706_S2006_Exam4key - Name: _KEY_ 7.06 Cell Biology EXAM #4...

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Name: ____________KEY__________________ 1 7.06 Cell Biology EXAM #4 KEY May 23, 2006 This is an OPEN BOOK exam, and you are allowed access to books, a calculator, and notes BUT NOT computers or any other types of electronic devices. Please write your answers in PEN (not pencil) to the questions in the space allotted. Please write only on the FRONT SIDE of each sheet, as we will Xerox all of the exams and thus only grade writing on the front. And be sure to put your NAME ON EACH PAGE in case they become separated! There are SEVEN pages including this cover sheet. Question 1. 21 pts __________ Question 2. 22 pts __________ Question 3. 20 pts __________ Question 4. 20 pts __________ Question 5. 17 pts __________ TOTAL 100 pts __________
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Name: ____________KEY__________________ 2 Question 1. (21 pts) Answer the following questions about actin and microfilaments. (a, 3 pts) 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? Net growth …a monomer concentration equal to the Cc for the (+) end? Net shrinkage …a monomer concentration equal to the Cc for the filament? No net change The net effects here should be the “sum” of the two net effects (plus and minus end) listed for each concentration in part b. (b, 6 pts) 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? …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, 4 pts) 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. Cytochalasin D binds to filamentous actin and prevents the addition of monomers. Thus monomers can only dissociate from filaments, leading to depolymerization. Latrunculin binds to actin monomers and sequesters them so that they cannot add to filaments. Thus monomers can only dissociate from filaments, leading to depolymerization. (d, 4 pts) How can you demonstrate experimentally that ATP hydrolysis by actin is not necessary for a subunit to add on to a microfilament? You would do an in vitro polymerization assay in which you added non- hydrolyzable ATP instead of ATP. You would incubate monomers and non- PLUS MINUS Net growth no net change No net change net shrinkage Net growth net shrinkage
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Name: ____________KEY__________________ 3 hydrolyzable GTP in a test tube and wait. Following certain incubation times, you would assay for filament length and see growth over time. You could assay for filament growth in a variety of ways: using EM microscopy, using fluorescent
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706_S2006_Exam4key - Name: _KEY_ 7.06 Cell Biology EXAM #4...

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