For each item, choose the most appropriate answer.

215.     The axoneme of a cilium
(A)     has 9 single microtubules arranged around a central pair of doublet microtubules
(B)     contains several ATPase, including myosin
(C)     is very similar to the oxoneme of a sperm flagellum
(D)     shortens by about 25% when stimulated by Ca 2+ ions
(E)      normally is activated by binding of acetylcholine at the tip of the   cilium

216.    The peripheral microtubules in a cilium are

(A)        about 10 nm diameter
(B)        constructed primarily from gamma-tubulin
(C)        stably sonnected to one another by dynein cross-links
(D)       disassembled when Ca 2+ levels are sufficiently elevated to bind to  calmodulin
(E)     linked to the central pair of microtubules by structures called radial spokes

217.     Dyneins

(A)       are ATPase with extensive sequence homology to myosins
(B)       are GTPase that bind transiently to the central pair of axonemal  microtubules
(C)        require association with actin filaments n order to effect undulator motion of cilia
(D)        bind to the plasma membrane surrounding the axoneme
(E)       go through cycles of attachment to and detachment from microtubules

ANSWERS AND TUTORIAL ON ITEMS 215-217

The answers are: 215-C;216-E;217-E. The movements of eukaryotic cilia and flagella are brought about by a cytoskeletal complex called the axoneme. The axonemes of  cilia and flagella are very similar. They are cylindrical structures with a central pair of single microtubules surrounded by nine doublet microtubules. This organization of the axonemal complex is referred to as the “9+2 arrangement”. Both the central singlets and the peripheral doublets are assembled from ?-? tubulin dimmers similar to those that form cytoplasmic microtubules: however, in contrast to cytoplasmic microtubules (which undergo various cycles of assembly/disassembly), the microtubules of the axoneme are stably assembled for the “life” of the oxoneme. As with other mrcrotubules, those of the axoneme are approximately 24 nm in diameter.

The outer doublets are joined tightly together by links made of a protein called nexin and are firmly connected to a proteinaceous sheath surrounding the central pair by protein structures called radial spokes (or radial links). The outer doublets have associated with them multiple copies of a high molecular weight protein complex called dynein. Dyneins are much larger than, and distinct from the muscle enzyme myosin (cilia and flagella contain neither myosin nor actin), but – like myosins – they hydrolyze ATP and the energy liberated by that hydroysis is used to power the bending motions of cilia and flagella. It appears that activation of cilia and flagella involves some sort of signal transmission, perhaps via the basal body, to the base of the axoneme. This activates the dynein side arms which hydrolyze ATP and transiently bind to the doublet microtubules adjacent to the doublet to which they are permanently attached.

This transient interaction between two adjacent doublets causes a sliding of one doublet relative to the other; the dynein-driven sliding motions cause the axoneme to bed. It is the bending movements of the axoneme that cause fluid to move past cilia and cause flagella to propel cells through fluid. Neither cilia nor flagella shorten to any significant extent during the cyclic association and dissociation of dynein bridges with adjacent microtubule doublest. Metallurgical Microscopes, Ore Microscopes Phase Contrast Microscopes