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mitosis(mītō`sĭs, mĭ–), process of nuclear division in a living cell by which the carriers of hereditary information, or the chromosomeschromosome
, structural carrier of hereditary characteristics, found in the nucleus of every cell and so named for its readiness to absorb dyes. The term chromosome
..... Click the link for more information. , are exactly replicated and the two copies distributed to identical daughter nuclei. Mitosis is almost always accompanied by cell division (cytokinesis), and the latter is sometimes considered a part of the mitotic process. The pattern of mitosis is fundamentally the same in all cells. However, while animal cells apparently divide by pinching into two separate cells, plant cells develop a cell plate, which becomes a cellulose cell wall between the two daughter cells. The importance of mitosis is the maintenance of the chromosomal set; each cell formed receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell.
The Stages of Mitosis
Mitosis is simply described as having four stages—prophase, metaphase, anaphase, and telophase; the steps follow one another without interruption. The entire four-stage division process averages about one hour in duration, and the period between cell divisions, called interphase or interkinesis, varies greatly but is considerably longer.
During interphase the chromosomes are dispersed in the nucleus and appear as a network of long, thin threads or filaments, called the chromatin. At some point before prophase begins, the chromosomes replicate themselves to form pairs of identical sister chromosomes, or chromatids; the deoxyribose nucleic acidnucleic acid,
any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
..... Click the link for more information. (DNA) of the chromosomes is synthesized only during interphase, not while mitosis is in process.
During prophase the two chromatids remain attached to one another at a region called the centromere, but each contracts into a compact tightly coiled body; the nucleolus and, in most cases, the nuclear envelope break down and disappear. Also during prophase the spindle begins to form. In animal cells the centrioles separate and move apart, and radiating bundles of fibers, called asters, appear around them. Some sets of fiber run from one centriole to the other; these are the spindle fibers. In plant cells the spindle forms without centrioles.
During metaphase the chromosomes congregate at a plane midway between the two ends to which the spindle tapers. This is called the equatorial plane and marks the point where the whole cell will divide when nuclear division is completed; the ends of the spindle are the poles to which the chromatids will migrate. The chromatids are attached to the spindle fibers at the centromeres.
During anaphase the two chromatids of each chromosome separate and move to opposite poles, as if pulled along the spindle fibers by the centromeres. During telophase new nuclear envelopes form around the two groups of daughter chromosomes (as they are now called), the new nucleoli begin to appear, and eventually, as the formation of the two daughter nuclei is completed, the spindle fibers disappear. The chromosomes uncoil to assume their dispersed distribution within the interphased nucleus. Cytokinesis, which may begin before or after mitosis is completed, finally separates the daughter nuclei into two new individual daughter cells.
A considerable variance in the degree and timing of these stages exists across species, and cells can be classified by their mitotic characteristics. Despite the relative ease of observation of the physical stages of mitosis under the microscope (primarily because the chromosomes stain readily when in their coiled state), the exact chemical and kinetic nature of mitosis is not yet fully understood. For instance, the spindle has been determined to consist largely of thin, elongate tubules called microtubules, but their functions have yet to be understood.
Meiosis and Amitosis
Mitotic division is the method of nuclear division of the somatic (body) cells, as distinguished from the gametes, or sex cells (eggs and sperm). In sexual reproduction, i.e., by the union of two gametes, the complex process of meiosismeiosis
, process of nuclear division in a living cell by which the number of chromosomes is reduced to half the original number. Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum and sperm), are being formed.
..... Click the link for more information. takes place, which produces cells that each contain only half the normal number of chromosomes. Direct cell division, in which the nucleus simply cleaves in two (sometimes but not always followed by division of the cytoplasm), is called amitosis and is very rare.
The series of visible changes that occur in the nucleus and chromosomes of non-gamete-producing plant and animal cells as they divide. During mitosis the replicated genes, packaged within the nucleus as chromosomes, are precisely distributed into two genetically identical daughter nuclei (see illustration). The series of events that prepare the cell for mitosis is known as the cell cycle. When viewed in the context of the cell cycle, the definition of mitosis is often expanded to include cytokinesis, the process by which the cell cytoplasm is partitioned during cell division.
Chromosome segregation is mediated in all nonbacterial cells (that is, eukaryotes) by the transient formation of a complex structure known as the mitotic spindle. During mitosis in most higher plants and animals, the nuclear membrane surrounding the replicated chromosomes breaks down, and the spindle is formed in the region previously occupied by the nucleus (open mitosis). In lower organisms, including some protozoa and fungi, the spindle is formed and functions entirely within the nucleus which remains intact throughout the process (closed mitosis).
All spindles are bipolar structures, having two ends or poles. In animal cells, each spindle pole contains an organelle, the centrosome, onto which the spindle focuses and terminates. The polar regions of plant spindles lack centrosomes and, as a result, are much broader. In animals the bipolar nature of the spindle is established by the separation of the centrosomes, which is critical for successful mitosis; the presence of only one pole produces a monopolar spindle in which chromosome segregation is inhibited. The presence of more than two poles produces multipolar spindles which distribute the chromosomes unequally among three or more nuclei. Centrosomes are duplicated during interphase near the time that the DNA is replicated, but then act as a single functional unit until the onset of mitosis. In plants, and during meiosis in some animals, the two spindle poles are organized by the chromosomes and by molecular motors that order randomly nucleated microtubules into parallel bundles. See Centrosome, Plant cell
Microtubules are the primary structural components of the mitotic spindle and are required for chromosome motion. These are 25-nanometer-diameter, hollow, tubelike structures. During interphase, microtubules are distributed throughout the cytoplasm, where they serve to maintain cell shape and also function as polarized roadways for transporting organelles and cell products. As the cell enters mitosis, the cytoplasmic microtubule network is disassembled and replaced by the mitotic spindle. The microtubules in animal cells originate from the centrosome which, like the chromosomes, was inherited during the previous mitosis where it functioned as a spindle pole. The motion associated with microtubules is mediated by several families of molecular motors which bind to and move along the wall of the microtubule. See Cytoskeleton
As mitosis begins, each replicated chromosome consists of two identical sister chromatids that are joined along their length. In most cells, chromosomes possess a unique region of highly condensed chromatin (DNA plus protein), known as the centromere, which forms an obvious constriction on the chromosome, referred to as the primary constriction. Spindle microtubules attach to a small specialized structure on the surface of the centromere known as the kinetochore. Fragments of chromosomes lacking a kinetochore do not move poleward; it is always the kinetochore that leads in the poleward motion of the chromosome. The centromere region of each replicated chromosome contains two sister kinetochores, one attached to each chromatid, that lie on opposite sides of the primary constriction.
Once initiated, mitosis is a continuous process that, depending on the temperature and organism, requires several minutes to many hours to complete. Traditionally it has been subdivided into five consecutive stages that are distinguished primarily by chromosome structure, position, and behavior. These stages are prophase, prometaphase, metaphase, anaphase, and telophase. In prophase, cell chromosomes condense within the nucleus. By late prophase/early prometaphase, the nuclear envelope breaks down; kinetochore-containing primary constrictions are sometimes visible; the cytoplasmic microtubule complex is replaced by two radial astral microtubule arrays; centrosomes separate; and microtubules in each aster grow and shorten at their ends away from the centrosome. By mid-prometaphase, the kinetochores on the chromosomes interact with the asters to form the spindle. In metaphase, all of the chromosomes are aligned on the spindle equator; sister kinetochores are attached to opposite poles by kinetochore fibers. In anaphase, the sister chromatids separate and move toward their respective spindle poles; at the same time the spindle poles move farther apart. In telophase, the two groups of sister chromosomes become two well-separated sister nuclei, and the cytoplasm of the cell divides (cytokinesis). See Cell (biology), Cell nucleus
(karyokinesis, indirect cell division), the most common mode of cell reproduction, which ensures the identical distribution of genetic material between the daughter cells and the continuity of the chromosomes in successive cell generations. Mitosis is biologically important because it involves both the doubling of the chromosomes by longitudinal splitting and the even distribution of chromosomes between the daughter cells. The beginning of mitosis is preceded by a preparatory period during which energy is stored and deoxyribonucleic acid (DNA) and the centrioles are replicated. Energy-rich macroergic compounds are the source of energy for mitosis. Because oxidative processes occur during interphase and the ‘’energy reservoir” is filled, mitosis is not accompanied by an intensification of respiration. The periodic filling and emptying of the energy reservoir provides the energy for mitosis.
Stages. The mitotic process is usually divided into four stages: prophase, metaphase, anaphase, and telophase. The preprophase (antephase, or resting stage), another stage sometimes observed, precedes the prophase and is the synthetic stage of mitosis, or the end of interphase (S-G2 periods)—that is, the synthetic through the premitotic stages). During preprophase the DNA molecules are replicated, and the materials of the mitotic apparatus are synthesized.
During prophase, the nucleus is reorganized. Condensation and spiralization of the chromosomes occur, the nuclear membrane disintegrates, and the mitotic apparatus is formed by the synthesis of proteins and their assembly into the cell division spindle, which consists of oriented fibers. Metaphase includes the movement of the chromosomes to the equatorial plane (meta-kinesis or prometaphase), the formation of the equatorial plate (mother star), and the disjunction of the chromatids, or sister chromosomes.
At anaphase the chromosomes move to the poles of the cell. This motion is caused by a lengthening of the central fibrils of the spindle, which pushes back the mitotic pole, and by a shortening of the chromosomal microtubules of the mitotic apparatus. The central fibrils of the spindle are lengthened either by polarization of the “reserve” macromolecules that complete the microtubules of the spindle or by dehydration of the spindle. The chromosomal microtubules become shorter because of the special properties of the contractile proteins of the mitotic apparatus, which are capable of contracting without thickening.
Telophase involves the construction of daughter cells from the chromosomes gathered at the poles, the division of the cell body (cytotomy, or cytokinesis), the disintegration of the mitotic apparatus, and the formation of the interphase body. The daughter nuclei are constructed by despiralization of the chromosomes and restoration of the nucleoli and nuclear membranes. Plant cells divide by forming a cell plate, and animal cells, by forming a division furrow. According to the “contractile ring” hypothesis, cytokinesis is the result of the contraction of the gelatinized ring of cytoplasm that surrounds the cell equator. The “membrane enlargement” hypothesis argues that cytokinesis occurs as a result of the enlargement of the cell surface as the loop-shaped protein chains become straight.
Duration. The duration of mitosis varies with the size of the cell, the number of chromosomes and nuclei, and environmental conditions, especially temperature. Mitosis takes 30–60 minutes in animal cells and two to three hours in plant cells. The longest stages are those involved in synthesis (preprophase, prophase, and telophase). The spontaneous movement of the chromosomes (metakinesis and anaphase) occurs rapidly.
Regulation. Mitosis is neurohumorally controlled by the nervous system; the adrenal, pituitary, thyroid, and sex hormones; and intracellular factors, such as the products of tissue break-down and the functional activity of the cell. The interaction of various regulatory mechanisms induces both general and local changes in mitotic activity. Mitosis of tumor cells is not subject to neurohumoral regulation.
The daily rhythm of cell division reflects the relationship between the regulation of mitosis and the interaction of the organism with its environment. In most organs of nocturnal animals, mitosis is most intense in the morning and least intense at night. In diurnal animals and man, the opposite is true. The daily rhythm of mitosis is the consequence of a chain reaction involving rhythmic changes in the environment (light, temperature, and nutrition, for example), the rhythm of the cells’ functional activity, and changes in metabolic processes.
Disruption. The normal course of mitosis may be disrupted by various pathological processes, of which there are three main categories. First, the chromosomes may be injured. This may involve swelling, adhesion, fragmentation, formation of bridges, injury to the centromeres, lagging of individual chromosomes during movement, disturbance of their spiral structure or despiralization, premature disjunction of the chromatids, or formation of micronuclei. Second, the mitotic apparatus may be injured. This may result in the delay of mitosis in metaphase; in multipolar, monocentric, and asymmetrical mitosis; and in tripolar or “hollow spindle” metaphase. Of particular significance in this group of mitotic pathologies is colchicine mitosis, or c-mitosis, which can be induced experimentally by the alkaloid colchicine, as well as by colcemide, vinblastine, vincristine, acenaphthene, and other stathmokinetic poisons used as mutagens. C-mitoses may also arise spontaneously in tissue culture and in tumors. In c-mitosis, the separation of the centrioles and their polarization of the cell division spindle are impaired, the mitotic apparatus is disorganized, and chromatid disjunction (c-pairs) does not occur.
The third major category of mitotic pathology includes disturbances of cell division, which may arise after exposure to mitotic poisons, toxins, extreme environmental factors (ionizing radiation, anoxia, and hypothermia) or in connection with viral infections and tumors. A sharp increase in pathological mitosis is typical of malignant tumors.
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I. A. ALOV