Centrosome: Definition, Structure, Cycle, Functions & Sample Question

Centrosome is the organelle containing two centrioles. Centrosome contains a complex of proteins that helps in the arrangement of extra microtubules. Before cell division begins, the centrosome duplicates, as division begins, the two centrosomes move to opposite ends of the cell.

Key Terms: Centrosome, Cell Organelles, Centrioles, Eukaryotic Cells, Microtubules, Cell Division, Spindle Fibers, Centrosome, Prophase, Mitosis, Mtoc 

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What Is Centrosome?

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Centrosomes are cell organelles, that are the fundamental microtubule structures that play an important role during the process of cell division. They consist of two mutually perpendicular cylindrical structures called ‘centrioles’ and a complex of highly dense proteins that help the tubules to shape.

It serves the main purpose as the MTOC (microtubule organizing center) in Eukaryotic cells and is also a regulator of cell cycle progression. It exists only in animal cells and is not present in plant cells or even fungi cells. Hence plant and fungi cells use alternative structures in place of centrosomes to organize their microtubules. 

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Structure Of The Centrosome 

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Structure of the centrosome is as classified: 

• The cell organelle centrosome contains two cylindrical structures called centrioles.
• These centrioles lie perpendicular to each other one is called the daughter centriole which is also referred to as the immature centriole and the other is called the mother centriole of the mature centriole. 
• The two centrioles are structurally similar but not identical and also differ in age. 
• Each of these centrioles is made up of nine peripheral microtubule fibril triplets of tubulin which are evenly spaced and organized in a cylindrical shape. 
• The central part of the centriole also consists of proteinaceous material called the hub and is connected to the tubules of the peripheral triplets by radial spokes also made of protein. 
• This is also known as the PCM- Pericentriolar material which is responsible for microtubule nucleation and anchoring of cytoplasmic microtubules. 

Structure Of Centrosome

Centrosomes are of variable sizes but centrioles are usually of size 0.2 μm in diameter and 0.5 μm in length and are embedded in the PCM. The Centrosome is usually found very close to the Golgi apparatus 

During cell division, one centrosome from the parent cell is moved to each daughter cell. In cells that multiply, the centrosome starts separating before the S-stage starts. 

The newly shaped centrosome helps in arranging the mitotic axles. And during the interphase stage, the centrosome is composed if an astral beam of microtubules that facilitate in intercellular dealing, cell extremity, cell grip, etc. 

Also Read: Nucleoplasm

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Centrosome Cycle

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Centrosome cycle consists of 4 major stages 

1. The G1 stage wherein the duplication of the centrosome occurs. 
2. The G2 stage wherein the centrosome develops further is termed maturation. 
3. Mitotic stage wherein the centrosome division occurs. 
4. Late mitotic stage wherein chromosome confusion or disorientation occurs. 

Centrosome Cycle

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Functions Of The Centrosome 

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Functions of the centrosome include the following: 

1. Centrosomes play a significant role in cell division. 
2. It also provides structure to the cell. 
3. They maintain the number of chromosomes during the process of cell division. 
4. They also encourage the changes in the shape of the cell membrane during cell division through the process of phagocytosis. 
5. The centrioles of the centrosomes give rise to the basal body precursors. 
6. The centrosome nucleates the polymerization of tubulin subunits into microtubules. 
7. The most important function of the centrosome is that it organizes the microtubules into functional units, for example, the mitotic spindle. 
8. This function of the centrosome ensures that the centrosomes are distributed evenly to the daughter cells. 
9. The centrosome also regulates the movement of the microtubules and the
10. cytoskeletal structure which helps shape the membrane of the animal cell. 

Also Read: Endoplasmic reticulum

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Centrosomes In Animal Cells 

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Centrosomes in animal cells are as classified:

• Centrosomes are present in the animal cell, but they are not actually an essential part of the cell. Hence certain species of flies such as the drosophila (fruit flies) and flatworms like Planaria do not possess centrosomes. 
• In differentiated animal cells, like the muscle cells, neurons, and the epithelial cells the centrosome is not the MTOC and is mostly inactive because upon differentiation the centrosomes lose the PCM components which often get delocalized to other parts of the cell in the cytoplasmic membrane. And the nuclear membrane which beave as the MTOC. 
• Sometimes, centrosomes are not essential for the assembly of mitotic spindles even in cells that possess them. 
• The presence of centrosomes, in the process of cell division increases the effectiveness of the process of Mitotic Spindle arrangement. 

For example, centrosomes contribute to mitotic fidelity, cytokinesis, and asymmetrical cell division but aren't necessary for the development and proper functioning of the flies. 

In human cells, dysfunction of the centrosome can result in cancer due to the instability in the chromosomes or even due to the metastatic capacity of cancer cells. This is more of a speculation than a fact.

Centrosomes In Animal Cell

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Centrosomes In Plant Cells

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Centrosomes in plant cells are specified as:

• Centrosomes do not exist in plant cells and hence use alternative MTOC structures to coordinate the Microtubules. 
• Plant cells do not particularly have axle post bodies or centrioles exception being the flogging male gametes that are present in certain species of blooming plants(conifer). 
• The functions of the MTOC appears to be taken up the atomic envelope during mitosis in plant cells.
• Plant and fungi cells do not possess centrosomes. Plant cells also do not have spindle pole bodies exception being the flogging male gametes. 
• The functions of the MTOC of spindle organization and microtubule nucleation appear to be taken up by the nuclear membrane during mitosis in plant cells. 
• Animal and plant cells being different do not share the same cytoskeletal elements. 
• Plants do not have centrosomes resembling organelles but they possess the ability to build spindles and possess developed cytoskeletal arrays like the prophase band.
• The cortical array and the phragmoplast plays a fundamental role in the growth process. 

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Things To Remember 

• A centriole is a cell organelle that is comprised of two cylindrical mutually perpendicular structures called centrioles. 
• Centrioles are made up of 9 microtubule triplets which are also surrounded by a dense proteinaceous matrix called PCM, expanded as pericentriolar material. 
• Functions of the centriole include facilitating cell division, maintaining the number of chromosomes during cell division, organizing the microtubules, regulating the movement of microtubules, and providing the cytoskeletal structure of the cell. 
• Plant and fungi cells do not possess centrosomes whereas animal cells do. 
• In animal cells the centrosomes act as the MTOC- microtubule organizing center. 

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Important PYQS Based On Centrosomes

1. Cellular Organelles With Membranes Are.... [NEET- 2015]
2. Cell recognition and adhesion occur due to biochemicals of cell membranes named as..... [NEET- 1993]
3. A bivalent of meiosis I consists of.... [KCET- 2013]
4. The major event that occurs during the anaphase of mitosis which brings about equal distribution of chromosomes is..... [KCET- 2006]