5.18 : Centrioles and Centrosomes

Most animal cells comprise a pair of centrioles together called a centrosome. The cell duplicates its centrosome and contains two centrosomes side-by-side, which begin to move apart during the prophase. As the centrosomes migrate to two different sides of the cell, microtubules start extending from each centrosome toward the other end. The mitotic spindle is composed of the centrosomes and their emerging microtubules.

Near the end of the prophase, also called late prophase or "prometaphase," microtubules invade the nuclear area from the mitotic spindle to indicate the transition between prophase and metaphase. The nuclear membrane disintegrates, and the microtubules attach themselves to the centromeres that adjoin pairs of sister chromatids. The kinetochore is a protein structure on the centromere that is the point of attachment between the mitotic spindle and the sister chromatids. As the sister chromatids, with their attached microtubules, line up along a linear plane in the middle of the cell, it initiates the metaphase. Next, a metaphase plate forms between the centrosomes that are now located at either end of the cell. The metaphase plate is the name for the plane through the center of the spindle on which the sister chromatids are positioned. The microtubules are now poised to pull apart the sister chromatids and bring one from each pair to each side of the cell. Anaphase takes place over a few minutes when the pairs of sister chromatids are separated from one another, forming individual chromosomes once again. These chromosomes are pulled to opposite ends of the cell by their kinetochores as the microtubules shorten.

In addition to cell division, centrioles are also involved in mechanisms, such as motility and sensing due to their similarity in structure to the basal bodies found at the bases of cilia and flagella.

Unlike animal cells, plants do not have centrosomes and basal bodies but use other mechanisms to nucleate the assembly of microtubules.

This text is adapted from https://openstax.org/books/anatomy-and-physiology/pages/3-5-cell-growth-and-division