What causes promote cell division
Mitosis, the process of division between cells and chromosomes
Research Report 2011 - Max Planck Institute for Molecular Physiology
Mitosis and meiosis are critical stages in the eukaryotic cell cycle. In mitosis, the duplicated chromosomes of a mother cell are evenly separated from one another in order to be able to form two daughter cells with full genetic, complementary equipment. On the other hand, during meiosis, the doubled chromosomes are divided by reduction in such a way that the homologous maternal and paternal chromosomes are randomly distributed to the gametes.
From a molecular point of view, mitosis and meiosis are similar, amazingly complex processes. Polymeric structures known as microtubules play a central role in mitosis and meiosis. Microtubules are dynamic biomolecules that result from the polymerisation of a building block called tubulin. They interact with a rich palette of different proteins, including molecular machines (motors) that use the energy provided for the intracellular transport of various loads and use the microtubules as “rails”.
Mitosis and meiosis have far-reaching effects on human health. Aneuploidy (the presence of a surplus chromosome) is far too common an abnormality in tumors (Fig. 1). It is believed that aneuploidy is a result of errors made during mitosis. It is therefore important to understand the cause of such errors and to study the mechanisms that protect normal cells from these errors occurring. The problems that can arise during meiosis lead to other types of genetic abnormality, such as the existence of an extra copy of chromosome 21 leads to Down syndrome.
The mitotic spindle
In mitosis and meiosis, nanometer-sized tubulins and their binding partners organize themselves into a structure known as a spindle, which is several micrometers in size. The main function of the spindle is to capture the chromosomes and align them accurately at the equator of the cell in the so-called metaphase plate. This capture begins in the prometa phase and continues until all of the chromosomes are arranged in the metaphase plate. After the metaphase, the chromosomes migrate to opposite poles of the cell, an event known as anaphase.
In mitosis, it is fundamental to understand that the goal of this process is to separate the replicated chromosomes. After DNA replication, the chromosomes consist of two identical copies of the same chromosome that are "glued" together. These “glued” chromosomes are called sister chromatids. A protein complex called cohesin exerts the attractive forces that hold the sister chromatids together. The sister chromatids in each pair attach to microtubules that arise at opposite poles of the mitotic spindle in a process called biororientation. In the transition from meta- to anaphase, the sister chromatids lose their bond when the cohesin complex is cleaved by molecular scissors (a protease called separase), allowing the separated sisters to migrate to the opposite cell poles. This just ostensibly simple trick allows cells to inherit chromosomes of exactly the same number and type. The basics of this process, in turn, are quite complex and poorly understood.
Kinetochores connect chromosomes with microtubules
How microtubules capture and stable bond with sister chromatids is currently under intense investigation. Protein frameworks known as kinetochores play an important role in this .
Kinetochores form on specialized regions of the chromosome called centromeres because they are generally located in the center of the chromosome. Like other regions of the chromosome, the centromere contains genetic material called DNA. In the chromosomes, the DNA is organized with histones in so-called nucleosomes, as molecular “pearls” around which the DNA is tightly wrapped due to complex molecular interactions. At centromeres, one of the “pearl proteins”, histone 3 (H3), has been replaced by a molecular brother named CENP-A. CENP-A marks the centromere and is ultimately responsible for assembling the kinetochore.
With over 100 different proteins, each of them in multiple copies, kinetochores represent extraordinary complexes. In kinetochores, in turn, the KMN network, a component of ten proteins, represents the nuclear binding site for the microtubules (KMN stands for the first letters of three sub-complexes, Knl1 , Mis12 and Ndc80) (Fig. 2), [2-4]. Within the KMN network, the Ndc80 complex contains a microtubule binding site that is essential for the separation of the chromosomes. Extensive structural and functional analyzes have led to an advanced understanding of the molecular mechanisms of microtubule binding by the Ndc80 complex and its regulation.
Recently, the interface between the KMN network and the chromosomes, which is provided by a macromolecular complex called CCAN (constitutive centromere associated network), has moved into the focus of intensive research .
Kinetochores and the Cell Cycle Checkpoints
An astonishing aspect of kinetochores is their ability to coordinate microtubule binding with the formation of feedback mechanisms that control the continuation of the cell cycle on the one hand and the accuracy of the attachment of the kinetochore to the microtubule on the other. Such feedback mechanisms are known as the “spindle assembly checkpoint” or “error correction”. Their importance is easy to explain: Turning off error correction and checkpoints leads to errors in chromosome separation, as a result of which daughter cells inherit the wrong number of chromosomes.
The spindle control point measures the occupation of the kinetochores with microtubules or the absence of them [6-9]. There is reason to believe that the error correction in turn monitors the tensile forces of the spindle on the kinetochores. Connections of the kinetochore with non-tension microtubules activate a pathway of correction that eventually removes the mis-bound microtubules, thereby eliminating the defect and promoting the formation of new, exact attachments.
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