Centrioles PDF

Centrioles  Cytoplasm of some eukaryotic cells contain two cylindrical, rod-shaped, microtubular structures known as centrioles Introduction Centrioles do not have limiting membrane, own...

Centrioles  Cytoplasm of some eukaryotic cells contain two cylindrical, rod-shaped, microtubular structures known as centrioles Introduction Centrioles do not have limiting membrane, own DNA or RNA They form spindle microtubules Centrioles get sometimes arranged beneath the plasma membrane to form or bear a cilia or flagella; they are known as basal bodies.  Occurrence Present in Absent in Most algal cells Red algae Moss cells Prokaryotes Fern cells Yeast Most animal cells Cone-bearing and flowering plants (conifers and angiosperms) Amoeba  They are cylindrical structures Diameter is about 0.15 to 0.25 µm Structure Length is about 0.3 to 0.7 µm In some cases length may vary from 0.16 to 8 µm All have the same ultrastructure, which is as follows: Ultrastructure Cylinder wall Triplets Linkers Cartwheel Ciliary rootlets Basal feet and satellites  The most striking feature of the ultra-structure is array of nine triplet microtubules equally spaced around the perimeter of an imaginary cylinder Cylinder The space between and immediately around the triplet is filled with an amorphous, electron dense wall material In transverse section the triplets are arranged like blades of pinwheel or turbine. Each triplet is angled inward to the central axis. Within each bade the tubule twist from one end to another and a helical structure is formed. The centrioles do not have a defined outer membrane and therefore the triplets are considered to form the wall.  The nine triplets found in basal bodies and in centrioles are similar in nature. The three subunit microtubules are designated as A, B and C, Innermost is Triplets A. A is complete and other two are incomplete C-shaped sharing their walls Diameter 200-260 Ao = 20-26 nm These tubules are very similar to microtubules The triplets are thought to run parallel to each other and to long axis of the cylinder, but it not always the case. E.g. In basal bodies the triplets get closer to each other at proximal end.  ‘A’ tubule of each triplet is linked to ‘C’ tubule of next triplet by protein linkers at intervals along their entire length. Linkers The linkers hold the cylindrical array of the microtubules and main the typical radial tilt of the triplets  There are no central microtubules in the centrioles and no special arms. However, often faint protein spokes radiate Cartwheel out to each triplet from a central core, forming a pattern like cartwheel. Such a cartwheel configuration determines the proximal end of a centriole and thus provides a structural and functional polarity to it. The growth takes place from distal end, in basal bodies also the cilia or flagella grows from the distal end.  Ciliary In some cells, from the basal ends of rootlets the basal bodies originate the ciliary rootlets which are of two types: 1. Tubular root fibrils 2. Striated rootlets  The basal feet are dense processes that are arranged perpendicular to the basal Basal Feet body. and Satellites Impose structural asymmetry on the basal body that is related with the direction of the ciliary beat The satellites of perpendicular bodies are electron dense structures lying near the centriole that are probably nucleating sites for the microtubules  Formation of basal bodies and cilia Functions The pair of centrioles act as a focal of point in centrosome which organises the array of cytoplasmic microtubules centrioles during interphase and duplicates at mitosis to nucleate the two poles of the mitotic spindle The basal bodies can form centrioles prior to cell division Involved in ciliary or flagellar beat  Cilia and Flagella Flagella is present in Cilia is present in Class Flagellata Class Ciliata Sponges Larvae of certain Gamete cells of plants and Platyhelminthes, algae Echinodermata, Mollusca Gamete cells of animals and Annelida Respiratory tracts of animals  Locomotion to cell or organism Functions of cilia and Creates food currents in lower aquatic animals flagella In respiratory tracts ciliary movement helps in elimination of solid particles Eggs of amphibians and mammals are driven out of oviduct because of cilia