Centrosome and Centriole

Introduction:

• Centrosome is an organelle usually containing two cylindrical structures called centrioles.
• They are membraneless structures.
• They are surrounded by amorphous pericentriolar materials.
• The clear cytoplasm (Zone of exclusion) around centriole is called centrosphere or kinoplasm or cytocentrum.
• Both centrioles are commonly called Diplosomes.
• Centriole does not bear an intracellular compartment.
• Centrioles are usually found in all the animal cells except Amoeba.
• Centrioles are absent in higher plants. Although centriole is found in those plants that bear flagellate stage in the life cycle. e.g. Many green algae, Bryophytes, pteridophytes, cycads.

Detailed Explanation:
 

Structure of centriole:

• The pair of centrioles is often called diplosome.
• Diplosome lies in a common specialized part of the cytoplasm called the centrosphere or kinoplasm.
• Centrosphere is devoid of any other cell organelle and contains fibrous material.
• The complex, formed of centrioles and centrospheres, is called a centrosome or central apparatus.
• Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel.
  
  
  
  
   
• Centrioles are capable of replication. Centriole replication is coordinated in animal cells with cell division. It occurs during the S phase.
• Prior to nuclear division, the two centrosomes separate and move to the opposite ends where spindle poles are to be established subsequently.
• The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells
• A centriole possesses a whorl of nine peripheral fibrils.
• Fibrils are absent in the centre and it shows 9 + 0 arrangement.
• Fibrils run parallel to one another angled 40° to each other.
• Each fibril is made up of three sub fibres. Therefore, it is called triplet fibril.
• The three subfibres are in reality microtubules joined together by their margins and, therefore, sharing the common walls made of 2-3 protofilaments.
• From outside to inside the three sub-fibres of a triplet fibril are named as C, B and A. Subfibre A is complete with 13 protofilaments while B and C subfibres are incomplete due to sharing of some microfilaments.
• The adjacent triplet fibrils are connected by C—A proteinaceous linkers.
• The centre of the centriole possesses a rod-shaped proteinaceous mass known as a hub.
• From the hub, develops 9 proteinaceous strands towards the peripheral triplet fibrils. They are called spokes. Each spoke has a thickening called X before uniting with A sub-fibre.
• Another thickening known as Y is present nearby and is attached both to X thickening as well as C—A linkers by connectives.
• Due to the presence of radial spokes and peripheral fibrils, the centriole gives a cart wheel appearance.
• On the outside of the centriole are present dense, amorphous, protoplasmic plaques in one or more series. They are called massules or pericentriolar satellites.
• Their position is change-able with different states of the cell. Massules act as a microtubule organizing center or MTOC.
• They function as nucleating centres for the growth of microtubules during aster formation and formation of new centrioles.

The function of the centrioles:

1. The centriole acts as MTOC (microtubule-organizing center) that arranges the microtubules array based on its ability to anchor, release, or nucleate microtubules.
2. Centrioles can be transformed into basal bodies.
3. Basal bodies formed from centrioles gives rise to cilia and flagella.

Evolution of centrosome:

• The evolution of centrosomes in eukaryotes reflects the variation in eukaryotic cells among different organisms in terms of sensory reception, locomotion, or division with respect to the natural evolutionary process and adaptation of the organism.
• The centrosome is considered to be the organelle that coordinated cytokinesis and karyokinesis.
• The position and the polarity of the centrosome control the positioning of microtubules.

Similarities between the Centrioles, Cilium and Flagellum:

• Centriole, cilium and flagellum resemble one another in their broad structure and function.
• All of them are made up of microtubules.
• The three possess nine peripheral fibrils of microtubules. Fibril organization is 9 + 0 in centrioles and 9 + 2 in case of cilia and flagella.
• Basal granule present at the base of a cilium or flagellum is derived from a centriole and resembles the same in structure.
• All the three are capable of movements. Centriole does so to a limited extent inside the cytoplasm. A cilium or flagellum produces a current in an external liquid medium for locomotion, feeding, aeration and circulation.
• Centrioles are parent organelles which produce basal bodies, cilia and flagella. They have nucleating centres or massules for the growth of microtubules.

Frequently Asked Questions (FAQs)
 

Q1. What is centrosome ?
Answer:

• The pair of centrioles is often called diplosome.
• Diplosome lies in a common specialized part of the cytoplasm called the centrosphere or kinoplasm.
• Centrosphere is devoid of any other cell organelle and contains fibrous material.
• The complex, formed of centrioles and centrospheres, is called a centrosome or central apparatus.
• Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel.

Q2. What is the main function of a centrosome?
Answer: The major functions of the centrosomes are as follows:

• The centriole acts as MTOC (microtubule-organizing center) that arranges the microtubules array based on its ability to anchor, release, or nucleate microtubules.
• The position and the polarity of the centrosome control the positioning of microtubules.
• Centrioles can be transformed into basal bodies.
• Basal bodies formed from centrioles gives rise to cilia and flagella.

Q3. What Role Do Centrioles play in Cell Division and Mitosis?
Answer:

• The centrosome and centrioles replicate and migrate during the cycle of cell division.
• The centrioles arrange the microtubules (spindle fibers) which support the movement of chromosomes.
• This ensures that each daughter cell receives the appropriate number of chromosomes.

Q4. Describe the resemblance between the Centrioles, Cilium and Flagellum?
Answer: Similarities between the Centrioles, Cilium and Flagellum:

• Centriole, cilium and flagellum resemble one another in their broad structure and function.
• All of them are made up of microtubules.
• The three possess nine peripheral fibrils of microtubules. Fibril organization is 9 + 0 in centrioles and 9 + 2 in case of cilia and flagella.
• Basal granule present at the base of a cilium or flagellum is derived from a centriole and resembles the same in structure.
• All the three are capable of movements. Centriole does so to a limited extent inside the cytoplasm. A cilium or flagellum produces a current in an external liquid medium for locomotion, feeding, aeration and circulation.
• Centrioles are parent organelles which produce basal bodies, cilia and flagella. They have nucleating centres or massules for the growth of microtubules.

Q5. Which structure is used for the formation of cilia and flagella?
Answer:

• The basal bodies are the structure formed from the centrioles.
• This centriole gives rise to cilia and flagella.