Centrioles

Centrioles
Centrioles are cylindrical structures that are composed of groupings of microtubules arranged in a ring of nine triplets. They are found in animal cells and help to organize the assembly of microtubules during cell division. Credit: Stocktrek Images/Getty Images

Centrioles are cylindrical structures that are composed of groupings of microtubules arranged in a 9 + 3 pattern. The pattern is so named because a ring of nine microtubule "triplets" are arranged at right angles to one another.

Centrioles are found in animal cells, but not plant cells. They help to organize the assembly of microtubules during cell division. Centrioles are found within cell structures known as centrosomes.

During cell division, the centrosome and centrioles replicate and migrate to opposite ends of the cell. Centrioles help to arrange the microtubules that move chromosomes during cell division to ensure each daughter cell receives the appropriate number of chromosomes. Centrioles are also important to the formation of cell structures know as cilia and flagella. These structures aid in cellular locomotion and are formed from centrioles called basal bodies.

Centrioles in Mitosis

Centrioles are located outside of, but near the cell nucleus. They replicate during interphase, prior to the start of mitosis and meiosis in the cell cycle.

In prophase, each centrosome with centrioles migrates toward opposite ends of the cell. A single pair of centrioles is positioned at each cell pole. The mitotic spindle initially appears as structures called asters which surround each centriole pair. Microtubules form spindle fibers that extend from each centrosome, thereby separating centriole pairs and elongating the cell.

In metaphase, centrioles help to position polar fibers as they extend from the centrosome and position chromosomes along the metaphase plate.

In anaphase, polar fibers connected to chromosomes shorten and separate the sister chromatids (replicated chromosomes). The separated chromosomes are pulled toward opposite ends of the cell by polar fibers extending from the centrosome.

In telophase, the spindle fibers disperse as the chromosomes are cordoned off into distinct new nuclei. After cytokinesis, each daughter cell contains one centrosome with one centriole pair.