Centrioles are detected only in animal cells and some lower plants, a centriole has belonged to short lengths of microtubules deceiving parallel to one another and organized around a central cavity to form a cylinder. In animal cells, centrioles are detected in and form part of, the centrosome where they have combined structures existing at right angles to one another. In this context, they are probably involved in spindle assemblage during mitosis. The centrosome is located in the cytoplasm outside the nucleus but often close to it.
A single centriole is also to be found at the basal end of cilia and flagella. In this connection, it is called a ‘basal body’ and is associated with the growth and application of the microtubules in a cilium or flagellum. Centrioles are present in (1) animal cells and (2) the basal place of cilia and flagella in animals and lower plants (e.g. chlamydomonas). In cilia and flagella, centrioles are named ‘basal bodies’ but the two can be considered inter-convertible.
Centrioles are absent from the cells of higher plants. When animal cells go through mitosis they are considered by some to improve from the presence of centrioles which arrive to control spindle fiber formation and which later has an effect on chromosome separation. Research however has displayed that mitosis can take place in animal cells after centrioles have been eliminated.
Sometimes this seems to be at the consumption of abnormalities in spindle development and successive complications with chromosome separation. Recent research also advises that embryos of Drosophila arrest very early if centriole replication cannot take place. In higher plants, mitosis takes place completely convincingly with microtubules forming spindle fibers but without the help of centrioles. The function of centrioles, therefore, remains something of a mystery.
Do Plant Cells Have Centrioles?
Found only in animal cells, these dichotomized organelles are typically detected together near the nucleus in the centrosome, a granular mass that delivers as an organizing center for microtubules. Within the centrosome, the centrioles are located so that they are at right angles to each other. Each centriole is constructed of nine bundles of microtubules (three per bundle) organized in a ring.
Centrioles play a notable role in cell division. During the interphase of an animal cell, the centrioles and other components of the centrosome are cloned, though scientists are not yet sure how this coping takes place. At first, the two pairs of centrioles go on in close closeness to each other, but as mitosis initiates, the authentic centrosome divides, and the pairs are broken up so that one set of centrioles is detected in each of the new microtubule-organizing centers.
These new centers radiate microtubules in star-shaped clusters identified as asters. As the asters drive to oppose poles of the cells, the microtubules, with the help of the centrioles, become organized into a spindle-shaped formation that spans the cell. These spindle fibers develop as guides for the alignment of the chromosomes as they split later during the mechanism of cell division.
Though centrioles perform an appearance in the mitosis of animal cells, plant cells are capable to recreate without them. Researchers have, therefore, been eminently interested in determining completely how important the organelles really are. Studies have displayed that certain animal cells, especially female gametes (oocytes), can strongly divide even when their centrioles are destroyed.
Some inspectors have also found, however, that the absence of centrioles in animal cells is correlated with an increased number of divisional errors and substantial postponements in the mitotic process, especially before chromosome segregation. Consequently, it has been suggested that centrioles evolved as a clarification of the cell, making mitosis a much more efficient and less error-prone process.
Centrioles in Animal Cell Mitosis
In cells that emphasize cilia or flagella, basal bodies, which exhibit the same structural form as centrioles, are present. These assemblies are located, however, near the cell exterior at the base of each cilium or flagellum, rather than in the centrosome near the nucleus. Basal bodies are anchored in their cytoplasmic positions by what is called a rootlet system in the cell. In some organisms, such as the unicellular Chlamydomonas, basal bodies change their location and are functionally transformed to centrioles before the mitotic process.
Structure
A centriole is composed of short lengths of microtubules formed in the form of an open-ended cylinder about 500nm long and 200nm in diameter. The microtubules constructing the wall of the cylinder are gathered into nine sets of bundles of three microtubules each.
In cilia and flagella where centrioles are at the base of the design and are called basal bodies, the wall and crater architecture is slightly different. In addition to cylinder walls composed of nine sets of bundles of three microtubules, there are walls of nine sets of two bundles. In both forms, there is a central matrix from which spokes radiate as in a cartwheel.
In animal cells, centrioles commonly locate in pairs with the cylindrical centrioles at right angles to each other.
Centrioles organize a ‘cloud’ of protein ingredients around themselves. This is the pericentriolar element (PCM). Together the two establish the essential centrosome.
Function
Centrioles function as a pair in most cells in animals but as a single centriole or basal body in cilia and flagella.
Cells arriving mitosis have a centrosome consisting of two pairs of centrioles and joined pericentriolar material (PCM). During prophase, the centrosome breaks down into two elements and a centriole pair migrates to each end or pole on the outside of the nuclear membrane or envelope. At this point, microtubules are constructed at the outer boundary of the pericentriolar material and grow out in a radial form.
The centriole pair and PCM are called an aster. Microtubules from the aster at one pole grow towards the aster at the opposite pole. These microtubules are called spindle fibres. Some of these will become attached by centromeres to chromosomes lined up on the ‘equator’ of the dividing cell. Others, though not attached to chromatids/chromosomes by centromeres, will cooperate in depressing apart the two parts of the dividing cell.
At the base of each cilium or flagellum, there is a single centriole. This structure and associated pericentriolar material, construct microtubules in a linear direction. These microtubules form most of the inside of cilia and flagella and are broadly responsible, using protein motors, for the mechanical aspects of their movement. The centriole at the base of each one also emerges to exert some degree of direction and control over the migration of the cilia and flagella.
Replication
In cells where centrioles are present as a pair, reproduction takes place during the whole of the cell cycle. In phase G1 the two centriole cylinders move very slightly apart from one another. During the S phase new cylinders of microtubules form near, and at right angles to, the two ‘mother’ cylinders. The two combinations of centrioles keep very close to one another until the prophase stage of mitosis. At this point they separate with both pairs of centrioles moving over the outer surface of the nuclear envelope to opposite ends or ‘poles’ of the cell, to form the astral poles of the dividing cell.
Conclusion
- Centrioles occur as paired cylindrical organelles together with pericentriolar material (PCM) in the centrosome of an animal cell.
- Centrioles are found as single structures in cilia and flagella in animal cells and some lower plant cells.
- Centrioles are constructed of microtubules.
- In animal cells, centrioles organize the pericentriolar material to produce microtubules including mitotic spindle fibers.
- Centrioles present something of an enigma; they appear to have an effect on the outcome of mitosis in animal cells. Centrioles are absent from the cells of higher plants but normal mitosis takes place and with satisfactory results.