Centrioles

• Centrioles is also called cell centre.
• Centrosome discovered by Benden. T. Boveri called it centrosome.
• Centrosome is present in all animal eukaryotic cells but absent in higher plants.
• H. Fol discovered spindle fibres.
• Two centrioles located just outside nucleus, lie at right angle to each other.
• Surrounding cytoplasm is clear called "Centrosphere" or "Kinoplasm".
• Centrioles + centrosphere = Centrosome or Microcentrum or Diplosome.
• Due to clear and shining centrosphere, centrioles are previously thought as light artifact. One end of centriole is open and other end is closed.
• Centrioles are membraneless elongated structure.
• Centrioles exhibit "Cart Wheel Structure" (Just like Basal body of Cilia). Basal body is also a type of centriole.

• Each centriole consists of 9 peripheral microtubules and each microtubule made up of three sub tubules which are called A, B and C. So centriole is made up of peripheral triplet.
• Each subtubule is made up of 13 protofilaments.
• Each protofilament is a dimer of a and b tubulin protein.
• Microtubules of centriole are made up of tubulin protein.
• Central microtubules are completely absent in centriole but in central part Hub protein is present.
• Each peripheral microtubule connects with hub protein with the help of radial spoke.
• Terminal end of radial spoke (towards peripheri) forms a dense structure called X-body, which is present near the A-subtubule.
• Between two X bodies, a Y-body is also present.
• Each Y-body connected to X-bodies on its both sides.
• A-C linker connects C- sub tubule of a microtubules with A-subtubule of another microtubule.
• Randell and Disprey discovered DNA in centriole, which is called centriolar DNA. It has not been isolated till now but its presence is confirmed. Centriolar DNA is probably circular, naked and double stranded. Centriolar DNA helps in duplication of centriole during G phase of cell division but recently it is beleived that Massules or Pericentriolar Satellites help in duplication of centriole.
• Around each peripheral microtubule a spherical structure is present, which is called Pericentriolar Satellites or Massules. These are 9 in number.
• Centrioles are self duplicating units, contain DNA and RNA and are capable of protein synthesis.