Centrioles: Definition, Structure, Function, Parts, Roles

What are Centrioles?

Centrioles are cylindrical organelles of eukaryotic cells which are made of microtubule triplets of a particular kind. It has a critical function in cell division, where they position microtubules to create the spindle fibres during mitosis and meiosis. This is to ensure proper distribution of chromosomes. They anchor sites for ciliation and fractionation, crucial for cell locomotion and instinctual apparatuses like cilia and flagella.

Centrioles are usually double, provided they are in the centrosome area which is close to the nucleus of the cell. Like replication, their reproduction is very controlled, having clear and strict structures that are fundamentally important for cell health and functionality in different species and at different stages of development.

Structure of Centrioles

The structure of centrioles is discussed below:

Basic Structure

Centrioles are cylindrical structures usually ranging in diameter to about 200 nm and in length to 500 nm in the human cell. Each centriole is made of nine triplets of microtubules disposed of in a circle, thus it represents a barrel shape.

Ultrastructure

Centrioles are composed of nine microtubule triplets. In each triplet, one is a complete microtubule called the A-tubule, while the other two demarcated microtubules, the B-tubule and C-tubule, are incomplete. Tubulin proteins form the main structural component of centrioles, while other proteins like pericentrin and centrin help in their stability and function.

Functions of Centrioles

The functions of centrioles are discussed below:

Role in Cell Division (spindle fibre formation, chromosome segregation)

The main function of centriole is in the formation of spindle fibres, which is instrumental in the separation of chromosomes during the division period.

In mitosis and meiosis, centrioles play a significant role in assembling the spindle apparatus thereby affecting proper orientation and segregation of chromosomes as well as cell division.

Formation of Cilia and Flagella

Centrioles act as basal bodies to help in the process of growth of cilia and flagella which are important in the movement and sensory organelles of the cell.

Organisation of the Mitotic Spindle

Centriole replication is semiconservative in which each new pair of centrioles consists of one old, or mother, centriole, and one new, or daughter, centriole.

Centriole Duplication and Cycle

The centrosome duplication is discussed below:

Duplication (S-phase, semiconservative process)

Replication starts in the S phase of the cell cycle for new centrioles to start forming alongside older ones.

Phases: In the G2 phase, the new centrioles begin to grow and reach maturation during this phase. In the M phase, a centriole pair gets separated so that each of the two daughter cells gets a centrosome.

Regulation of Duplication (proteins SAS6, STIL)

Proteins, including SAS6 and STIL, are important in the start and correct development of new centrioles in the course of duplication.

The centrosome cycle is tightly controlled and accurately coordinated with the cell cycle for achieving accurate centriole duplication and transportation, which is very important for genome stability as well as the efficient functioning of the cell.

Centrioles and Centrosomes

The centrosome is a small organelle located in the cytoplasm and consists of two centrioles surrounded by a granule material called pericentriolar material (PCM). Centrioles are part of centrosomes and play a crucial role in centrosome duplication.

Centrioles are also involved in the organisation of microtubules in the centrosome contributing to the formation of mitotic spindles during cell division and structural framework of the cell.

The centrosome and centrioles are closely related but not the same. The table below highlights their structural and functional differences for better understanding.

Centriole Abnormalities and Diseases

The abnormalities and diseases related to centrioles are discussed below:

Defects → Aneuploidy, genomic instability

Damage to centrioles causes improper cell division and aneuploidy or genomic instability, which are related to different diseases. Any alteration in the ultrastructure of centriole may influence cilia development and the latter’s performance, which results in numerous cellular abnormalities.

Microcephaly and cancers

Defects are associated with several diseases such as microcephaly, which is a result of reduced brain size by improper cell division, and different types of cancer which involve abnormal cell division and instability of the genome.

Centrosome Amplification → multipolar mitosis

Centrosome duplication in which at least one cell has more than two centrosomes can cause multipolar mitoses that in turn cause chromosome segregation disorders causing aneuploidy. This condition is frequently detected in cancer cells and is linked with the advancement of the disease and worse outcomes.

Centrioles NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Structure of Centrioles

• Function in cell division

• Cenrosome vs Centriole

Practice Questions for NEET

Q1. The major function of centrioles is

1. Cell division

2. Cell metabolism

3. Cell respiration

4. All of these

Correct answer: 1) Cell division

Explanation:

The major function of centrioles is to organize the microtubules during cell division, specifically in the formation of the spindle fibres. These spindle fibres help separate the chromosomes to opposite poles of the cell during mitosis and meiosis. Centrioles are located in the centrosome, which acts as the microtubule organizing centre (MTOC). They play a key role in maintaining the cell's shape and structure by anchoring microtubules. In addition, centrioles are involved in the formation of cilia and flagella, which are essential for cell movement and fluid movement across surfaces. Centrioles duplicate during the cell cycle to ensure that each daughter cell has the necessary components for proper division.

Hence, the correct answer is option 1) Cell division

Q2. Which of the following is a mismatched pair?

1. Plant cell - cell wall

2. Animal cell - centrioles

3. Plant cell - centrioles

4. Animal cell - lysosome

Correct answer: 3) Plant cell - centrioles

Explanation:

Centrioles are typically found in animal cells, not plant cells. Plant cells do not have centrioles, while animal cells do contain centrioles, which are involved in cell division and the organization of the cytoskeleton.

Hence, the correct answer is option 3) Plant cell - centrioles

Q3. How many centrioles does a centrosome have?

1. One

2. Zero

3. Two

4. Nine

Correct answer: 3) Two

Explanation:

A centrosome contains two centrioles. These tiny, cylindrical centrioles are made of microtubules. They are placed so that they are perpendicular to each other. The centrosome arranges the microtubules, which resemble the skeleton of the cell. During cell division, the centrosome helps produce the mitotic spindle. Chromosome separation depends on this spindle. All things considered, maintaining the cell's structure and ensuring proper division depend on the centrosome.

Hence, the correct answer is option 3)Two.

Also Read:

• MCQs on Centrioles and Centrosomes

• Endoplasmic Reticulum

• Plasma Membrane

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