Centrosome

The centrosome is the main microtubule organizing center (MTOC) of the cell as well as a regulator of cell-cycle progression. It was discovered in 1888 by Theodor Boveri and was described as the 'special organ of cell division.' Most plant and fungal cells lack centrosomes. Although the centrosome has a key role in efficient mitosis in animal cells, it has been recently shown that it is not necessary.

Centrosomes are composed of two orthogonally arranged centrioles surrounded by an amorphous mass of pericentriolar material (PCM). The PCM contains proteins responsible for microtubule nucleation and anchoring^[1] including γ-tubulin, pericentrin and ninein. Each centriole generally comprises nine triplet microtubule blades in a pinwheel structure as well as centrin, cenexin and tektin^[2].

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Roles of the centrosome

Centrosomes are often associated with the nuclear membrane during interphase of the cell cycle. In mitosis the nuclear membrane breaks down and the centrosome nucleated microtubules can interact with the chromosomes to build the mitotic spindle.

The mother centriole, the one that was inherited from the mother cell, also has a central role in making cilia and flagella^[2].

The centrosome is duplicated only once per cell cycle so that each daughter cell inherits one centrosome, containing two centrioles. The centrosome replicates during the S phase of the cell cycle. During the prophase of 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. ^[3] 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. ^[4]

Interestingly, centrosomes are not required for the progression of mitosis. When the centrosomes 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^[5]. In the absence of the centrosome the microtubules of the spindle are focused by motors allowing the formation of a bipolar spindle. Many cells can completely undergo interphase without centrosomes^[2].

Figure 1: Role of the centrosome in cell cycle progression^[6].

Although centrosomes are not required for mitosis or survival of the cell, they are required for survival of the organism. Acentrosomal cells 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 as it is not necessary but greatly increases the efficacy. Some cell types arrest in the following cell cycle when centrosomes are absent. This is not a universal phenomenon.

When the nematode C. elegans egg is fertilized the sperm delivers a pair of centrioles. These centrioles will form the centrosomes which will direct the first cell division of the zygote and this will determine its polarity. It is not yet clear whether the role of the centrosome in polarity determination is microtubule dependent or independent.

Centrosome Genome

Recent research in 2006^[7] indicates that centrosomes may have their own genome, previously known only in nuclei, mitochondria and chloroplasts. Unlike the latter, it is RNA-based rather than DNA-based, and apparently includes an RNA sequence capable of duplicating the centrosome genome. The putative centrosome genome RNA sequences were purified from surf clam eggs, were found in "few to no" other places in the cell, and do not appear in existing genome databases.

The existence of nucleotides associated with the centrosome remains controversial. Many studies have investigated whether nucleotides associate with the centrosome with varying results.

References

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