Metaphase

The nuclear division (mitosis) of cells of eukaryotic organisms with replication of the DNA can be divided into four main phases. The second main phase is called Metaphase in the course of which the chromosomes contract in a spiral and position in the equatorial plane with approximately the same distance to both opposite poles. The spindle fibers are connected to the centromeres of the chromosomes from both poles.

What is the metaphase?

The metaphase is the second of a total of four main phases into which the nucleus division of eukaryotic cells, called mitosis, can be divided. During the metaphase, the arrangement of the chromosomes in the so-called equatorial plane or metaphase plate is characteristic.

Every single chromosome consists of four chromatids, two of which are "identical". The chromatids are initially held together by their common centromere. Small protein structures form on the centromeres, to which the fibers of the spindle poles attach in order to pull the sister chromatids to the opposite poles. The pulling apart of the chromatids is already part of the anaphase that follows the metaphase.

During the metaphase all preparations are made which are necessary to detach the chromatids from the centromeres in order to be able to be drawn to the poles. Only when all centromeres are connected with the corresponding pole fibers or microtubules, the bonds of the chromatids on their centromere are released so that their displacement to the respective pole begins.

Function & task

In the human body there is an ongoing need for growth based on cell reproduction, which mostly follows the principle of cell division. In nucleated cells of single and multicellular organisms (eukaryotes), the divisions include the division of the cytoplasm and their cell nuclei.

The two daughter cells that result from division are identical in their diploid chromosome sets with the respective "mother cell", so that the growth of certain tissues in the body is theoretically unlimited on the basis of non-sexual cell division, provided that the division process is not interrupted or terminated by growth-inhibiting substances.

The process of cell division is also linked to the process of nuclear division, which is known as mitosis. Within mitosis, the second of a total of four main phases is known as the metaphase. It is an important link in the core division process. The metaphase is important in order to position the chromatids of the double set of chromosomes in the equatorial plane or metaplate in such a way that they can be drawn by the microtubule filaments in the direction of the two poles in the subsequent anaphase.

A particularly important function of the metaphase is to check (checkpoint) and monitor the spindle fibers (microtubules) emanating from the poles. It must be ensured that the microtubules are connected to the "correct" centromere. This ensures that the two sets of chromosomes that are grouped at the poles during the subsequent anaphase are absolutely identical. This can only be achieved by having a chromatid of a chromosome at each of the two poles after the nucleus has been divided.

If, for example, two identical sister chromatids were to be found at one of the two poles and were missing at the other pole, this would lead to considerable disturbances with the impossibility of further cell growth or unchecked growth. In the case of parenchyma cells, there would be a loss of the specific functionality of the cells.

Illnesses & ailments

Mitosis embodies a very complex process that involves the risk of errors within the replication of the DNA strands and the distribution of the chromatids on the two poles, with sometimes far-reaching consequences. For example, “incorrect” attachment of microtubules to the kinetochores of the centromeres can occur relatively often. For example, certain kinetochores can remain free, i.e. not connected to a microtubule, or both chromatids are connected to microtubules of the same pole at their centromeres. One of the most important functions of the metaphase lies in checking for “correct” and complete attachment of the microtubules to the kinetochore.

The pulling apart of the chromosomes in the anaphase is normally only released when the check of the spindle fibers is successful and all kinetochores signal the correct connection. The mitotic checkpoint is implemented by a group of specialized proteins that suppress or cash in the transition to the anaphase if the adhesion does not correspond to the target value. The process is somewhat comparable to a pit stop in a Formula 1 race, when all four fitters have to report completion after changing wheels before the Formula 1 driver can start again.

Another bigger problem arises when mistakes are made in breaking the DNA strands. This can lead to a loss of function of the cells and to continuous, fast or slow progressing further mitoses that no longer react to the body's own growth inhibitors. The uninhibited growth characterizes benign (benign) or malignant (malignant) tumors.

Other problems can arise from DNA methylation. When the DNA strands are split up, the activity of DNA methyltransferases can lead to the addition of methyl groups (-CH3) to the DNA. Although the process does not correspond to a gene mutation in the conventional sense, it does correspond to an epigenetic change in the affected gene. The "gene methylation" usually leads to phenotypically recognizable changes in the affected individual and is mostly passed on to the next generation of cells - similar to an inheritance.

The extent to which the development of benign and malignant tumors and DNA methylation can be traced back to processes within the metaphase has not been adequately researched.