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A centriole is a tiny structure found near the nucleus (a cell's control center) of most animal cells that plays an important role during cell division. Shaped like long, hollow tubes, centrioles help the X-shaped chromatids (duplicated chromosomes) to split apart when a cell divides in two.

All animal cells must divide in order to repair themselves and to grow. Just before a cell divides and produces an identical cell (a process called mitosis), it duplicates or copies its chromosomes so that the new cell will have the same deoxyribonucleic acid (DNA) code as the old cell. Chromosomes contain DNA, which is the chemical that holds the code for all of an individual's inherited traits.

Most of the time, chromosomes are long and thin and appear as a tangled mass of thin threads in the cell nucleus. However, after the chromosomes make an exact copy of themselves and just before cell division is about to take place, the chromosomes begin to shorten and thicken and continuously fold in upon themselves. As they get shorter and thicker, the copy is attached to the original, and together they form a X-shaped structure. Each separate strand of this X-shaped structure is called a chromatid.

While this is going on inside the nucleus of the cell, outside the nucleus small cylindrical tubes called centrioles are preparing to go to work. The centrioles soon move to opposite ends of the nucleus and fibers begin to form between the centrioles as they move away from each other. These fibers make up a structure called a spindle. As division continues to progress, the two connected chromatids are pulled apart by the spindle.

Each spindle pulls a chromatid toward it, separating them and splitting each chromosome away from its duplicate. The two sets of chromosomes continue to move away from each other and toward a centriole. As division nears its end, a membrane forms between the two groups of chromosomes and there are now two new identical cells. Although centrioles are still somewhat mysterious to biologists, it is known that they play an important role during cell division since they act as organizing centers for the spindles that actually separate chromosomes.

[See alsoCell; Cell Division; Chromatin; Mitosis ]

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centriole A structure associated with the centrosome and found mainly in animal cells. It consists of two short cylinders, orientated at right angles to each other and composed of microtubules. When present, the centriole replicates during the nondividing phase of the cell cycle, and during the prophase of mitosis a centriole migrates with each centrosome to lie at opposite poles of the cell. It was formerly believed that the centrioles were involved in assembly of the spindle microtubules, but this role is now in doubt – they are not present in the cells of most higher plants, and their removal from cells does not affect spindle formation. See also undulipodium.

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centriole A hollow, cylindrical structure, normally one of a pair lying at right angles to one another, adjacent to the nuclear envelope in animal cells, and composed of nine sets of microtubules, each set arranged in triplets. Centrioles are thought to be organizers of microtubular structures in these cells, and during cell division a pair is found at each pole of the mitotic spindle.

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centriole In plants, a cylindrical organelle occurring in flagellated or ciliated cells, where it acts as a precursor to the basal body of each flagellum or cilium. Centrioles are absent from higher plants.

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centriole (sen-tri-ohl) n. a small particle found in the cytoplasm of cells, near the nucleus. Centrioles are involved in the formation of the spindle and aster during cell division.