Near the nucleus, eukaryotic cells have two centrioles, which are two cylindrical, rod-shaped microtubular structures.
They are present in the majority of mammalian cells, moss cells, certain fern cells, and most algal cells, with the remarkable exception of red algae. They do not have DNA or RNA or a limiting membrane.
Prokaryotes, red algae, yeast, cone-bearing and flowering plants (conifers and angiosperms), as well as some non-ciliated or flagellated protozoans don’t have them (such as amoebae).
Centrioles can get positioned just under the plasma membrane to create and carry flagella or cilia in flagellated or ciliated cells. Centrioles form a spindle of microtubules, the mitotic apparatus during mitosis or meiosis.
The basal body of a centriole is referred to when it has a flagellum or cilium.
Structure of Centrioles
Centrioles and basal bodies are cylindrical structures that typically measure 0.3 to 0.7 um in length, but some can be as small as 0.16 um and others as long as 8 um. They have a diameter of 0.15 to 0.25 um.
Although viewable using a light microscope, only an electron microscope was able to uncover the specifics of the centriole structure.
In the centrosome, a location close to the nucleus, each cell possesses two centrioles.
Each pair of centrioles has members that are perpendicular to one another.
Without DNA and a membrane covering, they are tiny, sub-microscopic microtubular subcylinders with a pattern of nine triplet fibrils that may replicate themselves to generate astral poles and basal bodies.
A whorl of nine peripheral fibrils makes up a centriole. There are no fibrils in the middle. Thus, the configuration is known as 9 + 0. Although parallel, fibrils face one another at an angle of 40°. Three sub-fibers make up each fibril.
As a result, it is referred to as a triplet fibril.
The three sub-fibers are really microtubules that are connected by their edges and share walls formed of two to three proto-filaments.
The diameter of each sub-fiber is 25 nm. The three subfibers that make up a triplet fibril are designated C, and A from outside to inside. While both sub-fibers are lacking because of the sharing of certain microfilaments, sub-fibre A is complete with 13 proto-filaments.
A proteinaceous linker known as —A connects the neighbouring triplet fibrils. The hub, a rod-shaped proteinaceous mass, is located at the core of the centriole. The hub is 2.5 nm in diameter. In the direction of the peripheral triplet fibrils, nine proteinaceous strands branch out from the hub. They’re known as spokes.
Before joining with the A sub-fiber, each spoke has an X-shaped thickening. Nearby, there is another thickening called Y. It is joined via connectives to both the X thickening and the C-A linkers.
The centriole seems to be spinning in a cartwheel shape in T.S. owing to the existence of radial spokes as well as peripheral fibrils.
Functions of Centrioles
The creation of the spindle machinery, which is essential for cell division, involves centroles.
Centriole deficiency results in divisional mistakes and delays in the mitotic process.
Each cilium or flagellum’s anchor point or basal body is made up of a single centriole.
Cilia and flagella are also formed under the direction of basal bodies.
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