247.    Which statement is LEAST ACCURATE?

(A)    The polarity of microtubules is related to the fact that the tubulin subunit is a heterodimer of ? and ? subunits.
(B)    Actin filaments often associate with special patches of the plasma m mbrane.
(C)    Kinesin is an ATPase that may interact with microtubules.
(D)    Intermediate filaments are assembled in the intermediate zone of the mitotic spindle.
(F)    While some microtubules may be rapidly assembled or disassembled, those of cilia and flagella are quite stable once assembled to form the axoneme.

248.     Which of the following is most directly involved in causing axoplasmic transport?

(A)    mylen sheath
(B)    neurofilaments
(C)    synapse
(D)    neurotubules
(E)    myosin

249.    Dyein and kinesin

(A)    are both active in the axoneme of cilia
(B)    hydrolyze ATP to power movement of membrane bound organelles in axons
(C)    drive the transfer of actin from areas rich in microtubules to sites where intermediate filaments form
(D)    both require troponin for activation
(E)    serve to phosphorylate microfilaments and microtubules

250.    Tubulozole is a recently discovered drug that causes the depolymerization     of microtubules. Which of the following is LEAST likely to be a direct result     of exposure to tubulozole?

(A)    Resuciton in the rate of mitosis needed to replace damaged epithelial cells in the intestinal mucosa.
(B)    Diminished rate of elongation of cilia forming on the apices of regenerating respiratory epithelium.
(C)    Functioning of the contractile ring during cell division in early embryogenesis.
(D)    Movement of secretory vesicles along axons.
(E)    Blockage of spermatogenesis at metaphase of meiosis I.

251.    Of the following, the organelle MOST DIRECTLY RESPONSIBLE for organizing the microtubules in cells is the
(A)    plasma membrane
(B)    centrosome
(C)    chromatin
(D)    nucleus
(E)    terminal web

252.    The protein MOST LIKELY to be found in the centriole is
(A)    tubulin
(B)    dynein
(C)    alpha actinin
(D)    kinesin
(E)    actin

253.    Which organelle is MOST SIMILAR in structure and function to centrioles?

(A)    microfilaments
(B)    desmosome
(C)    axoneme
(D)    basal body
(E)    nuclear pore

ANSWERS AND TUTORIAL ON ITEMS 247-253

The answers are: 247-D; 248-D; 249-B; 250-C; 251-B; 252- A; 253-D. Microtubules, made of  the protein tubulin, are cytoskeletal important in a large number of cellular functions. Ali known microtubules are formed of tubulin dimmers, each made of one ?- and one ?-tubulin molecule. Because the dimers assemble head-to-tail (i.e., ?-?-?-?), each microtubule is a polarized structure and this polarity is important in the many microtubule-based events involving directed movement. Microtubules are assembled (from the tubulin dimmers) at or near structures called microtubule organizing centers (MTOCs). The most common MTOCs are parts of cell regions called centrosomes. Centrosomes are areas, usually near the nucleus (thus often called cytocenters), that contain one or two centrioles and a series of ill-defined dense areas or satellite bodies. It is the material of the satellite bodies that serve as MTOCs and from which microtubules elongate, usually outward toward the cell periphery. All microtubules assembles such that their “minus” end is at the MTOC and the “plus” end is near the cell periphery. This assembly polarity is important because the enzymes that move organelles along mircrotubule tracks are of two distinct classes. Articulating microscope  Boom Mounted Microscopes.

Microtubule-based motors that move materials from the minus (cytocenter) ends to the plus (cell peripheries) ends of microtubules are kinesins. The transfer of materials along axons toward the axon terminus (anterograde exoplasmic transport) is powered by kinesins (ATPase) moving alon neurotubules is powered by dyneins, a separate and distinct set of mictrotubule motors. The importance of microtubule-based cellular functions is revealed by the wide range of processes that can be inhibited by agents – such as the drug tubulozole – that discrupt microtubules. Of the processes listed as possible “answers” in Item 250, only the contractile ring required for cytokinesis (and made up of actin microfilaments) would not be directly affected by tubulozole. Although not involved in cytokinesis, microtubules are crucially involved in karyokinesis (the part of mitosis in which chromosomes are separated to the forming daughter cells) because they are the principal cytoskeletal component of the mitotic spindle.

The spindle poles are composed of centrioles and surrounding MTOCs. Although the microtubules of the centrioles are not essential for assembly of mitotic spindle microtubules, the microtubular nature of the centrioles is presumably important to the functioning of the cellular organelles that are most closely related to centrioles – the basal bodies. Assembly of the axonemal complex of cilia and flagella requires the presence of basal bodies, organelles that appear to be identical to centrioles, but moved to the cell periphery. It is not clear what changes are affected when centrioles are repositioned to function as basal bodies, but the microtubules of cilarary and flagellar axonemes do assemble on the microtubules that form the wall of the basal bodies.