BIO 320 Spring 2008 - Discussion 8 key

BIO 320 Spring 2008 - Discussion 8 key - CELL BIOLOGY (BIO...

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CELL BIOLOGY (BIO 320) SPRING 2008 DISCUSSION SHEET 8 Key _______________________________________________________________________ _ 1. In cells most of the actin subunits are bound to thymosin, which locks actin into a form that cannot hydrolyze its bound ATP and cannot be added to either end of a filament. Thymosin reduces the concentration of free actin subunits to around the critical concentration. Actin subunits are recruited from this inactive pool by profilin, whose activity is regulated so that actin polymerization occurs when and where it is needed. The advantage of such an arrangement is that the cell can maintain a large pool of subunits for explosive growth at sites and times of its choosing. 2. Kinesin molecules need to be highly processive because a small number of kinesin molecules must be able to transport a mitochondrion, for example, all the way down to a nerve axon. Their high processivity translates into high efficiency of transport. By contrast, it is essential for muscle function that myosin II molecules not be processive. Because myosin II motors in skeletal muscle always function as part of a large array, it doesn’t matter that individual motors let go; others will always be bound. In fact, if myosin II is bound to actin tightly enough to be highly processive, it would inhibit muscle contraction, whose speed depends on the low processivity of its motors. 3. Cytochalasin B interferes with actin filament formation, and its effect on the cell demonstrates the importance of actin to cell locomotion. The experiment with colchicine shows that microtubules are required to give the cell a polarity, which then determines the end that becomes the leading edge. In the absence of microtubules, cells still go through the motions normally associated with cell movement, such as the extension of lamellipodia, but in the absence of cell
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This note was uploaded on 05/06/2008 for the course BIO 320 taught by Professor Staff during the Spring '08 term at University of Texas at Austin.

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BIO 320 Spring 2008 - Discussion 8 key - CELL BIOLOGY (BIO...

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