pset4(2) - Problem Set 4 7.06 Spring 2007 Question 1....

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Problem Set 4 7.06 Spring 2007 Question 1. Answer the following questions about actin and microfilaments. (a) What happens (net growth, net shrinkage, or no net change) to filament length when microfilaments are incubated at each of the following concentrations of monomer? …a monomer concentration equal to the Cc for the (–) end? …a monomer concentration equal to the Cc for the (+) end? …a monomer concentration equal to the Cc for the filament? (b) What happens (net growth, net shrinkage, or no net change) at each end when microfilaments are incubated at each of the following concentrations of monomer? PLUS MINUS …a monomer concentration equal to the Cc for the (–) end? …a monomer concentration equal to the Cc for the (+) end? …a monomer concentration equal to the Cc for the filament? (c) Cytochalasin D and latrunculin both lead to the destabilization of microfilaments. However cytochalasin D binds to F-actin and latrunculin binds to G-actin. Explain how this difference in mechanism can still lead to the same effect.
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(d) How can you demonstrate experimentally that ATP hydrolysis by actin is not necessary for a subunit to add on to a microfilament? (e) Why is it, if the concentration of ATP is much higher in the cell than ADP and the ATP-bound form of actin is more likely to polymerize, that not all the actin in the cell is polymerized? Question 2. Embryonic epithelial cells grown in rich culture medium appear round and symmetrical. Moving the cells to nutrient-poor medium will cause the cells to differentiate and become polarized. (a) When you move embryonic epithelial cells to nutrient-poor medium in the presence of cytochalasin D, you find that the cells remain round and symmetrical. What can you conclude from these results?
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(b) You wish to visualize the organization of actin in polarized epithelial cells. How could you visualize actin? (c) Microvilli are cellular projections located on the apical surface of some epithelial cells (such as intestinal epithelial cells) that increase surface area. They contain a core of actin filaments arranged in bundles. You suspect that these bundles are maintained by actin- bundling proteins. Actin-bundling proteins are one of the many kinds of actin-binding proteins we learned about in class. In general, how might you isolate and identify actin- binding proteins? (d) Once you have identified actin-binding proteins by way of part (c) , what experiments might you use to see whether it is possible that these proteins are specifically actin- bundling proteins? Question 3. You are a UROP student working in a cancer research lab. You were told that, except for blood cells, all normal cells in your body are anchorage-dependent, meaning that they
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have to grow in or on some sort of extracellular matrix. If cells are deprived of extracellular matrix, they will not proliferate in the presence of growth factors, and these cells will die by apoptosis. (a)
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This note was uploaded on 01/07/2011 for the course BIOLOGY 7.012 taught by Professor Ericlander during the Spring '04 term at MIT.

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pset4(2) - Problem Set 4 7.06 Spring 2007 Question 1....

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