Introduction
Most of the body’s tissues renew themselves continuously. This renewal is achieved by a constant formation of new cells. This new formation is achieved by a division of cells.
This cell division requires that the cells are capable of dividing. The cells capable of division in adults are called adult stem cells. The actual division of the cell, which is also called cytokinesis, is preceded by the division of the cell nucleus.
The cell nucleus contains mostly DNA. The DNA contains the genetic information. To ensure that the resulting cells contain all the information, the containing DNA is doubled before the division of the cell nucleus. The process of the division of the cell nucleus is also called mitosis.
Sequence of a cell division
The cell nuclear division takes place in 5 phases. At the end of these 5 phases, instead of one nucleus, there are two fully functional and identical cell nuclei. It is important for the understanding of nuclear division that the DNA is organized in chromosomes.
The entire genetic information in humans and animals is therefore divided into several chromosomes. In all body cells, except for the egg and sperm cells, humans possess 2 copies of the entire genetic information. One copy from the mother and one from the father.
In total, the DNA in the cell nucleus is divided into 46 chromosomes. Mitosis is preceded by the doubling of the genetic information in the so-called cell cycle, i.e. the life cycle of a cell. Before doubling, the chromosomes are present as one-chromatid chromosomes, after doubling, they are present as two-chromatid chromosomes.
After the cell nuclei have divided, there are again single-chromatid chromosomes. This is to illustrate that the genetic information is doubled before the cell nuclei divide and no information is lost. Nuclear division begins when the chromosomes are packed more tightly together.
Actually, these are present in the cell nucleus unsorted. Through this condensation, the individual chromosomes can be recognized under the light microscope. This is not possible before, because the chromosomes are previously unsorted and fill the cell nucleus.
At the same time, the shell surrounding the cell nuclei decays. Then the chromosomes are arranged in a line by the spindle apparatus. The spindle apparatus consists of protein structures that are arranged in a thread-like fashion, the microtubules.
These protein structures can move chromosomes and thus arrange them in one plane for the next steps. Once the chromosomes are correctly arranged, the spindle apparatus pulls the two identical chromatids apart. Thus, single chromatid chromosomes have now been created again.
In the end, the shell of the cell nucleus is rebuilt and two identical nuclei are present. The cell then divides and the nuclei are distributed to two newly formed cells. However, this process is not part of the cell nucleus division, but a separate step and is called cell division or cytokinesis.
Nuclear division can be divided into 5 phases. The phases are called prophase, prometaphase, metaphase, anaphase and telophase. In the first phase, the prophase, mainly the condensation of the chromosomes takes place.
Before this phase, the individual chromosomes cannot be distinguished from each other under the light microscope. Only in the condensed state they become visible as individual chromosomes. Besides condensation, the decay of the shell surrounding the nucleus begins.
In the next phase, the prometaphase, the nuclear envelope decays completely, and the spindle apparatus is formed. The spindle apparatus becomes important in the subsequent phase, the metaphase. In this phase the chromosomes are ordered.
The following phase is called anaphase. In this phase, the chromosomes are separated so that 2 identical daughter chromosomes are formed. The resulting chromosomes also move apart.
The last phase of mitosis is the telophase, in which the nuclear shells are restored. In addition, the condensation of the chromosomes is reversed. At the end of the telophase there are two functional cell nuclei. This topic might also interest you: Tasks of the cell nucleus
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