Mitosis - Simply Explained!

What is Mitosis?

Mitosis describes the process of cell division. Cell division begins with the doubling of the DNA and ends with the pinching off of the new cell. In this way, two identical daughter cells are created from a mother cell that contain the same genetic information. During the entire mitosis, both the mother cell and the two daughter cells that arise have a double (diploid) Set of chromosomes. In addition to the interphase, mitosis is part of the cell cycle and is used to multiply body cells, such as skin cells. Mitosis can be divided into different stages and always proceeds in the same way.

Abandonment of mitosis

The task of mitosis is cell division and thus the multiplication of body cells. The precondition for the process of mitosis is the preceding interphase, in which the DNA is doubled. A mother cell with a double (diploid) set of chromosomes results in two identical daughter cells in a process that is always the same. These also have a double set of chromosomes, which however only consists of one chromatid. The doubling of the DNA takes place again in the interphase. However, mitosis does not take place in all cells in our body. A distinction is made between body cells and germ cells, which are not created by mitosis but by meiosis. The result of meiosis are four daughter cells with a simple (haploid) set of chromosomes that are ready for fertilization. Another specialty are cells that have reached a highly specialized form and then no longer divide. These include, for example, nerve cells or red blood cells. However, mitosis plays an extremely important role in cells that limit surfaces, such as skin cells or surface cells (epithelial cells) in the gastrointestinal tract. These cells need to be renewed regularly, which is the job of mitosis. The constant process of mitosis in different stages and several control points within the interphase ensures that no errors occur during cell division.

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What are the phases of mitosis?

The cell cycle, which is responsible for cell division and thus also for cell reproduction, can be divided into interphase and mitosis. In the interphase, the DNA is doubled and the cell is prepared for the upcoming mitosis. This phase of the cell cycle can vary in length and varies greatly depending on the cell type. Mitosis is the second phase of the cell cycle and involves the division of genetic material and the formation of two identical daughter cells from a common mother cell. This cell division process can be divided into different stages in which characteristic processes always take place. Depending on the source, a distinction is made between four to six stages.

At the beginning there is the prophase, in which the two chromosomes condense and the spindle apparatus also arises. Next, the two maximally condensed chromosomes line up in the equatorial plane, which is described as metaphase. Between these two stages, some authors mention the prometa phase. Next, the two sister chromatids are separated in the anaphase. Finally, a new nuclear membrane forms in the telophase and the chromosomes loosen up again. In some books, the so-called cytokinesis is still regarded as a separate phase. During cytokinesis, the new cell body constricts, so that ultimately two identical daughter cells are created.

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What is the metaphase?

The metaphase is part of mitosis and thus a stage in the division of cells in the body. It is the third phase of mitosis and follows the prometa phase. After the chromosomes have condensed and the nuclear membrane has dissolved, the double set of chromosomes is arranged in the equatorial plane. The metaphase is also the only phase of mitosis in which the chromosomes are clearly visible under the microscope. This is because DNA has assumed its most compact form in this phase of cell division. The two 2-chromatid chromosomes are now next to each other on the equatorial plane of the cell. This plane is approximately the same distance from both cell poles. This position is guaranteed by the built-up spindle apparatus, which separates the sister chromatids from one another in the further course of mitosis.

What is the anaphase?

The anaphase is the fourth phase of mitosis and thus a step in the cell division of nucleated cells. After the chromosomes have condensed and have arranged themselves in the metaphase in the equatorial plane, the anaphase follows. In this step, the sister chromatids are separated from each other by the spindle apparatus and drawn to opposite cell poles. Thus, the actual chromosome division begins in the anaphase. In this way, a still double set of chromosomes is created from an original mother cell with a double set of 2 chromatid chromosomes. However, this now only consists of two 1-chromatid chromosomes. The anaphase is followed by the telophase.

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What is the telophase?

Telophase describes the last step of mitosis, in which the genetic information of nucleated cells is shared in order to enable cells to multiply. The telophase follows the anaphase. The sister chromatids were drawn from the equatorial plane to the opposite cell poles with the aid of the spindle apparatus. In telophase, the chromosomes have each reached their cell pole and the spindle apparatus dissolves. At the same time, a new nuclear envelope is formed from the fragments of the disintegrated nuclear membrane. This chromosome division is now followed in a further step by cytokinesis. Here, a cell body constricts, so that two independent but identical daughter cells are created.

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Duration of mitosis

Mitosis lasts about an hour on average, so that one can speak of rapid cell division. Compared to interphase, mitosis takes relatively little time. In addition, depending on the cell type, the interphase can last from several hours to several months or even years. The G1 and G0 phases in the interphase are particularly responsible for this. In the G1 phase various proteins and cell organelles are produced and in the G0 phase the cell goes into a kind of dormant mode. Many cells remain in the G0 phase for years or even decades.

What is the mitotic rate?

The rate at which cells divide can be described with the help of the mitotic rate. This allows conclusions to be drawn about the speed of reproduction of certain tissues. The mitotic rate is determined with the help of a microscope. With a certain number of cells, for example 1,000 cells, it is determined how many of them are in a mitotic stage. The mitotic rate is given as a percentage and is therefore a relative figure. Tissue that is renewed particularly often has a high mitotic rate. These include the bone marrow, the skin (epidermis) and the mucous membrane of the small intestine. The bone marrow is responsible for the formation of blood and continuously produces new blood cells. The skin and the mucous membrane of the gastrointestinal tract are also regularly renewed, so that a high mitotic rate can also be found here. However, high rates of mitosis can also indicate malignant tumors that are growing rapidly. These degenerated cells evade the control points in the interphase and mitosis and can grow unhindered. The increased mitotic rate can also be used as a therapeutic approach, since fast-growing tumors are particularly sensitive to mitosis inhibitors and can be treated with a higher chance of recovery.

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What are mitosis inhibitors?

Mitosis inhibitors are substances that inhibit the process of mitosis. Mitosis inhibitors prevent the nucleus from dividing and consequently stop cells from multiplying. These toxins are used as cytostatics in tumor treatment. Lymphomas and leukemia in particular respond well to this form of chemotherapy. The mechanism of the mitosis inhibitor consists of binding to the tubulin, which is needed to build up the spindle apparatus. Tubulin is the protein from which the microtubules of the spindle apparatus are composed. If this protein is not available due to the binding of a mitosis inhibitor, no spindle apparatus can be built up and the cell nucleus does not divide. However, mitosis inhibitors such as vinca alkaloids or taxanes can have dangerous side effects that can damage the nervous system in particular.

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What is the difference between mitosis and meiosis?

Both mitosis and meiosis are responsible for core divisions, with both processes differing in their course and result. Mitosis creates two identical daughter cells with a double (diploid) set of chromosomes from a mother cell. In contrast to meiosis, only one chromosome division is necessary. Overall, mitosis has the function of distributing the entire genetic information in the form of DNA between two identical cells and is therefore essential for cell reproduction. In contrast, meiosis is important for the formation of the germ cells for sexual reproduction. Since germ cells have a simple (haploid) set of chromosomes, meiosis requires two nuclear divisions. In the first meiosis, a double set of chromosomes is formed. The second equivalent division now separates the sister chromatids from one another, so that we have a total of four daughter cells, each with a simple set of chromosomes. Thus, mitosis and meiosis differ in the number of divisions, in the number and type of daughter cells and in their duration. Mitosis takes about an hour to complete. Meiosis, on the other hand, takes much longer. The prophase of meiosis alone takes around 24 hours in men (sperm formation) and in women several years or even decades (formation and maturation of the egg cell).

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What is the interphase?

Besides mitosis, interphase is the second part of the cell cycle. It always lies between two mitotic divisions and has different tasks. During the interphase, the DNA halved in mitosis is doubled again. In addition, there is general cell growth of the two daughter cells and they are prepared for renewed mitosis. Just like mitosis, the interphase can be divided into several stages. Immediately after mitosis, the G1 phase follows the interphase. The double set of chromosomes in the daughter cells consists of only one chromatid each. In this phase the daughter cells grow and many proteins and enzymes are produced. The next phase is the so-called S-phase (synthesis phase). Here the DNA is doubled so that we still have a double set of chromosomes, which now also consists of two chromatids. In the last phase of the interphase, the G2 phase, both daughter cells grow again and will prepare for the upcoming mitosis. The two daughter cells have now created new mother cells that can be divided in mitosis. The interphase lasts about 18 hours on average and thus takes much more time than mitosis (duration about one hour). Two control points are important in the interphase, which are located at the transition from the G1 phase to the S phase and from the G2 phase to mitosis. Here the cell and in particular the genetic information are checked for possible errors. If an error is found, it is fixed first before the cell divides. If the error were not recognized and eliminated, it would continue to reproduce in many cells through mitosis.

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