Examine the high power transmission electron micrograph in Tissue-culture microscope 3.2 below. It is an array of long hollow structures cut perpendicular to their long axis. The diameter of each hollow structure is 25 nm.

218.     Each individual hollow structure is a

(A)    microfilament
(B)    ciliary axoneme
(C)    microtubule
(D)    thick filament of muscle
(E)    intermediate filament

219.     The most abundant protein found in these structures is

(A)    tubulin
(B)    actin
(C)    keratin
(D)    desmin
(E)    myosin

220.     Which of the following proteins is also likely to be present in abundance?

(A)    actin
(B)    vimentin
(C)    dynein
(D)    ?-actinin
(E)    tropomyosin

221.     Which description is most appropriate for the structures?

(A)    The thick filaments are composed of aggregated of myosin molecules.
(B)    Myosin side-arms are involved in their movement.
(C)    They consist of globular actin molecules arranged in two intertwined helices with troponin and tropomyosin molecules arrayed along the helix.
(D)    They consist of 13 protofilaments composed of alternating ?- and ?-tubulin subunits arranged like a string of beads.
(E)    They after abundant in the cores of microvilli.

ANSWERS AND TUTORIAL ON ITEMS 218-221

The answers are: 218-C;219-A;220-C;221-D. Tissue-culture microscope 3.2 is a high power transmission electron micrograph of an array of cytoplasmic microtubules cut perpendicular to their long axis. Microtubules are long hollow structures with an outside diameter of 25 nm. They consist of 13 protofilaments. Each protofilament consist of alternating subunits of ?- and ?- tubulin. The tubulins have similar molecular weights (55 kD) but have different amino acid sequences and subtle functional differences.

In addition to tubulin, microtubules have a variety of microtubule-associated proteins (MAPs) that are involved in the interaction of microtubules with other cytoskeletal elements; one of the most important MAPs is dynein, the ATPase responsible for the sliding of axonemal outer doublets that result in movements of cilia and flagella. Microtubules are important cytoskeletal structures that function in maintenance of cell morphology. They are also major constituents of cilia, flagella, centrioles and the mitotic spindle. Microtubules play a central role in the movements of entire cells as well as in such intracellular movements as the redistribution of chromosomes during mitosis. Polarizing Microscopes, Portable Field Microscopes