Mitosis

As mitosis ( to Greek μίτος = Thread ) (also Mitosis or indirect nuclear division, from Greek κάρυον = core; κίνησις = movement ) refers to the process of mitosis in cells of a eukaryotic organism. Following the nuclear division in most cases the division of the cell occurs abdomen ( cytokinesis ) so that from a cell two daughter cells. Mitosis and cytokinesis is also described as M- phase (mitosis phase). With in each case situated between two M-phase interphase form the cell cycle, if a few cell divisions take place in succession.

While a portion of the interphase, S phase, the DNA double-strand contained in the chromosomes is doubled, according to which each chromosome consists of two sister chromatids, both of which contain a copy of the DNA double strand and associated at the centromere. In the subsequent mitosis each chromosome is divided so that both daughter nuclei of each chromosome obtain a sister chromatid. Both cores obtained by a complete copy of the previously present in the mother cell genome. In contrast to meiosis, the ploidy of a cell does not change through mitosis.

• 3.3.1 Resolving the nuclear envelope
• 3.3.2 Completion of the spindle apparatus

• 3.5.1 Chromatidenwanderung
• 3.5.2 anaphase I and anaphase II
• 3.5.3 Introduction of cell division

History

In 1835, the botanist Hugo von Mohl Tübingen observed for the first time the division of a plant cell ( Cladophora glomerata algae ) under the microscope. Ernst Julius Remak managed this on embryonic blood cells in 1841. During the following years other cell researchers saw the same procedure to the cells of many plants and animals. Hugo von Mohl had made an important for the understanding of life processes of discovery in retrospect. The Berlin physician Rudolf Virchow she expressed in 1855 in a theorem from:

" Omnis cellula e cellula: Cells only arise from cells. "

But still reigned vague ideas about the fine structure of the known cellular components and their function. This was particularly the nucleus and its role in the division. Only with the development of microscopes and the staining techniques in the second half of the 19th century, the researchers were able to gain new insights. So far saw the Bonn botanist Eduard Strasburger, 1874 in a preparation of dividing cells unknown stages of division, in which elongated, curved or unwound structures were visible instead of a normal nucleus. Due to their strong stainability she called the Berlin physician Heinrich Wilhelm Waldeyer in 1888 as chromosomes. Closer microscopic examination it was found that each chromosome consists of two identical halves, the chromatids. They lie close together, but are only in one place, the centromere, connected to each other. The term " mitosis " was coined by the cell biologist Walther Flemming.

Chromosomes were discovered not only in plants but also in animal cells. Over time, it was found that each plant species and in all body's cells have a species-specific number of chromosomes. The number is between two chromosomes during Pferdespulwurm ( Ascaris megalocephala univalens ) and a few hundred in some plants.

Function of mitosis

Mitosis allows both daughter nuclei receive the same number of chromosomes and thus the same genetic information. To distribute the genotype of a mother cell to two daughter cells, the chromosomes are in interphase, a preceding phase of the cell cycle, doubled. Each chromosome, which initially consists of a core according to division of a chromatid has, after doubling two identical Schwesternchromatiden associated at the centromere. Mitosis is triggered by so-called mitogens.

In unicellular eukaryotes ( protists ) mitosis is cytokinesis together with the basis of the propagation. In many protists mitosis proceeds as in multicellular eukaryotes open mitosis, i.e., the core sleeve is temporarily released. An exception are the dinoflagellates, in which a closed mitosis occurs without dissolution of the nuclear membrane.

In multicellular eukaryotes, mitosis is a prerequisite for the formation of a new nucleus, and therefore commonly used for the formation of new cells. Some exceptions to this rule are described in the article about cell nuclei. In multicellular organisms such as human cell division during the development no longer takes place in all cells. For example, nerve cells and muscle cells no longer proliferate after completion of differentiation. Here, the cell remains in the so-called G0 phase, so that the DNA is not only replicated (refer to the cell cycle ). Finished Red blood cells can no longer divide because its nucleus is missing and thus no mitosis can be initiated. Epidermal cells V.A. in the gut and the epidermis, however, proliferate significantly more likely than average.

The actual nuclear division takes in human cells usually about an hour (compared with the interphase lasts continuously dividing cells a total of 24 hours on average ). When flying mitosis however, is partly just 8 minutes.

( In the formation of gametes occurs a completely different form of nuclear division in which one meiosis called and arise in the normally haploid from a diploid cell into two division steps four cells. Here only the second division corresponds closely to a normal mitosis. )

Phases

Summary

Mitosis is divided into 4 or 5 interflowing phases. ( The prometaphase is seen particularly in the English literature as a distinct phase during metaphase followed in the classical German literature directly to the prophase. )

• In the prophase of the animal cell to separate the two centrosomes and migrate to opposite poles of the cell. The centrosomes act as microtubule organizing centers ( MTOC English: Microtubule organ- ising center) and are the starting point of the Mitosespindelbildung. ( The cells of higher plants do not possess centrosomes, here act other cell components as MTOCs. ) Condense the chromosomes become visible (only now a mostly shown X - Fom can be seen, in interphase they are not visible with light microscopes because Although it may be several centimeters long in its extended form, but are much too thin ). Since the chromosomes have previously been doubled in interphase, they consist of two identical sisters chromatids that are related only at the centromere. The end of prophase is achieved when the nuclear envelope fragmented ( English literature ), or if the condensation of chromosomes is complete (classical German literature).
• In prometaphase, the nuclear envelope breaks down and the spindle fibers penetrate into the area of ​​the now casingless core. At centromeres set to the three-layer Kinetochor-/Chromosomenmikrotubuli through which the chromosomes can be aligned in the center of the cell in the metaphase and pulled apart at anaphase using the pile fibers.
• In metaphase, the condensed metaphase chromosomes are aligned by the microtubules of the spindle between the spindle poles in the equatorial plane. The metaphase is complete when all the chromosomes have arrived at this metaphase.
• In anaphase the two chromatids of a chromosome are separated by the spindle fibers and drawn ( with the centromere progress ) towards the spindle poles apart. Thus each pole receives a complete chromatid. Thus, the basis for the two future daughter cells is created. The anaphase is considered finished when no longer move the chromosomes of the two future daughter cells further apart.
• Telophase as the final stage of mitosis will be referred to. It follows without transition to the preceding anaphase. The Kinetochorfasern depolymerize, the nuclear envelope is again formed and the chromosomes decondense. After completion of decondensation genes can be read back, the core is re- work form.

The telophase follows in most cases the cytokinesis, the cell division. However, this is not part of the mitosis.

Prophase

Following the interaction phase and thus almost complete replication of DNA condensed chromatin, so that the chromosomes can be seen. This fold and the chromatids condense the extent that the genetic code is not expressible. Now the chromosome structure usually represented with the Chromatidenpaar and the central centromere is visible. In this phase also breaks up the nucleoli ( nucleolus ), as well as the production of ribosome components can no longer take place because of the chromosome compaction. ( At this point, replication is not yet complete. )

Condensation of chromosomes

During interphase the chromosomes are in the nucleus before decondensed. During prophase and condense the chromatin shorten probably by back off and folding. It created light microscopically visible structure, the core loops or chromosomes. They are not fundamentally new structures, but they represent only a more compact, suitable for the transport form of chromatin dar. In this state, the DNA is not expressible.

Each chromosome now shows a clear longitudinal gap. It consists of two longitudinal structures namely, the chromatids, the copies of the original chromosome. At a constriction, the centromere, the chromatids are held together.

Polfaserbildung

Of each of the formed by doubling over the S- phase of the two interphase centrosomes ( Zentriolenpaare ) one moves on opposite sides of the core and so as to form the poles of the spindle. The centrioles, which have also doubled in the S- phase of interphase, organize the formation of the spindle apparatus. Here, from the tubulin subunits of the depolymerized microtubules of the cytoskeleton (see more events) established the elements of the spindle, the first arrange themselves radially around the centrioles. This is also known by the switch.

( The centrioles are not the sole centers of Mikrotubuliorganisation. Too, the associated to them, amorphous mass of centrosomes seems to be responsible for the function of the spindle. According destruction of centrioles by a laser spindle functionality remains namely receive. )

Plant cells need not itself centrioles for their cell division.

More events

It depolymerize the microtubules of the cytoskeleton, whereupon finishing touch is the cell.

Prometaphase

The nuclear envelope is beginning to disintegrate by phosphorylation of the lamins. After the centrosomes have organized at the opposite poles, penetrates the resulting spindle into the nucleoplasm. Now form three-layered kinetochores to which attach appropriate Kinetochormikrotubuli to the centromeres of the chromosomes. These are responsible for the transport of the later separate chromosome fragments and to align parallel to the pile fibers.

Dissolution of the nuclear envelope

The prometaphase begins with the dissolution of the nuclear envelope. The remaining parts are barely distinguishable to the endoplasmic reticulum, as both are homologous origin.

For some single-celled eukaryote ( protozoa ), the nuclear envelope during mitosis process is included, such as the dinoflagellate. There, the spindle is at outside of the nuclear membrane, where it forms kinetochorähnliche structures.

Completion of the spindle apparatus

The outer star fibers contact with the cytoskeleton. Also emerging fiber formations from one pole of the cell to another, pile fibers. So-called three-layered kinetochores form on the centromeres of the chromosomes. These special structures trigger a polymerization of microtubules, after which each form three Kinetochorfasern towards the poles. They allow the movement and orientation and pitch of the chromosomes at the position of the centromeres.

Metaphase

The chromosomes are now shortened maximum. By train and push the spindle apparatus, the chromosomes are transported into the equatorial plane between the poles. This puts them almost exactly between the centrioles in a starting position from which the chromatids of the chromosomes can be pulled apart out. ( Meiosis I differs substantially from mitosis. During mitosis in the two sister chromatids of a chromosome are separated, the homologous pairs of chromosomes separated in meiosis. )

The Kinetochormikrotubuli lie parallel to the pile fibers. According to recent research, it is assumed that no tensile forces from the pole directions are crucial for the drifting apart of the chromatids, but proteins to the centromere locations, which migrate to the microtubule toward the centrosomes. This mechanism then works on the same principle as the dynein microtubule or Kinesinproteine ​​follow. The chromatids are pulled apart by her slowly from the central position. ( The metaphase occurs precisely at the moment in the anaphase about by separating the chromatids of the chromosomes at the centromere location and the two daughter chromosomes, which consist now of only one chromatid, migrate to the individual poles. )

Anaphase

Both chromatids of a chromosome can be separated from each other. These daughter chromosomes ( A chromatid chromosomes) to be transported to the opposite poles of the cell. Here, the Kinetochorfasern shorten.

At the same time the pile fibers, causing the poles repel one another extend.

Chromatidenwanderung

Thus, the two Chromatidenteile can move to the poles, the Kinetochorfasern be depolymerized. The associated reduction in walking the kinetochores to the spindle fibers in the direction in which the density of the pile fibers is increased. The bound to the kinetochore on the centromere Chromatidenteile are thereby pulled along by the plasma as small arms.

Anaphase I and anaphase II

At anaphase is between the moving apart of the chromosomes ( anaphase I) and the moving apart of the spindle poles ( anaphase II) distinguished.

Initiation of cell division

At the same time the pile fibers extend, with the effect that the two polar regions, which have formed in the cell, repel each other and so the condition of cytokinesis, so enter the actual cell division. The subsequent cell nuclear division final telophase occurs with the arrival of the chromosomes at the poles.

Telophase

Reaching the chromosomes, the poles, the ever- shortened Kinetochorfasern depolymerize eventually. Nuclear envelope ( largely from fragments of the old nuclear membrane of the mother cell ) and nucleoli are formed again and the spindle apparatus disintegrates. The chromosomes decondense again. In addition, the pile fibers extend even further, to the poles have reached the maximum repulsion from each other.

( The actual division of the cytoplasm of the cell, and thus is described by the cytokinesis ).

Mitosis and cell cycle

In most cases, after the completed core division is a division, cytokinesis. You will not be counted for mitosis. In animal cells, even during telophase anaphase or even a contractile ring of actin fibers is formed, which is narrowed along with myosin so far that the plasma membranes fuse and the daughter cells separate.

In the following phase, the interphase, the chromosomes can replicate again to allow a new mitosis.

Special

Following mitosis does not have to take place in any case, a cytokinesis.

Thus muscle fiber cells are like liver cells, osteoclasts and some other body cells polynuclear.

The nucleus of the ciliate ( Ciliata ) passes through the conjugation of two mitosis without dividing the cytoplasm.

The flagellates ( Flagellata ), it happens that the nucleus up to 8 times shares, so that a cell occurs with 16 nuclei. Such a cell is called Plasmodium. Until the cell is separated in a step 16 in daughter cells.

Plasmodia of the slime molds can even have thousands of nuclei. However, some of these collections come into existence rather by cell fusions or aggregations. In this case, we speak rather of a syncytium, a giant cell whose cell nuclei taken from many different cells and not from an original cell.