Structure of the Cilia and Flagella: The diameter and internal structure of cilia and flagella is similar to each other but these structures differ in length. Cilia are generally less than 15 μm in length while flagella may be as long as 200 μm. Virtually all eukaryotic cilia and flagella are remarkably similar in their organization, possessing a central bundle of micro-tubules, called the axoneme, in which nine outer doublet micro-tubules surround a central pair of singlet micro-tubules. This characteristic “9 + 2” arrangement of micro-tubules is seen when the axoneme is viewed in cross section with the electron microscope. As shown in the figure below, each doublet micro-tubule consists of A and B tubules, or subfibres. The A tubule is a complete micro-tubule with 13 protofilaments, while the B tubule contains 10 protofilaments. The bundle of micro-tubules comprising the axoneme is surrounded by the plasma membrane.
At its point of attachment to the cell, the axoneme connects with the basal body. Each A tubule bears two side arms called dynein arms that project laterally towards the B tubule of the next doublet. Doublet micro-tubules, which represent a specialized polymer of tubulin, are found only in the axoneme. The outer doublets are connected circumferentially by nexin links. The axoneme is held together by three sets of protein cross-links. Within the axoneme, the two central singlet and nine outer doublet micro-tubules are continuous for the entire length of the structure.
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