706_F06pset5_ans - 7.06 Fall 2006 Problem Set 5 1.a...

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7.06 Fall 2006 PS 5-KEY 1 of 5 7.06 Fall 2006 11/1/06 Problem Set 5 1.a. Describe the structure and functions of the three types of cytoskeletal filaments described in class: What types of molecules compose the different types of filaments? What are the functions of each? Microfilaments: actin polymers involved in organizing the plasma membrane and in cell movement. Microtubules: Polymers of alpha and beta tubulin heterodimers; establish cell polarity and maintain organization of membranous organelles, form mitotic spindles. Intermediate filaments: many types eg. keratin, lamins; maintains integrity of cell contacts and tissues. b. Describe three similarities in the structural/functional properties of actin and tubulin. - They form polymers. -The polymers have a distinct polarity. -Can hydrolyze NTPs and this affects polymer stability. -Both can bind motor proteins. c. Treatment of cells with taxol causes cell cycle arrest in mitosis. Why? How does taxol affect the microtubules? Cells arrest in mitosis. This checkpoint is activated because it senses attachment/tension defects caused by the affect of taxol on the microtubules in the mitotic spindle. Taxol stabilizes the MT polymers. (Note: taxol does not work in yeast) 2. What happens to a microtubule that loses its “GTP cap”? Loss of the GTP cap will cause a MT to convert from growth to shortening. Shortening will continue until the MT disappears or a new cap forms. 3. You are interested in studying the properties of skeletal muscle using mouse as a model system. a. You notice the dead mice are stiff/in rigor mortis. Based on what you know about actin/myosin interaction in muscles, explain the molecular basis of rigor mortis:
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7.06 Fall 2006 PS 5-KEY 2 of 5 The muscles of the dead mouse are depleted of ATP, because normal metabolic processes have stopped. As a result, the myosin-ADP remains bound to the actin and cannot dissociate (because there is no ATP to be bound). Myosin and actin remain tightly bound, making the muscle stiff and inflexible. b. You isolate tissue from these stiff mice and treat it with a non-hydrolyzable ATP analog. What do you expect to happen to the muscle and why? When myosin replaces ADP with nonhydrolyzable-ATP, myosin will release actin,
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706_F06pset5_ans - 7.06 Fall 2006 Problem Set 5 1.a...

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