Name: ____________KEY_____________ 1 2006 7.012 Problem Set 7 KEY ** Due before 5 PM on FRIDAY, December 8, 2006. ** Turn answers in to the box outside of 68-120. PLEASE WRITE YOUR ANSWERS ON THIS PRINTOUT. 1. Because producing effective HIV vaccines is problematic, the most common treatments for HIV infections are currently are anti-viral drugs. (a) Why do you think that there are no serious side effects to humans being treated for HIV by drugs that target the enzyme reverse transcriptase? Reverse transcriptase is specific to HIV; humans don’t have the reverse transcriptase enzyme. (b) The drugs used to treat AIDS that are protease inhibitors need to be specific for HIV proteases, and cannot inhibit proteases in general. Why do you think this is? We need proteases to degrade proteins on a regular basis in our own cells, so we don’t want to target all proteases with the drug. By doing so, we would be inhibiting proteases that are essential for the host’s cell survival. (c) You are attempting to come up with new ideas for HIV drugs. As a start you’d like to try to develop a drug you call anti-CD4, which would inhibit the function of CD4 proteins on the surface of helper T cells. Why would anti-CD4 be useful for preventing HIV infection? The HIV virus binds to the CD4 protein on the surface of TH cells so by inhibiting the ability of CD4 to bind molecules, you prevent its binding to HIV and thus HIV’s ability to enter human T cells. (d) What is the most major disadvantage of anti-CD4 treatment? The ability of CD4 to recognize and bind MHCII molecules is critical for the human immune system and so, by disrupting CD4 activity, this treatment has a negative effect on the patient’s immune system. (e) HIV viral particles cannot dock onto hamster cells, although it can dock onto human cells. You want to study HIV entry into host cells in lab using hamster cells. How do you think you might genetically manipulate hamster cells such that HIV viral particles can dock onto them? You could genetically manipulate hamster cells so that they contain the human CD4 gene and thereby express the human CD4 protein on the cell surface.Name: ____________KEY_____________ 2 2. You are working in a lab that is trying to perform stem cell therapy on mice with Parkinson’s Disease by inducing stem cells to become nerve cells in vitro, and then delivering these nerve cells to the diseased mouse’s brain. The mouse strain you are trying to treat has a form of Parkinson’s Disease that is autosomal recessive and is caused by loss-of-function in the PARK2 gene. (a) You initially decide that you want to do the stem cell treatment with stem cells harvested from a wild-type mouse. What is a potential problem with treating the diseased mouse with stem cells taken from another mouse that is not related at all to the diseased mouse? The mouse receiving the transplant will recognize the transplanted cells as non-self and mount an immune response against these cells. (b) You decide to try the kind of therapy discussed in part (a) anyway, regardless of the problem listed above. You decide that you will do the therapy using adult stem cells harvested from a wild-type mouse. What is one major problem with trying to use adult stem cells specifically for this therapy? Adult stem cells have a more limited differentiation potential than embryonic stem cells and so they may not appropriately generate new nerve cells in the host mouse. Adult stem cells are also very rare, and hard to isolate from the adult. (c) You change your mind, and decide that you want to work with ES cells that are genetically identical to those of the diseased mouse, due to the problems listed in part (a) and (b). Why can’t you directly isolate ES cells from the diseased mouse? Embryonic stem cells are derived from the inner cell mass of a developing blastocyst. Since the diseased mouse is a fully developed adult, ES cells do not exist in this adult. (d) You decide to use Somatic Cell Nuclear Transfer in order to derive ES cells that are genetically identical to the diseased mouse. List the steps of the procedure you would need to do to accomplish this. 1. Remove nucleus from an unfertilized mouse egg 2. Remove a diploid nucleus from a cell from the adult diseased mouse 3. Put the diploid nucleus into the enucleated egg 4. Allow the newly nucleated egg to grow up to the blastocyst stage 5. Remove ES cells from the inner cell mass of the developing blastocyst 6. In culture, induce the ES cells to form neurons (e) Before you induce the ES cells you derived in part (d) to become nerve cells, what must you do to the ES cells to make the stem cell treatment actually beneficial to the diseased mouse? (Hint: re-read the introduction to this question.)Name: ____________KEY_____________ 3 You first have to give a wild-type copy of the PARK2 gene to the ES cells. (f) You do your treatment described in parts (c) – (e) on a female mouse, and the treatment is successful. Your mouse patient has now received the stem cells and has been relieved of its symptoms. For each cell in the treated mouse described below, state how many total alleles of the PARK2 gene are in that cell, and how many are wild-type versus mutant: -- a nerve cell originally present in the mouse: 2 alleles; both mutant. Since this form of Parkinson’s is an autosomal recessive disease the cells of the brain that are affected must carry a mutation in both of their alleles. -- a nerve cell given to the mouse during the treatment: This depends on whether you have simply given the mouse an extra copy of wild-type PARK2 (which would integrate into the mouse genome at a random location) or whether you have targeted the mouse’s mutant copy of PARK2 with a wild-type copy of PARK2. If you did the former, the answer would be: 3, two mutant, one wild-type. OR if you did the latter, the answer would be: 2, one mutant, one wild-type. -- an egg cell from the mouse: 1 allele; mutant. The mouse affected with Parkinson’s would be of the genotype PARK2–/PARK2– so each of its egg cells would contain one mutant allele of PARK2. 3. Organismal cloning proves that an adult cell’s nucleus contains all of the genetic material necessary to generate every cell type in an organism. (a) All nucleated cells in the body of an adult human female contain the same DNA except for which three cell types? B cells and T cells
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