basal body

Meaning: A basal body sits just under the plasma membrane at the base of a eukaryotic cilium or flagellum. It is built like a centriole: nine microtubule triplets in a ring with no central pair (9+0). It anchors the appendage and acts as the template from which the shaft (axoneme) grows. The axoneme itself has nine outer microtubule doublets plus two central microtubules (9+2). Motor protein dynein, using ATP, slides neighboring doublets, which bends the shaft.

On the MCAT: Expect a cross-section diagram or electron micrograph where you must tell the basal body (9+0, triplets) from the shaft (9+2, doublets). Passages may describe cells with defective cilia or flagella, such as immotile sperm or poor clearance of mucus in the airway, and ask you to predict loss of movement or a problem with microtubule structure. Basal bodies are made of tubulin, not actin or intermediate filaments.

Common confusion: Students mix up the basal body with the centrosome. Centrioles in a centrosome organize the spindle during division, while a basal body is a centriole-like structure anchoring a cilium or flagellum. Also, bacterial flagella have a basal body too, but it is a different structure, a rotary motor with no microtubules, and it is driven by a proton gradient.