Cilia and flagella are fine hair-like structures responsible for motility in prokaryotic microorganisms, protozoans as well as in eukaryotic sperm. The axoneme, the structural core, is made of microtubule bundles and associated proteins like nexins and dyneins. These microtubules are arranged in a 9+2 arrangement, where nine pairs of doublet microtubules form an outer ring surrounding a central pair of singlet microtubules. The doublets have two structurally different microtubules— A and B. A has thirteen protofilaments, while B has ten. Microtubule A has an extended radial spoke that connects with the singlet microtubule pair at the center and helps regulate the activity of axonemal dyneins. Axonemal dyneins have their stem embedded in microtubule A and their globular head and their stalk projected toward microtubule B of the neighboring microtubule doublet. These dyneins are primarily responsible for ciliary motion. Adjacent microtubule doublets are connected by nexin, while central singlet microtubules are connected by central bridges made up of tryptophan and aspartic acid-rich protein PF20.