Spindle apparatus
The spindle apparatus of a nuclear division spindle is a cellular structure made up of many spindle fibers and forms during mitosis and meiosis for the division of the cell nucleus . The spindle fibers in turn consist of tiny tubes called microtubules .
Microtubules are structures that are made up of the protein tubulin and are important for the development of the spindle apparatus. You can switch back and forth between the phases of growth and decay; the cell uses this property in the formation of the core division spindle. The spindle apparatus forms two poles in a cell, from which star-shaped microtubules arise. The microtubules bind to the kinetochores of the two-chromatid chromosomes of the cell and play an important role in the separation of the sister chromatids , which as daughter chromosomes are divided equally between the two newly emerging nuclei of daughter cells.
There are three types of organization that microtubules of the spindle fibers combine to form:
- Polar microtubules are called microtubules which, starting from the poles, can extend far beyond the equatorial plane. They overlap each other, so that under the light microscope it looks as if they reached from pole to pole (English overlap microtubuli ).
- Kinetochore microtubules attach to the kinetochores - the protein structures that sit on the centromere of a chromosome - and reach close to the poles or are associated with the polar microtubules.
- Astral microtubules, on the other hand, form a star around the poles and make contact with elements of the cell skeleton .
The chronological sequence of mitosis is regulated by a control mechanism - the mitotic checkpoint . This delays the initiation of the anaphase as long as there are still kinetochores that are not yet attached bipolarly from both sides of the mitotic spindle.
Most animal cells organize the spindle apparatus with the help of centrosomes . A centrosome consists of a pair of centrioles ; For the division of the nucleus, two centrosomes are needed, each representing the polar organization centers for microtubules ( English microtubule organizing center , MTOC ). In the interphase of a cell cycle, each of the two centrioles of the one centrosome present as the central body is therefore doubled and then the centrosome is divided. At the beginning of the prophase of mitosis, microtubules are then formed, which push the two centrosomes into opposite areas of the cell - as the two poles of the spindle.
Each pair of centrioles or centrosomes of a pole is surrounded by a matrix from which microtubules radiate out. Special binding sites for the growing microtubules can be found on the kinetochores of the centromere of a chromosome (which can also be further distributed in chromosomes with a “diffuse” centromere). The migration of attached and divided chromosomes to the poles of the nuclear spindle requires a conductive structure of spindle fibers. Its structure is established and elaborated through interactions between kinetochore, polar and astral microtubules. The latter can be in contact with cytoskeletal parts of the cell cortex near the membrane. The actual translocation of chromosomes is mediated by various motor proteins , including a. Kinesins that are bound to kinetochores and migrate on the microtubules. According to previous knowledge, both pull and push.
In the telophase of mitosis, the nuclear spindle is finally broken down. In plant cells, the microtubules collect in the area of the phragmoplast , from where the organization of an interphase cytoskeleton and the new cell wall begin. The microtubules lie in the phragmoplastic plane in close proximity to the microfilaments.
Individual evidence
- ↑ Alberts et al .: Molecular biology of the cell . 5th edition, Garland Science, New York 2008, pp. 1075ff.