Centrosomes are microtubule organizing centers for animal cells. These organelles are also known as MTOC or “Microtubule Organizing Centres”.
Structure of Centrosome
They are made up of two perpendicular centrioles, a daughter centriole, and a mother centriole, linked together by interconnecting fibers. It consists of a complex of proteins that helps in the formation of additional microtubules. An amorphous pericentriolar matrix surrounds the centrioles. It is involved in the nucleation and anchoring of cytoplasmic microtubules.
The centrosomes in the animal cells are very much like DNA. During cell division, one centrosome from the parent cell is transferred to each daughter cell. In proliferating cells, the centrosome starts dividing before the S-phase begins. The newly formed centrosomes participate in organizing the mitotic spindles. During Interphase, the centrosome organizes an astral ray of microtubules that help in intracellular trafficking, cell adhesion, cell polarity, etc.
In post-mitotic cells, the centrosome consists of a mature centriole and an immature centriole, known as the mother centriole and daughter centriole respectively.
Centrosomes are not present in plant and animal cells. This is because there is no change in the shape of the cell membranes of these organisms during cell division.
The centrosome cycle consists of four phases:
- G1 phase where the duplication of centrosome takes place.
- G2 phase where the centrosome maturation takes place.
- Mitotic phase where the centrosome separation takes place.
- Late mitotic phase where the chromosome disorientation takes place.
- Assist in cell division during mitosis.
- They direct the movement of microtubules and cytoskeletal structures, thereby, facilitating changes in the shapes of the membranes of the animal cell.
- They play a role in mitosis in organizing the microtubules ensuring that the centrosomes are distributed to each daughter cell.
- They also stimulate the changes in the shape of the cell membrane by phagocytosis.
- They maintain the chromosome number during cell division.
Centrosomes In Animal Cells
In most animal cells, centrosomes are not required in the cell division process even though they add to the effectiveness of the mitotic spindle arrangement. In humans, dysfunctioning of centrosomes can stimulate cancer as a result of an increase in the levels of instability in chromosomes or due to the metastatic capability of cancer cells. However, the study on this lacks evidence.
Centrosomes In Plant Cells
Plants and fungi do not possess centrosomes hence make use of MTOC structures to coordinate their microtubules. Plant cells do not possess spindle pole bodies or centrioles except in flagellate male gametes which are completely present in a few flowering plants(conifers). The primary function of the MTOC for spindle organization and microtubule nucleation appears to be taken up by the nuclear envelope while the mitosis of the plant cell.
A rare pathway has been developed by higher plants to regulate the dynamics and assembly of the cytoskeleton. The microtubules are nucleated at the organizing and nucleation centres in many other eukaryotes which are committed to establishing polarity. Though animal and plant cells share the main cytoskeleton elements that imply controlled working, plants do not exhibit centrosome resembling organelles but yet are capable of building spindles thus have developed cytoskeletal arrays such as the preprophase band, the cortical arrays and the phragmoplast that participate in fundamental growth processes.
Certain elements such as the gamma-tubulin etc have a major role to play in the microtubule nucleation taking place at the surface of the nucleus which is referred to as the major operative plant microtubule-organizing centre.
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