2005_pset8_ans - 7.06 Problem Set#8 Spring 2005 1 a You inject a significant amount of fluorescently labeled tubulin into a cell and wait until all

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1 7.06 Problem Set #8, Spring 2005 1. a. You inject a significant amount of fluorescently labeled tubulin into a cell, and wait until all microtubules are labeled with fluorescence. You then use a laser to irradiate a small region of one of the microtubules, thereby bleaching the fluorescent dye so that it is no longer fluorescent. You then perform a timecourse in which you measure the recovery of fluorescence into the bleached area. For cells in interphase, you obtain the following data. Superimpose on this graph a drawing of the data you would expect to obtain if you repeated the experiment, but using cells proceeding through prophase. Explain your reasoning. Flourescence is reincorporated into the bleached areas of the microtubules when new subunits are added to these polymers. Thus measuring fluorescence recovery time is, in a way, measuring the half-life of these polymers. Since microtubules are so much more dynamic/unstable during mitosis than during interphase, fluorescence will be recovered much faster during mitosis. 100% % of fluor. recovered time 60 min 100% % of fluor. recovered time 60 min
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2 b. Taxol is a drug that binds to and greatly stabilizes assembled microtubules; after taxol treatment, the microtubules are stable to all but the harshest denaturating conditions. This activity is in a sense the opposite of that seen with the drugs we discussed in class, like colchicines, which are microtubule-depolymerizing drugs. It turns out that both kinds of drugs – taxol and colchicines – are anti-cancer drugs. Why do you think that each of them works as an anti-cancer drug? Anti-cancer drugs generally attack cells that are undergoing the cell cycle. Both taxol and colchicines inhibit proper progression of the cell cycle because they both interfere with microtubules, which make up the mitotic spindle. When cells are treated with taxol, the microtubule polymers would be unable to depolymerize. Therefore, the mitotic spindle would form, but would be unable to shrink. This would make it difficult to attach to kinetochores and align chromosomes on the metaphase plate, and would make it impossible to separate the sister chromatids. Therefore, the taxol-treated cells would arrest in prometaphase. When cells are treated with colchicines, the microtubule polymers would all depolymerize. Therefore, the mitotic spindle would not form and chromosomes could neither line up nor separate. c. The frog oocyte (unfertilized egg) is a single cell that has asymmetry. The asymmetry of the frog oocyte is reflected in the asymmetric distribution of many mRNAs and proteins within the oocyte cytoplasm. The localization of one specific mRNA called Vg1 changes as the oocyte develops: - In early stage oocytes, Vg1 is found throughout the cytoplasm. - In intermediate stage oocytes, Vg1 is found traveling towards one end of the
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This note was uploaded on 01/23/2012 for the course BIOLOGY lsm1301 taught by Professor Seow during the Spring '11 term at National University of Singapore.

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2005_pset8_ans - 7.06 Problem Set#8 Spring 2005 1 a You inject a significant amount of fluorescently labeled tubulin into a cell and wait until all

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