centrioles

Meaning: A centriole is a short cylinder built from nine triplets of microtubules (a 9+0 pattern, with no central pair). In animal cells a pair of centrioles lies at right angles inside the centrosome, the main microtubule-organizing center. The centrosome is duplicated in S phase. In prophase the two centrosomes move to opposite poles and organize the spindle, whose microtubules separate chromosomes in meiosis I and sister chromatids in mitosis and meiosis II. Centrioles also give rise to basal bodies, which anchor cilia and flagella. Spindles can form without centrioles in some cells, such as higher plant cells, so the centrosome material around them does much of the organizing.

On the MCAT: Passages ask where spindle fibers originate, when centrosomes duplicate (S phase), and what happens if microtubule assembly or centrosome separation is blocked (chromosomes fail to segregate, so the cell stalls in mitosis or becomes aneuploid). Cytoskeleton questions ask you to identify microtubules (tubulin) as opposed to actin filaments or intermediate filaments. You may also be asked to compare structures: centrioles and basal bodies have 9 triplets, while the axoneme of a cilium or eukaryotic flagellum has 9 doublets plus 2 central microtubules (9+2).

Common confusion: Centriole, centrosome and centromere are different. The centrosome is the organizing center that contains the centrioles. The centromere is the chromosome region that joins sister chromatids and assembles the kinetochore, where spindle microtubules attach.