Centriole: Structure, Function, and How It Differs from the Centrosome

The centriole is a tiny, barrel-shaped structure that plays an outsized role in the cell: it organizes the microtubules, builds the poles of the spindle that pull chromosomes apart during cell division, and forms the base of cilia and flagella. It is also one of the most confused topics in cell biology, mainly because students mix it up with the centrosome. This article explains what the centriole is, its distinctive 9+0 structure, what it does, and how it relates to the centrosome.

What is a centriole?

A centriole is a small, cylindrical, barrel-shaped structure found in animal cells, built entirely from microtubules. It is about 0.2 micrometers wide and 0.4 micrometers long. Its job is to help organize the cell's microtubules, and it is central to cell division.

Two features are important to note straight away, because they are common exam points:

• A centriole has no membrane. It is not a membrane-bound organelle; it is a bundle of microtubules.
• A centriole has no DNA or RNA. It is made of protein (tubulin) only.

Centrioles usually occur in pairs, positioned at right angles to each other.

Centriole vs centrosome: the key distinction

This is the single most confused point in the topic, so it is worth settling first.

A centriole is the barrel of microtubules itself.

A centrosome is the cell's main microtubule-organizing center, and it is made of a pair of centrioles surrounded by a cloud of protein called the pericentriolar material (PCM). So the centrosome is the larger structure, and the centrioles are the core inside it.

In short: a centrosome contains two centrioles. They are not the same thing. When a question asks how many centrioles are in a centrosome, the answer is two (one pair).

Structure of the centriole: the 9+0 arrangement

The defining feature of the centriole is how its microtubules are arranged.

A centriole is built from nine sets of microtubules, spaced evenly around the wall of the barrel. Each set is a triplet, meaning three microtubules joined together (named A, B, and C). Because there are nine triplets around the outside and nothing in the center, this is called the 9+0 arrangement (nine outer groups, zero in the middle).

So a single centriole contains 9 triplets, which is 9 × 3 = 27 microtubules in total. The triplets are held in their circular arrangement by protein links, and viewed end-on the arrangement looks like the blades of a pinwheel or turbine. A structure at one end called the cartwheel gives the centriole its polarity (a defined proximal and distal end).

The 9+0 arrangement is worth contrasting with cilia and flagella, which is a favorite exam comparison:

• Centriole and basal body: 9+0 (nine triplets, no center).
• Cilium and flagellum: 9+2 (nine doublets around the outside, plus two single microtubules in the center).

Occurrence: which cells have centrioles

Centrioles are found in animal cells and in some lower plants and algae. They are typically absent from the cells of higher plants (flowering plants and conifers), which organize their microtubules using other structures instead. They are also absent from most fungi and from prokaryotes (bacteria have no centrioles).

The fact that higher plant cells manage cell division without centrioles is a useful reminder that the centriole is helpful but not strictly essential for making a spindle.

Functions of the centriole

Organizing microtubules. As the core of the centrosome, centrioles help organize the network of microtubules that runs through the cell during its normal (non-dividing) life.

Forming the spindle poles in cell division. Before a cell divides, the centrosome duplicates, and the two centrosomes move to opposite ends of the cell. They become the two poles of the mitotic (or meiotic) spindle, the framework of microtubules that separates the chromosomes into the two daughter cells. This is the centriole's most important role.

Forming basal bodies for cilia and flagella. A centriole can move to the cell surface and become a basal body, the anchor from which a cilium or flagellum grows. The 9+0 basal body gives rise to the 9+2 structure of the cilium or flagellum above it. This is how centrioles underlie cell movement and the beating of cilia.

How to Remember

Centriole is the barrel; centrosome is the barrel plus its cloud. The centriole is the microtubule cylinder. The centrosome is a pair of centrioles wrapped in pericentriolar material. A centrosome contains two centrioles.

9+0 for the centriole, 9+2 for the cilium. Nine triplets and nothing in the center (9+0) is the centriole and basal body. Nine doublets plus a central pair (9+2) is the cilium and flagellum. The centriole has no center.

Nine triplets, twenty-seven microtubules. Nine sets of three equals 27 microtubules in one centriole. If asked how many triplets, the answer is nine.

No membrane, no DNA. A centriole is just tubulin arranged in a barrel. It has no membrane and no genetic material. Do not call it a membrane-bound organelle.

Animals yes, higher plants no. Animal cells have centrioles; flowering plants and conifers do not. If a question contrasts plant and animal cells, centrioles are on the animal side.

Key exam facts

Where Students Get Confused

The centriole and the centrosome are the same thing. They are not. The centriole is the microtubule barrel; the centrosome is the whole organizing center, made of a pair of centrioles plus the surrounding pericentriolar material. A centrosome contains two centrioles.

A centriole has a membrane. It does not. A centriole is a bundle of microtubules with no membrane at all. The answer to "how many membranes surround a centriole" is zero.

Centrioles and cilia have the same 9+2 structure. No. The centriole (and the basal body) has the 9+0 arrangement: nine triplets, no center. The cilium and flagellum have 9+2: nine doublets plus a central pair.

All cells have centrioles. Not true. Animal cells and some lower plants have them, but higher plants (flowering plants, conifers), most fungi, and bacteria do not. Higher plants divide without centrioles.

Centrioles have DNA like mitochondria do. They do not. Centrioles contain no DNA or RNA. They are made of tubulin protein.

References

1. Alberts B, Heald R, Johnson A, et al. Molecular Biology of the Cell. 7th ed. New York: W. W. Norton; 2022.
2. Iwasa J, Marshall W. Karp's Cell and Molecular Biology. 8th ed. Hoboken: Wiley; 2016.
3. Reece JB, Urry LA, Cain ML, et al. Campbell Biology. 12th ed. New York: Pearson; 2021.