Book : Cell Biology
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Centrosome

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In cell biology, the centrosome is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell as well as a regulator of cell-cycle progression. The centrosome is thought to have evolved only in the metazoan lineage of eukaryotic cells. Fungi and plants lack centrosomes and therefore use other MTOC structures to organize their microtubules.  Although the centrosome has a key role in efficient mitosis in animal cells, it is not essential in certain fly and flatworm species.

Centrosomes are composed of two orthogonally arranged centrioles surrounded by an amorphous mass of protein termed the pericentriolar material (PCM). The PCM contains proteins responsible for microtubule nucleation and anchoring including γ-tubulin, pericentrin and ninein. In general, each centriole of the centrosome is based on a nine triplet microtubule assembled in a cartwheel structure, and contains centrin, cenexin and tektin. In many cell type the Centrosome is replaced by a Cilium during cellular difrentiation . However, once the cell start to divide, the cilium is replaced back by the Centrosome.

The centrosome has evolved in multicellular organisms from the basal body/axoneme of the unicellular ancestor. It plays a major role in organizing the microtubule cytoskeleton in animal cells. During interphase, the centrosome organizes an astral array of microtubules (MTs) that participate in fundamental cellular functions such as intracellular trafficking, cell motility, cell adhesion and cell polarity. In proliferating cells, the centrosome starts duplicating just before, or at, the onset of S phase and the two newly formed centrosomes participate in the assembly and organization of the mitotic spindle, its orientation with respect to cortical cues, and the late events of cytokinesis.

The animal centrosome consists of a pair of centrioles linked together through their proximal regions by a matrix consisting in part of large coiled-coil proteins of the pericentrin family, which anchor other matrix components. The centrioles contain cylindrical arrays of triplet MTs organized with nine-fold radial symmetry and the proximal region is structurally similar to the basal bodies of cilia and flagella. In animals, centrioles retain the ability to act as basal bodies by templating the assembly at their distal end (the plus ends of the centriole MTs) either of a primary cilium or of beating cilia during ciliogenesis in specialized cells. Recent discoveries have revealed that cilia have crucial roles in an increasing number of cellular and developmental processes, establishing a link between dysfunctional cilia and several genetic diseases.

In post-mitotic cells, the centrosome contains a mature centriole called the mother centriole and an immature centriole assembled during the previous cell cycle, the daughter centriole, which is about 80% the length of the mother centriole Mother centrioles are distinguished by two sets of nine appendages at their distal ends, which are thought to be required for anchoring microtubules at the centriole and for docking of centrioles at the plasma membrane during ciliogenesis.

Function of Centrosome:

Centrosomes are associated with the nuclear membrane during prophase of the cell cycle. In mitosis the nuclear membrane breaks down and the centrosome nucleated microtubules (parts of the cytoskeleton) can interact with the chromosomes to build the mitotic spindle.

The mother centriole, the older of the two in the centriole pair, also has a central role in making cilia and flagella.

The centrosome is copied only once per cell cycle so that each daughter cell inherits one centrosome, containing two structures called centrioles (see also: centrosome cycle). The centrosome replicates during the S phase of the cell cycle. During the prophase in the process of cell division called mitosis, the centrosomes migrate to opposite poles of the cell. The mitotic spindle then forms between the two centrosomes. Upon division, each daughter cell receives one centrosome. Aberrant numbers of centrosomes in a cell have been associated with cancer. Doubling of a centrosome is similar to DNA replication in two respects: the semiconservative nature of the process and the action of cdk2 as a regulator of the process. But the processes are essentially different in that centrosome doubling does not occur by template reading and assembly. The mother centriole just aids in the accumulation of materials required for the assembly of the daughter centriole.

Interestingly, centrioles are not required for the progression of mitosis. When the centrioles are irradiated by a laser, mitosis proceeds normally with a morphologically normal spindle. Moreover, development of the fruit fly Drosophila is largely normal when centrioles are absent due to a mutation in a gene required for their duplication. In the absence of the centrioles the microtubules of the spindle are focused by motors allowing the formation of a bipolar spindle. Many cells can completely undergo interphase without centrioles. Unlike centrioles, centrosomes are required for survival of the organism. Acentrosomal cells (i.e. cells without centrosomes) lack radial arrays of astral microtubules. They are also defective in spindle positioning and in ability to establish a central localization site in cytokinesis. The function of centrosome in this context is hypothesized to ensure the fidelity of cell division because it greatly increases the efficacy. Some cell types arrest in the following cell cycle when centrosomes are absent. This is not a universal phenomenon.



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