Definition of centrioles
- Eukaryotic cells contain two rod-shaped, microtubular, cylindrical structures, called centrioles, near the nucleus.
- They lack a limiting membrane and no DNA or RNA and are found in most algae cells (a notable exception being red algae), moss cells, some fern cells, and most animal cells.
- They are absent in prokaryotic plants, red algae, yeasts, cones, and flowers (conifers and angiosperms) and in some non-flagellated or ciliated protozoa (such as amoebae).
- Centrioles form a microtubule spindle, the mitotic apparatus during mitosis or meiosis, and are sometimes arranged just below the plasma membrane to form and carry flagella or cilia in flagellated or hair cells.
- When a centriole carries a flagellum or cilium, it is called the basal body.
What is centriole?
A centriole is a small structure made of microtubules that exist as part of the centrosome, which helps organize microtubules in the body. A centriole is the main unit that creates and anchors microtubules in the cell. Typically, a eukaryotic cell has a centriole that is at right angles to a second centriole at the centrosome. The microtubules extend from each centriole and use the centriole as an anchor for the base of each microtubule. Each cell can have hundreds or thousands of microtubules that spread to various parts of the cell. During normal cellular functions, motor proteins bind to both microtubules and an article for transport. Motor proteins creep along the microtubules, dragging any substance or compartment with them. Not all cells contain centrioles, and other methods are used to create microtubules.
- Centrioles and basal bodies are cylindrical structures that are 0.15–0.25 um in diameter, generally 0.3–0.7 um in length, although some are as short as 0.16 um and others are as long as 8um.
- They are visible under an optical microscope, but the details of the centriole structure were revealed only under an electron microscope.
- The members of each pair of centrioles are at right angles to each other.
- They are tiny submicroscopic microtubule sub-cylinders with a configuration of nine triple fibrils and the ability to form their own duplicates, astral poles, and basal bodies, without DNA and a membranous envelope.
- A centriole has a spiral of nine peripheral fibrils. Fibrils are absent in the center. The arrangement, therefore, is called 9 + 0. The fibrils run parallel to each other but at an angle of 40 °. Each fibril is made up of three sub-fibers. Therefore, it is called a fibril triplet.
- The three sub-fibers are actually microtubules joined by their margins, and thus share common walls made of 2-3 proto-filaments.
- Each sub-fiber has a diameter of 25nm. From the outside in, the three sub fibers of a triple fibril are called С, В and A. Subfiber A are complete with 13 proto-filaments, while the sub fibers В and С are incomplete due to the shared use of some microfilaments.
- The adjacent triple fibrils are connected by С-A protein bonds. The center of the centriole has a bar-shaped protein mass known as the center. The cube has a diameter of 2.5 nm. From the center, it develops 9 protein strands towards the peripheral triple fibrils. They are called radios.
- Each radio has a thickening called X before joining with a sub-fiber. Another thickening known as Y is present nearby. It is connected to both the X thickening and the C-A linkers using connectors.
- Due to the presence of radial rays and peripheral fibrils, the centriole gives a somersault appearance in T.S.
- Centrioles are involved in the formation of the spindle apparatus, which functions during cell division.
- The absence of centrioles causes divisional errors and delays in the mitotic process.
- A single centriole forms the anchor point, or basal body, for each individual cilium or flagellum.
- The basal bodies also direct the formation of cilia and flagella.
Cells form a microtubule endoskeleton complex that allows substances to be transported anywhere in the cell. The products are labeled with special glycoproteins (sugar and proteins) that act as signals for specific motor proteins. These proteins bind to the product or vesicle in which the product is stored, and also bind to a microtubule. The microtubules are arranged in the centriole, of which each centrosome has two. The centrioles anchor the microtubules that extend from there and contain the factors necessary to create more tubules.
During mitosis, the centrosomes replicate by duplicating each centriole. The 4 centrioles are divided into two centrosomes, each with a centriole at right angles to the second centriole. The microtubules extend between the centrosomes that separate the sets of centrioles. The centrioles will be separated, at opposite ends of the cell. Once established, each centriole will extend the microtubules into the cytoplasm that searches for chromosomes. Microtubules bind to chromosomes in their centromeres, which are parts of DNA specially formulated to allow the binding of proteins and special microtubules. The microtubules are then disassembled from the centriole, which attracts the microtubules to the centriole, as the motor proteins separate the chromosomes.
Definition of centrioles, Function, Structure