Centrioles



CENTRIOLES:

The centrioles are noticeable with both light and electron microscopes.It is present in all the animal cells ( but not in Amoeba), located just outside the nucleus. It is cylindrical, 0.5 ųm in length and without a membrane. It has 9 sets of peripheral triplet tubules but none in the centre (9+0) . Each set has three   tubules arranged at definite angles. It has its own DNA and RNA and therefore it is self duplicating. 






Location:

The centrioles occur in nearly all animal cells and in motilie plant cells, such as zoospores of algae, sperm cells of ferns, and motile algae. They are absent in amoebae, prokaryotic cells,higher gymnosperms and all angiosperms. The centrioles commonly occur in pairs. A pair of centrioles is called a diplosome. An interphase ( undividing)  cell has a pair of centrioles  ( diplosome) usually near the nucleus.  They lie in a small mass of specialized, distinctly straining  cytoplasm that lacks other cell organelles and is called  centrosphere, or Kinoplasm or cytocentrum.The centrioles and the centrosphere are together referred to as centrosome. Before   cell division, the centrioles at each pole  of  the spindle. Some cells, such as the giant cells of  bone marrow, have numerous centrioles. 


Structure:

The centrioles appear as two minute, dark_ staining granules under a light microscope ( G, Kentron= a point, centre). An electron microscope shows, them as two short, hollow cylinders usually lying at right angles to each other. They are about 0.3 to 0.5 ųm long and 0.15 ųm thick. Each centriole is made up of 9 microtubule  triplets, which lie evenly spaced in a ring. There is no microtubule at  the centre, giving the "9 + 0" pattern for the centriole. Each microtubule in a triple is about 250 A° wide. The triplets are tilted to form an angle of about 40° to the radius of the cylinder. There is no membrane around the centrioles. The three  microtubules of a triple are named A, B and C, beginning from the inside. The A microtubule is joined to the C microtubule of the adjacent triple by a dense A_ C linker. A fune radial fibre, or spoke, joins each A  microtubule to the central hub of the cylinder. Each radial fibre has a thickening, the foot, near the A microtubule. Near the distal end, the radial spoke forms branches that bear additional thickening and join to A_ C linkers.





          
     The three microtubules of a triplet are often called subtubles.The A substuble is complete. The B and C subtubles are C__ shaped and their walls are completed by adjacent subtubles. The wall of A subtuble is composed of 13 parallel protofilaments,and each protofilament is made up of a row of œß_ tubulin dimers.



      
     






Massules: Around the centriole are nine amorphous sphere of electron dense material with poorly defined limits. These are called pericentriolar satellites or massules. These help in the assembly of new centrioles in G2 phase of cell cycle.

Chemical Composition: 

The microtubules of the centriole are composed of a protein tubulin and some lipids. They are rich in ATPase  enzyme. 

  Origin: New centrioles are  synthesized¹ from their subunits, the œ,ß__ tubulin dimers. They may form in cells which have no preexisting centrioles. 

Functions:

The centrioles serve the following functions_

(i) They help organize spindle fibres and astral rays during cell division. Therefore, they are called microtubule_ organising centres ( MTOCs). However, the centrioles are not essential for this purpose. The cells of higher  plants lack centrioles and still form a spindle.

 (ii)  They provide basal bodies which  give rise to cilia and flagella.

( iii) The distal centrioles of a spermatozoon gives rise to the axial filament of the tail.

BASIAL BODIES

There are structure similar to centrioles. They have the same nine sets of triplet organization ( 9+0) , as in the centrioles. The cilia and flagella appear to arise from the basal bodies.


Location:

The basal bodies, also called basal granules, occur in  eukaryotic cells which bear cilia or flagella. They lie  close to the plasma membrane. They were formerly called kinetosomes ( G = kinetos = moving, movable; some = body) or blepharoplasts.

Structure:


The basal bodies have the same form, size structure and chemical  composition as the centrioles, consisting of a ring of nine microtubules triplets of tubulin ( 9+ 0 arrangement).In fact, a centriole can become a basal body, similarly a basal body may detach from a calcium or a flagellum and become a centriole in action and location. In many animals,  including humans, the basal body of the fertilizing sperm's flagellum enters the egg and becomes a centriole.

Functions:

The basal bodies have three main functions _ 
(i) They give rise to cilia and flagella. Thus, they act as MTOCs.

(ii) They also anchor and control the movements of cilia and flagella. 

( iii) They convert into centrioles at times.

(iv) Distal one of the two centrioles in a spermatozoan, gives rise to axial filaments or tail.


Resemblance among Centriole,  Cilium and Flagellum:

Common  features  among centriole, cilium and flagellum are

(i) All the them are made up of microtubules. 

(ii) All three possess  nine peripheral fibrils of microtubules with a fibril organisation 9+0 in centrioles and 9+ 2 in case of cilia and flagella. 

(iii) Basal granule at the base of a cilium or flagellum is derived from a centriole and resembles  the same in structure. 

(iv) All of them are capable  of movements. ( A centriol moves to a limited extent inside the cytoplasm. .Cilium and Flagellum have movement  as explained earlier).

( v) Centrioles are parent organelles producing basal bodies,  cilia and flagella  and they have nucleating centres or massules for the growth of microtubules.


































































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