Practice question

Axonemal architecture highly conserved across eukaryotes from unicellular Chlamydomonas algae to human respiratory epithelium reflecting efficient solution for producing bending from linear motor sliding requiring precise geometry. Transmission electron micrograph cross section shows nine microtubule doublets arranged in ring each doublet complete A tubule 13 protofilaments plus partial B tubule 10 protofilaments equally spaced 30 nm apart around circumference. In motile flagella two central single microtubules C1 C2 encircled by inner sheath and connected to outer doublets via radial spokes T shape complexes transmitting regulatory signals from central pair apparatus containing kinases and phosphatases. Outer and inner dynein arms nexin DRC links create functional bending unit converting ATP hydrolysis into sliding. This 9 plus 2 arrangement plus associated proteins forms functional bending unit generating waveform. Primary sensory nonmotile cilia lack central pair showing 9 plus 0 organization nonmotile lacking dynein arms. Cytoplasmic microtubules appear as 13 protofilament singlets not cilia actin not present core. Arrangement explains sliding filament model for ciliary beating and genetic diseases.