Cilia, Flagella, Centrosome and Centriole | Biology for ACT PDF Download

Cilia and Flagella

Cilia & Flagella are mechanical, hair like cellular appendages and locomotory structure. Flagellar apparatus is consist of following parts:

(a) Shaft or Ciliary Part : It is projecting hair like part of ciliary appartus. Cilium is composed of 11 microtubules. (9 doublet + 2 singlet)

• Bundle of microtubules is called as axonema. Nine microtubules are peripheral and each composed of two small tubules i.e. A tubule with two arms and B-Tubules without arms.

• Microtubules is consists of a contractile proteintubulin similar to actin of muscles.

• Arms of A tubules consist of an enzymatic protein dynien similar to myosin of muscle cells. Dynien have ability of hydrolysis of ATP & liberates energy for ciliary movement.

• The central tubules are connected by bridges and is also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublets by radial spoke. Thus there are nine radial spokes. The peripheral doublets are  also interconnected by linkers.

(b) Kinetosome or Basal Granule or Blepheroplast or Basal body : It is membraneless structure, lies immediately below the plasmamembrane. Basal body exhibit cart wheel structure similar to centriole. (9 triplet fibriles connected to a central hub in basal body).

• Arrangement of microtubules is 9 ( triple) +0. In basal granule there occurs 9 microtubules on periphery and each microtubule is composed of three tubules i.e. A-tubule, B-tubule and C-tubule .

• Central part of basal granule is composed of semisolid cytosol called "Central Hub".

• Microtubules connected to central hub with the help of protein fibres called primary fibres or spokes. 

• Secondary fibres connect microtubules with each other.

• Each primary fiber have a thickening called X-thickening. In between X-thickenings there occurs Y-thickenings. X and Y-thickening are inter connected.

(c) Rootlet or Rhizoplast : This is a conical bundle of protein fibers which arises from basal body to different directions. Rootlet have dark bands composed of ATPase.

Types of Flagella :

1) Whiplash – When the laterel hair like structures absent.

(2) Tinsel – When the flagella bears lateral hairs like structure (flimmers)

•  Cilia and Flagella are simialr in structure but some differences may observed –

Centriole

“Centriole is an organelle, cylindrical in shape, that is composed of a protein called tubulin.”

What is Centriole?

All animal cells have two centrioles. They help the cell during cell division. They work during the process of mitosis and meiosis. They could be found in some lower plants such as Chlamydomonas, although they are not present in many of the fungi, angiosperms (flowering plants) and pinophyta (conifers). They are usually present near the nucleus but are not visible when the cell is not dividing.

Structure of Centriole

• All the centrioles are formed of 9 groups of microtubule triplets organized in a cylindrical shape. 
• The detailed structure of centrioles can be studied only under an electron microscope. 
• These are associated together at right angles to each other. The embryo of Drosophila melanogaster and C. elegans are exceptions to this organization. 
• The former forms 9 pairs instead of microtubule triplets, whereas the premature embryos and sperm cell of C. elegans have 9 single microtubules.
• Edouard van Beneden and Theodor Boveri observed and identified the centrioles for the first time in 1883 and 1888. The structure of duplication of centrioles was first given by Joseph G. Gall and Etienne de Harven in the 1950s.
• Centriole helps in organizing the mitotic spindle and complete the process of cytokinesis. However, centrioles were believed to be necessary for the formation of the mitotic spindle in the animal cell. 
• Although, several recent types of research have explained that the cell which does not have a centriole (surgically removed through laser) can function without it in the G1 level of interphase and can be formed later in a de novo manner.
• The location of the centrioles plays a key role in the three-dimensional organization of the cell as it also regulates the location of the nucleus.
• In flagellated and ciliated organisms the location of such organelle is decided after the mother centrioles that form the base.

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Centriole Function

Following are the important centrioles function:

• In spite of being devoid of DNA, the centrioles are capable of forming new centrioles.
• They can be transformed into basal bodies.
• The basal bodies give rise to flagella and cilia.
• They help in cell division by forming microtubule organising centres.
• Out of the two centrioles, the distal centriole forms the tail or axial filament.

Centrosome

“Centrosome is a microtubule-organizing centre in animal cells.”

Structure of Centrosome

                                                                                         Structure of Centrosome

• The centrosome is made up of two perpendicular centrioles, a daughter centriole, and a mother centriole, linked together by interconnecting fibres.
• It consists of a complex of proteins that helps in the formation of additional microtubules.
• An amorphous pericentriolar matrix surrounds the centrioles. It is involved in the nucleation and anchoring of cytoplasmic microtubules.
• Centrosome in the animal cells is very much like DNA. During cell division, one centrosome from the parent cell is transferred to each daughter cell.
• In proliferating cells, the centrosome starts dividing before the S-phase begins. The newly formed centrosomes participate in organizing the mitotic spindles.
• During Interphase, the centrosome organizes an astral ray of microtubules that help in intracellular trafficking, cell adhesion, cell polarity, etc.
• In post-mitotic cells, the centrosome consists of a mature centriole and an immature centriole, known as the mother centriole and daughter centriole respectively.
• The centrosome cycle consists of four phases:

(1) G1 phase where the duplication of centrosome takes place.

(2) G2 phase where the centrosome maturation takes place.

(3) The mitotic phase where the centrosome separation takes place.

(4) A late mitotic phase where the chromosome disorientation takes place.

Centrosome Function

The major functions of centrosome are listed below:

• The centrosomes help in cell division.
• They maintain the chromosome number during cell division.
• They also stimulate the changes in the shape of the cell membrane by phagocytosis.
• In mitosis, it helps in organizing the microtubules ensuring that the centrosomes are distributed to each daughter cell.
• They regulate the movement of microtubules and cytoskeletal structures, thereby, facilitating changes in the shapes of the membranes of the animal cell.

Centrosome in Animal Cells

• In most animal cells, centrosomes are not required in the cell division process even though they add to the effectiveness of the mitotic spindle arrangement. 
• In humans, dysfunctioning of centrosomes can stimulate cancer as a result of an increase in the levels of instability in chromosomes or due to the metastatic capability of cancer cells. 
• However, the study on this lacks evidence.

Centrosome in Plant Cells

• Plants and fungi do not possess centrosomes hence make use of MTOC structures to coordinate their microtubules. 
• Plant cells do not possess spindle pole bodies or centrioles except in flagellate male gametes which are completely present in a few flowering plants(conifers). The primary function of the MTOC for spindle organization and microtubule nucleation appears to be taken up by the nuclear envelope while the mitosis of the plant cell.
• A rare pathway has been developed by higher plants to regulate the dynamics and assembly of the cytoskeleton. 
• The microtubules are nucleated at the organizing and nucleation centres in many other eukaryotes which are committed to establishing polarity. 
• Though animal and plant cells share the main cytoskeleton elements that imply controlled working, plants do not exhibit centrosome resembling organelles but yet are capable of building spindles thus have developed cytoskeletal arrays such as the preprophase band, the cortical arrays and the phragmoplast that participate in fundamental growth processes.
• Certain elements such as the gamma-tubulin etc have a major role to play in the microtubule nucleation taking place at the surface of the nucleus which is referred to as the major operative plant microtubule