What Happens When Protein Fibers Radiate From Centrioles

The Mystery of Centrioles

Centrioles, a peculiar structure, can be found in animal cells and the basal region of cilia and flagella in animals and some lower plants. In cilia and flagella, they are known as “basal bodies,” but they can be considered interchangeable [^1^]. Interestingly, centrioles are absent from the cells of higher plants [^2^].

While some believe centrioles play a role in controlling spindle fiber formation during mitosis in animal cells, research has shown that mitosis can occur even after centrioles have been destroyed. However, this may lead to abnormalities in spindle development and problems with chromosome separation [^3^]. Additionally, recent studies suggest that embryos of Drosophila experience developmental issues if centriole replication is hindered [^4^].

On the other hand, higher plants can undergo mitosis successfully without the aid of centrioles. Microtubules perform the task of forming spindle fibers in these plants, leaving the function of centrioles somewhat of a mystery [^5^].

The Structure of Centrioles

A centriole is made up of short sections of microtubules arranged in an open-ended cylinder shape, approximately 500nm long and 200nm in diameter. The microtubules are organized into nine bundles, each consisting of three microtubules, forming the walls of the cylinder [^6^].

In cilia and flagella, where centrioles act as basal bodies, the architecture of the walls and cavity differs slightly. Apart from walls made up of nine bundles of three microtubules, there are also walls composed of nine bundles of two microtubules. Both types have a central matrix from which spokes radiate, similar to the structure of a cartwheel [^7^].

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In animal cells, centrioles typically exist in pairs, with the cylindrical centrioles perpendicular to each other. Alongside centrioles, the pericentriolar material (PCM) forms a protein “cloud,” together constituting the essential centrosome [^8^].

The Function of Centrioles

In most animal cells, centrioles function as pairs, while in cilia and flagella, a single centriole or basal body is present. During prophase, centrosomes containing two pairs of centrioles and associated PCM divide into two parts. Each part migrates to opposite ends or poles of the cell, located outside the nuclear membrane. At this stage, microtubules start to grow radially from the outer edge of the pericentriolar material, forming an aster. Some microtubules from one pole’s aster will grow towards the opposite pole’s aster, functioning as spindle fibers. These fibers connect to chromosomes aligned at the cell’s “equator” through centromeres. Other microtubules, although not attached to chromosomes, assist in separating the dividing cell’s two parts [^9^].

In the case of cilia and flagella, a single centriole located at their base, along with associated pericentriolar material, constructs microtubules in a linear direction. These microtubules comprise most of the internal structure of cilia and flagella and contribute significantly to their movement through the use of protein motors. The centriole at the base also appears to have an influence on the direction and control of cilia and flagella movement [^10^].

Centriole Replication

When centrioles exist as pairs, replication occurs throughout the entire cell cycle. During the G1 phase, the two centriole cylinders slightly move apart from each other. In the S phase, new microtubule cylinders form near the “mother” cylinders, perpendicular to them. The two pairs of centrioles remain close until the prophase of mitosis, when they separate. Both pairs of centrioles move along the outer surface of the nuclear envelope to opposite ends or “poles” of the cell, forming the astral poles of the dividing cell [^11^].

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Summary

To summarize, here are the key points about centrioles:

  • Centrioles occur as pairs in the centrosome of animal cells, along with pericentriolar material.
  • In cilia and flagella, centrioles exist as single structures.
  • Centrioles are composed of microtubules.
  • In animal cells, centrioles play a crucial role in organizing pericentriolar material to generate microtubules, including mitotic spindle fibers.
  • The presence of centrioles influences the outcome of mitosis in animal cells. Their absence does not prevent mitosis but can lead to unsatisfactory outcomes.
  • Centrioles are absent from the cells of higher plants, yet normal mitosis occurs successfully [^12^].

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Please note that while we strive to provide authoritative and up-to-date information, new discoveries may challenge current scientific thinking. The British Society for Cell Biology (BSCB) cannot be held responsible for any discrepancies between information found on this website and the accepted views of examining bodies [^13^].

Centrioles

Image source: CIMR GridPoint Technology

Reference:
[^1^] – Wikipedia
[^2^] – National Center for Biotechnology Information
[^3^] – ScienceDirect
[^4^] – BMC Biology
[^5^] – ThoughtCo
[^6^] – Microbiology Society
[^7^] – The American Journal of Pathology
[^8^] – The Cell: A Molecular Approach
[^9^] – Molecular Biology of the Cell
[^10^] – Cell
[^11^] – Journal of Cell Science
[^12^] – Plant Biology
[^13^] – British Society for Cell Biology

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