The division spindle is ... The description, structure and functions

The spindle of fission is a temporal structure formed during the processes of mitosis and meiosis, and provides chromosome segregation and cell division. It is bipolar: the microtubule system, formed in the space between the poles, resembles a spindle in shape. In the centromere region, microtubules of the spindle attach to the chromosome kinetochores. According to them, the chromosomes move to the poles.

Structure

The spindle of fission consists of three basic structural elements: microtubules, fission poles and chromosomes. The poles of division in animals are organized with the help of centrosomes, in which centrioles are contained. In the absence of centrosomes (in plants, and in oocytes in some animal species), the spindle has wide poles and is called the centrosome. In the formation of the spindle, another structure participates, motor proteins. They belong to dynes and kinesins.

The spindle of division is a bipolar structure. At both poles are centrosomes - organelles, which are the centers of organization of microtubules. In the structure of the centrosome, two centrioles are distinguished, surrounded by many different proteins. Condensed chromosomes, having the appearance of two chromatids attached to the centromere site, are located between the poles. In the centromere region, there are kinetochores to which microtubules attach.

Formation

Since the fission spindle is the structure responsible for cell division, the beginning of its assembly occurs in the prophase. In plants and in oocytes, in the absence of a centrosome, the center of the organization of microtubules is the core shell. Microtubules approach the nuclear envelope and at the end of the prophase their orientation ends and a "prophase spindle" is formed - the axis of the future spindle of division.

In view of the fact that in the animal cells it is the centrosome that serves as the center of organization, the beginning of the fission spindle formation is the divergence of the two centrosomes during the prophase period. This is possible due to motor protein dyneins: they attach to the outer surface of the nucleus, as well as to the inner side of the cell membrane. A group of dynes attached to the membrane connects to the astral microtubules and they begin to move toward the minus-end, as a result of which the centrosomes are diluted along the opposite sections of the cell membrane.

Finishing the assembly

The final formation of the fission spindle occurs at the stage of prometaphase, after the disappearance of the nuclear membrane it becomes full, after all, after this, the centrosomes and microtubules can access the components of the spindle.
However, there is one exception: in budding yeast, the formation of the fission spindle occurs within the nucleus.

The formation of filament spindle threads and their orientation is impossible without two processes: the organization of microtubules around the chromosomes and attaching them to each other at opposite poles of division. Many elements necessary for the final formation of the fission spindle, including chromosomes and motor proteins, are located inside the cell nucleus, and microtubules and, if it is an animal cell, centrosomes are contained in the cytoplasm, that is, the components are isolated from each other. That is why the formation of the spindle ends only after the disappearance of the nuclear envelope.

Accession of chromosomes

The formation of the spindle of fission involved protein, as well as many other structures, and in animal cells this process is well studied. During prophase, microtubules form a star-shaped structure around the centrosome, which diverges radially. After the membrane of the nucleus collapses, dynamically unstable microtubules begin to actively probe this area and the kinetochores of chromosomes can gain a foothold on them. Some of the chromosomes immediately appear on opposite poles, while the rest are first associated with the microtubules of one of the poles, and only then they begin to move toward the desired pole. When the process is completed, the chromosomes already associated with any pole begin to be attached by the kinetochores to the microtubules from the opposite pole, thus, during the metaphase process, ten to forty tubes are attached to the kinetochores. This formation is called a kinetochore bundle. Gradually, each of the chromosomes is connected with the opposite pole, and they form a metaphase plate in the central part of the spindle of division.

The second option

There is another scenario, according to which the spindle of division can form. This is possible for cells that have a centrosome, and for cells in which they are absent. A gamma-tubulin ring complex is involved in the process, due to which there is nucleation of short microtubules around the chromosomes. The tubes join the kinetochores with the plus-end, after which the polymerization of the microtubules begins, that is, the regulated growth. The minus ends "merge" and remain at the poles due to motor proteins. If a pair of centrosomes participates in the formation of the fission spindle, this facilitates the connection of the microtubules, but the process is possible without them.

Equally

A clear separation of chromosomes between two cells formed during division can occur only if the pair chromatids with their kinetochores have joined different poles. The bipolar discrepancy of chromatids is called amphithecic, but there are other variants that arise during the time when the fission spindle is collected. It is monotepic (one kinetochore joins one pole) and synthetic (both kinetochores of the chromosome are connected to one pole). When merotepic one kinetochore is captured immediately by two poles. Only the usual, bipolar binding that is due to tension forces from the poles is stable, the remaining bonding methods are unstable and reversible, but are possible due to the location of the kinetochore.