Nuclear division (mitosis) of cells of eukaryotic organisms with replication of DNA can be divided into four main phases. The second main phase is called metaphase, during which chromosomes contract in a spiral pattern and position themselves in the equatorial plane at approximately equal distances from both opposite poles. The spindle fibers, starting from both poles, are connected to the centromeres of the chromosomes.
What is metaphase?
Metaphase is the second of four major phases into which nuclear division of eukaryotic cells, called mitosis, can be divided. During metaphase, the arrangement of chromosomes in the so-called equatorial plane or metaphase plate is characteristic. Each individual chromosome consists of four chromatids, two of which are “identical in construction”. The chromatids are initially still held together by their common centromere. Small protein structures form at the centromeres to which the fibers of the spindle poles attach in order to pull the sister chromatids to the respective opposite poles. The pulling apart of the chromatids already belongs to anaphase, which follows metaphase. During metaphase, all preparations necessary to detach the chromatids from the centromeres in order to be pulled to the poles are in progress. Only when all centromeres are connected to the corresponding pole fibers or microtubules are the bonds of the chromatids at their centromere released so that their shipment to the respective pole can begin.
Function and task
In the human body, there is an ongoing need for growth based on cell proliferation, which usually follows the principle of cell division. In nucleated cells of unicellular and multicellular organisms (eukaryotes), divisions involve the division of the cytoplasm and their nuclei. The two daughter cells resulting from the division are also identical in their diploid chromosome sets to the respective “mother cell”, so that the growth of certain tissues in the body on the basis of non-sexual cell division is theoretically unlimited, provided that the division process is not interrupted or terminated by growth-inhibiting substances. Also associated with the cell division process is the nuclear division process known as mitosis. Within mitosis, the second of a total of four main phases is called metaphase. It is an important chain link within the nuclear division process. Metaphase is important for positioning the chromatids of the double set of chromosomes in the equatorial plane or metaplate in such a way that they can be pulled toward the two poles by the microtubule filaments in the subsequent anaphase. A particularly important function of metaphase is to check (checkpoint) and monitor the spindle fibers (microtubules) extending from the poles. It must be ensured that the microtubules are connected to the “correct” centromere in each case. This is to ensure that the two sets of chromosomes grouping at the poles during the following anaphase are absolutely identical. This can only be achieved by having one chromatid of a chromosome at each of the two poles after nuclear division has occurred. If, for example, two identical sister chromatids were located at one of the two poles and missing at the other pole, there would be considerable disturbances with the impossibility of further cell growth or unrestrained growth. In the case of parenchymal cells, there would be a loss of the specific functional capacity of the cells.
Diseases and disorders
Mitosis embodies a very complex process that, within the replication of DNA strands and the distribution of chromatids to the two poles, carries the risk of errors with sometimes far-reaching consequences. For example, “incorrect” attachment of microtubules to the kinetochores of centromeres can occur relatively frequently. For example, certain kinetochores may remain free, i.e. not connected to a microtubule, or both chromatids may be connected at their centromeres to microtubules of the same pole. In checking for “correct” and complete attachment of microtubules to kinetochores lies one of the most important functions of metaphase. The chromosomes in anaphase are normally not released until the check of the spindle fibers is successful and all kinetochores signal correct attachment.The mitotic checkpoint is realized by a group of specialized proteins that suppress the transition to anaphase or cash in if the adhesion does not correspond to the setpoint. The process is somewhat comparable to a pit stop at a Formula 1 race, when all four mechanics have to report full stop after changing the wheel before the Formula 1 driver can take off again. Another major problem arises when errors occur during the splitting of the DNA strands. This can lead to a loss of function of the cells and to continuous fast or slow further mitoses that no longer respond to endogenous growth inhibitors. Uninhibited growth characterizes benign (benign) or malignant (malignant) tumors. Further problems can arise from DNA methylation. During the splitting of DNA strands, the activity of DNA methyltransferases can lead to the addition of methyl groups (-CH3) to the DNA. The process does not correspond to a gene mutation in the conventional sense, but 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 usually passed on to the next cell generations – similar to an inheritance. The extent to which the development of benign and malignant tumors and DNA methylation can be attributed to processes within metaphase has not been adequately explored.
