centriólum

Centrioles are cylindrical organelles found in animal cells and in some lower plant forms. They are typically found in pairs, oriented perpendicularly to each other, and are part of the centrosome. Centrioles play a crucial role in cell division, acting as the organizing centers for microtubules that form the mitotic spindle. This spindle is responsible for separating chromosomes during mitosis and meiosis.

The structure of a centriole is highly organized, consisting of nine triplets of microtubules arranged in a pinwheel-like pattern. Each triplet is composed of one complete A microtubule and two incomplete B microtubules. This 9x3 arrangement is a defining characteristic of centrioles. The procentrioles, which are precursors to mature centrioles, are assembled near existing centrioles.

During the cell cycle, the centrosome duplicates, ensuring that each daughter cell receives a centrosome. Before cell division begins, the centrosomes migrate to opposite poles of the cell. Microtubules then emanate from these centrosomes, forming the spindle apparatus that guides chromosome movement. Centrioles are also involved in the formation of cilia and flagella, which are involved in cell motility and sensing the environment. While essential for many eukaryotic cells, centrioles are absent in higher plants and fungi.