interphase includes the G1 stage in which the newly divided cells grow in size, the S stage in which the number of chromosomes is doubled and appears as chromatin, and the G2 stage where the cell makes the enzymes and other cellular materials needed for mitosis.

Mitosis occurs only in eukaryotic cells and the process varies in different species. For example, animals undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae (yeast) undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus. (Prokaryotic cells, which lack a nucleus, divide by a process called binary fission, not mitosis.) Nuclear division in eukaryotes is often coordinated with cell division, however, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei. This occurs most notably among the fungi and slime molds, but is found in various groups.

The process of mitosis is fast and highly complex. The primary result of mitosis is the transferring of the parent cell's genome into two daughter cells. These two cells are identical and do not differ in any way from the original parent cell. Because each resultant daughter cell will be genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurred during the S stage of interphase. Each chromosome has an identical copy of itself, and together the two are called sister chromatids. The sister chromatids are held together by a specialized region of the chromosome: a DNA sequence called the centromere.

During mitosis the pairs of chromatids condense and attach to fibers that pull the sister chromatids to opposite sides of the cell. The cell then divides in cytokinesis, to produce two daughter cells which are identical genetically to the parent cell from which they came.Mitosis has four major stages Prophase, Metaphase, Anaphase, and Telophase.


Plant Cell Cycle Phases (Onion Root Tip)

Notice that some of the phases occur in stages, so there are what are considered early stages and late stages. 


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Animal Cell Cycle Phases (Whitefish Blastula)

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