Centriole

A centriole is a barrel shaped organelle found in most eukaryotic cells, though absent in higher plants and fungi. The walls of each centriole are usually composed of nine triplets of microtubules. Deviations from this include Drosophila melanogaster embryos, with nine doublets and Caenorhabditis elegans sperm cells and early embryos, with nine singlets. An associated pair of centrioles, arranged perpendicularly, constitutes the compound structure known as the centrosome.

Function
Centrioles are involved in the organization of the mitotic spindle and in the completion of cytokinesis.

The position of the centriole determines the position of the nucleus and plays a crucial role in the spatial arrangement of cell organelles. In organisms with flagella and cilia, the position of these organelles is determined by the mother centriole which becomes the basal body. Centrioles are an important part of centrosomes which are involved in organizing microtubules in the cytoplasm.

The inability of centrioles to properly migrate prior to ciliary assembly has recently been linked to Meckel-Gruber syndrome. Additionally, proper orientation of cilia via centriole positioning towards the posterior of embryonic node cells is critical for establishing left–right asymmetry during mammalian development.

Structure of centrioles and mechanism of their duplication
Cells usually contain two complete centrioles. During cell replication, a new centriole will grow from the side of each of the preexisting centrioles. The older of the two centrioles in a pair is called the mother centriole and the younger one is called the daughter centriole. The two centrioles in the centrosome are connected to each other by proteins. The mother centriole has radiating appendages at one (distal) end of its long axis and is attached to the daughter centriole at the other (proximal) end. Each daughter cell formed after cell division will inherit one of these pairs (one older and one newer centriole). Duplication of centrioles starts at the time of the G1/S transition and ends before the onset of mitosis.