This image shows an abnormal, tetrapolar mitosis. Chromosomes are highlighted pink. The cells shown are S3 tissue cultured cells from Xenopus laevis, African clawed frog.
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle – the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle.
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.
The process of mitosis is fast and highly complex. The sequence of events is divided into stages corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, anaphase and telophase. 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 identical daughter cells which are still diploid cells.
Because cytokinesis usually occurs in conjunction with mitosis, "mitosis" is often used interchangeably with "mitotic phase". 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 moulds, but is found in various groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development. Errors in mitosis can either kill a cell through apoptosis or cause mutations that may lead to certain types of cancer.
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. The genome is composed of a number of chromosomes—complexes of tightly-coiled DNA that contain genetic information vital for proper cell function. Because each resultant daughter cell should be genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurs during the S phase of interphase, the period that precedes the mitotic phase in the cell cycle where preparation for mitosis occurs.
Each chromosome now 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.
The "real" process of mitosis begins when the chromosomes condense and become visible. In most eukaryotes, the nuclear membrane which segregates the DNA from the cytoplasm disintegrates into membrane vesicles. The nucleolus which make ribosomes in the cell also dissolves. The chromosomes align themselves in a line spanning the cell. Microtubules — essentially miniature strings— splay out from opposite ends of the cell and shorten, pulling apart the sister chromatids of each chromosome. As a matter of convention, each sister chromatid is now considered a chromosome, so they are renamed to daughter chromosomes. As the cell elongates, corresponding daughter chromosomes are pulled toward opposite ends. A new nuclear membrane forms around the separated daughter chromosomes.
As mitosis completes,the cell begins cytokinesis. In animal cells, the cell pinches inward where the imaginary line used to be (the area of the cell membrane that pinches to form the two daughter cells is called the cleavage furrow), separating the two developing nuclei. In plant cells, the daughter cells will construct a new dividing cell wall between each other. Eventually, the parent cell will be split in half, giving rise to two daughter cells, each with a replica of the original genome.
Although errors in mitosis are rare, the process may go wrong, especially during early cellular divisions in the zygote. Mitotic errors can be especially dangerous to the organism because future offspring from this parent cell will carry the same disorder.
In non-disjunction, a chromosome may fail to separate during anaphase. One daughter cell will receive both sister chromosomes and the other will receive none. This results in the former cell having three chromosomes containing the same genes (two sisters and a homologue), a condition known as trisomy, and the latter cell having only one chromosome (the homologous chromosome), a condition known as monosomy. These cells are considered aneuploid, a condition often associated with cancer. Occasionally when cells experience nondisjunction, they fail to complete cell division and retain both nuclei in one cell, resulting in binucleated cells.
Mitosis is a demanding process for the cell, which goes through dramatic changes in ultrastructure, its organelles disintegrate and reform in a matter of hours, and chromosomes are jostled constantly by probing microtubules. Occasionally, chromosomes may become damaged. An arm of the chromosome may be broken and the fragment lost, causing deletion. The fragment may incorrectly reattach to another, non-homologous chromosome, causing translocation. It may reattach to the original chromosome, but in reverse orientation, causing inversion. Or, it may be treated erroneously as a separate chromosome, causing chromosomal duplication. The effect of these genetic abnormalities depends on the specific nature of the error.
When mutations occur in the genes that control the timing and number of mitotic cell cycles, cells may lose control of nuclear replication and cellular division. This can result in abnormal cell growth and the synthesis of excess tissue in a single organ. The excess tissue is a cancerous cell mass known as a tumor. Tumors that remain in their original location are called benign tumors and may not be harmful if they do not grow to excessive sizes. Tumors that leave their original location and invade other cells there are called malignant tumors. The migrating tumor cells may lodge in other parts of the body and form new tumors in a process known as metastasis.
Image Source: Gary Gorbsky, Oklahoma Medical Research Foundation
Credits: Tamara Potapova and Gary Gorbsky, Oklahoma Medical Research Foundation,
Research Stir, S.P.C.