A centriole is a tiny cylindrical structure found inside animal cells, built from nine sets of bundled protein tubes arranged in a pinwheel pattern. It measures roughly 250 nanometers across and 500 nanometers long, making it invisible without an electron microscope but one of the largest protein-based structures in a typical cell. Centrioles work in pairs: they form the core of the centrosome, which organizes the network of internal fibers that give a cell its shape and pull chromosomes apart when the cell divides.
How a Centriole Is Built
The defining feature of a centriole is its ninefold symmetry. Nine groups of three linked tubes, called microtubule triplets, are arranged in a ring to form a hollow barrel. Each triplet contains an A-tubule, a B-tubule, and a C-tubule. The A-tubule is a complete ring of 13 protein strands, similar to the microtubules found throughout the rest of the cell. The B- and C-tubules are incomplete, each made of 10 strands of their own while sharing a wall with the neighboring tubule. This interlocking design makes the triplet structure exceptionally stable.
The protein subunits that form these tubes are called alpha-tubulin and beta-tubulin. They snap together in pairs, then stack end-to-end like bricks in a column. Two additional forms of tubulin, delta and epsilon, interact with each other and play roles in maintaining the triplet arrangement. Various non-tubulin proteins act as internal scaffolding, bridging between strands and reinforcing the cylinder from the inside.
At the base of the centriole sits a structure called the cartwheel, a hub-and-spoke arrangement that establishes the ninefold symmetry early in assembly. A protein called SAS-6 is the key building block of this cartwheel, and without it, the characteristic nine-part pattern cannot form.
Centrioles vs. Centrosomes
These two terms are often confused, but they describe different levels of organization. A centriole is the barrel-shaped core structure. A centrosome is the larger organelle that contains two centrioles surrounded by a cloud of proteins called the pericentriolar material (PCM). The PCM is what actually anchors and launches new microtubules outward into the cell. Think of the centrioles as the structural skeleton and the PCM as the functional machinery wrapped around them. When a centrosome is actively generating microtubules, it functions as the cell’s microtubule organizing center.
The two centrioles in a centrosome are not identical. One is the older “mother” centriole, which carries special protein appendages at its tip and base. The other is the younger “daughter” centriole, which lacks these features until it matures. The mother’s appendages are important for anchoring microtubules at the centrosome and for building cilia, a function the daughter centriole cannot perform.
Role in Cell Division
Before a cell divides, it needs two centrosomes, one at each end, to set up the spindle of fibers that will pull chromosomes to opposite sides. The spindle’s bipolarity is essential: with exactly two poles, each daughter cell receives one complete copy of every chromosome. Centrioles ensure this by duplicating once per cell cycle, so that each centrosome contains a pair and each daughter cell inherits one centrosome after division.
That said, centrioles are not strictly required for building a spindle. Cells that have had their centrioles experimentally removed can still assemble spindle fibers and divide. What they lose is precision. Without centrioles, the spindle tends to drift and become mispositioned within the cell, which can cause problems for tissues that depend on cells dividing in a specific orientation.
How Centrioles Duplicate
Centriole duplication is tightly synchronized with the cell’s replication cycle. In late G1, just before the cell begins copying its DNA, a new “procentriole” starts forming at a right angle to each existing centriole. A master regulatory protein called PLK4 docks on the surface of the mother centriole and recruits a partner protein, STIL, which in turn brings in SAS-6 to begin constructing the cartwheel. Scaffold proteins restrict this process to a single site on each mother centriole, ensuring only one new centriole forms per parent.
During S phase and into G2, the new centrioles elongate by adding tubulin subunits to their growing microtubule walls. By the time the cell enters mitosis, each original centriole has a new daughter centriole attached, giving the cell two complete centrosomes ready to anchor opposite ends of the mitotic spindle.
Building Cilia and Flagella
Centrioles have a second life as basal bodies. When a cell stops dividing and enters a resting state, the mother centriole migrates to the cell surface, docks against the inner face of the membrane, and begins assembling a primary cilium: a slender, antenna-like projection that detects chemical and mechanical signals from the cell’s surroundings. During this conversion, the centriole’s triplet microtubules transition to doublet microtubules in the cilium’s shaft, and specialized fibers anchor the structure to the membrane.
Small transport vesicles cap the tip of the centriole as it approaches the membrane, helping to initiate cilium growth. The process is reversible. When the cell receives signals to divide again, the cilium is disassembled, the basal body detaches from the membrane, and it migrates back near the nucleus to resume its role in organizing the mitotic spindle. This back-and-forth between centrosome duty and cilium duty is a fundamental part of how animal cells balance growth and signaling.
Which Cells Have Centrioles
Centrioles are found across the animal kingdom and in many other eukaryotic lineages, including some fungi and protists. The ninefold triplet architecture appears even in early-branching organisms like Giardia, suggesting the structure evolved once and was inherited throughout the eukaryotic tree. Every species that has centrioles also uses cilia or flagella at some point in its life cycle, reinforcing the idea that centrioles originally evolved to build motile appendages rather than to organize cell division.
Higher plants are the most notable exception. Plant cells do not have centrioles and organize their mitotic spindles through a different mechanism that relies on the chromosomes themselves to guide fiber assembly. Several other lineages have also lost centrioles over evolutionary time, but all of them appear to descend from ancestors that once had them.
For a time, scientists debated whether centrioles might have originated as a symbiotic organism, similar to how mitochondria evolved from bacteria. This idea was partly motivated by the observation that centrioles duplicate as if they had their own genome. Extensive experiments have since confirmed that centrioles contain no DNA of their own and can even form from scratch under the right conditions, ruling out an endosymbiotic origin.
Centriole Errors and Cancer
Because centrioles control centrosome number, errors in their duplication can have serious consequences. If a centriole duplicates more than once in a single cell cycle, the cell ends up with extra centrosomes. Extra centrosomes create extra spindle poles during division, which means chromosomes get pulled in three or more directions instead of two. The result is daughter cells that receive the wrong number of chromosomes, a condition called chromosome instability.
This kind of instability is found in nearly all types of cancer and is considered a major driver of tumor progression. Cells with scrambled chromosome numbers can accidentally gain extra copies of growth-promoting genes or lose copies of tumor-suppressing genes, accelerating the accumulation of dangerous mutations. Mutations in well-known tumor suppressors like p53 and BRCA1 have been directly linked to centrosome amplification. Cells lacking functional BRCA1, for instance, develop excess centrosomes, which helps explain the genomic chaos seen in BRCA1-associated breast and ovarian cancers.