Centrioles

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Centrioles:

 A centriole is a barrel shaped polarized structure made up of microtubules. It is present double in number and is considered smallest cell organelles in animal cell. They usually participate in the process of cell division and are observed during the process of mitosis and meiosis. The granular mass which organizes at the center of a microtubule is called as centrosome. Centrioles consist of nine triplets of microtubules, all arranged in a ring like pattern.

Centriole definition:

Centrioles are present in most of the animal cells, cells of fungi and algae but not commonly in plants. The centrioles are cylindrical in shape and referred as cellular organelles. They are present in most of the eukaryotes. The centrioles consist of triplets of microtubules arranged in a ring pattern; these microtubules form an arrangement of 9+3. This arrangement of the microtubules in a ring like pattern is referred as triplets and is arranged perpendicular to one another. Centrioles organize and form the assembly of microtubules during cell division in animal cells. They also get replicated during Interphase of both mitosis and meiosis and get distributed equally among the daughter cells at the end of the process. In cilia and flagella, they form the basal bodies.

Centriole structure:

  • The walls of each centriole are composed of nine triplet microtubules (9+0) arrangement or, in case of Drosophila, nine doublet microtubules.
  • The central part of centriole is proteinaceous and called the hub, which is connected with peripheral triplets by radial spokes made up of protein.
  • In animal cells, a pair of centrioles present in MTOC (microtubule-organizing center).
  • MTOC with a pair of centrioles is known as centrosome.
  • Two centrioles are arranged perpendicular to each other in the center of centrosome.
  • The cloud of amorphous material that surrounds a pair of centrioles is called centrosome matrix or pericentriolar material.
  • Although the localization of centrioles within the centrosome together with their precise duplication prior to mitosis has suggested a role in bipolar spindle assembly or function, the actual role of centrioles in cell division remains unclear and controversial.
  • Cells from which centrioles and centrosomes are ablated can still form bipolar spindles via a centrosome-independent self-organization process.

Centriole functions:

  • Centrioles are involved in forming mitotic poles in higher animal cells.
  • They have their functions in MOC which also involves formation of flagella and cell division.
  •  Centriole duplicates within the cell and one of them migrate to the opposite poles during cell division where they organize themselves to execute mitotic spindle formation. They once again disassemble during G1/S phase transition in Interphase of cell cycle.
  • Each centriole duplicates to form a new centriole during cell division. The newly formed centrioles stay attached tightly to their parent centriole and further elongation occurs during S and G2 phase.
  • The migration and the arrangement of the centrioles govern the alignment of the spindle fibers. It also stimuluses the chromosomes attachment to the spindle.
  • The mitotic spindle fibers regulate for the separation of chromosomes into their daughter cells during cell division.
  •  At end of the division, each cell consists of two centrioles - one the parent centriole and the other newly formed daughter centriole.
  • After separation, the centrioles regulate the assembly of the nucleus and also determine the cellular organization in the daughter cells.
  • The centrioles also play role in the formation of flagella or cilia.
  • Centrioles also give rise to the fiber of the tail of sperms.
  • The malfunctioning of the centrosome leads to the formation of certain cancers.

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