The centriole is a small, cylindrical organelle found inside the cells of most animals and many lower eukaryotes, though it is notably absent in higher plants and most fungi. It is constructed primarily from a protein called tubulin and is a non-membranous structure that acts as a major organizer of the cell’s internal scaffolding system. The centriole’s primary functions revolve around organizing the cell’s internal architecture and ensuring the accurate division and movement of cells.
Anatomy and Location
The physical structure of a centriole is a highly conserved, short cylinder, typically about 0.2 micrometers in diameter and 0.4 to 0.5 micrometers in length. This cylindrical wall is composed of nine sets of microtubules. Each set consists of three fused microtubules, known as a triplet. These nine triplets form a distinct pattern referred to as the 9+0 arrangement because there are no microtubules present in the cylinder’s center.
A centriole is usually found in a pair, known as a diplosome, with the two cylinders positioned at a precise right angle to one another. This pair is embedded within a dense cloud of protein known as the pericentriolar material (PCM). This entire complex—the two orthogonal centrioles and the surrounding PCM—forms the centrosome, which is the main microtubule-organizing center of the cell.
The centrosome is typically situated in the cytoplasm near the nucleus. The centrioles within the centrosome serve as the core template for organizing the vast network of microtubules that radiate outward throughout the cell. This organizational role allows the centrioles to influence the position of the nucleus and the overall spatial arrangement of other organelles.
Role in Cell Division
The most recognized function of the centriole is its involvement in cell replication, specifically during mitosis and meiosis. Before a cell divides, the centrosome duplicates itself. These two centrosomes then move away from each other to establish opposite ends of the dividing cell.
As the cell enters division, the centriole pairs become the organizing centers for the formation of the spindle apparatus. This apparatus is a complex network of microtubules, or spindle fibers, which extends between the two poles of the cell. The centrioles at each pole help organize the microtubules that will eventually attach to the chromosomes.
The organized spindle fibers attach to the condensed chromosomes and physically pull them apart. This action ensures that the duplicated genetic material is precisely segregated. The accurate separation of chromosomes is necessary so that each resulting daughter cell receives a complete and identical set of genetic information.
Building Blocks for Motility
Beyond their role in cell division, centrioles perform a vital function by acting as the foundation for cellular appendages. When a cell is not dividing, one of the centrioles can migrate to the cell membrane. Once there, it anchors itself and is referred to as a basal body.
The basal body then serves as a structural template to grow an external projection called a cilium or a flagellum. These appendages are slender, hair-like structures that function in cellular movement, such as propelling sperm cells, or in sensory roles, like moving fluid across the surface of cells.
The centriole’s core structure, the 9+0 arrangement of microtubule triplets, templates the formation of the main shaft of the cilium or flagellum, known as the axoneme. This mature structure typically transitions into a 9+2 pattern, possessing nine outer pairs of microtubules and two central single microtubules. This transformation allows the basal body to generate the functional, motile appendage.
How Centrioles Duplicate
The process of centriole duplication is tightly regulated and occurs only once per cell cycle, beginning as the cell transitions from the G1 phase into the DNA synthesis (S) phase. This timing ensures that a cell has the correct number of centrioles before it begins to divide its genetic material. The duplication process is semi-conservative, meaning existing centrioles act as templates for the formation of new structures.
A new, small centriole, called a procentriole, begins to form near the proximal end of each existing “mother” centriole. It grows in a distinct orientation, perpendicular or orthogonal, to the mother structure. The formation of this new structure is regulated by a master protein kinase called Polo-like kinase 4 (PLK4).
Throughout the S and G2 phases of the cell cycle, the procentrioles elongate until they reach the full length of the parent centrioles. This precise, controlled duplication is necessary to prevent errors in chromosome segregation during the upcoming cell division. If this process is disrupted, it can lead to an abnormal number of centrioles, which is a condition often observed in cancer cells.