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Unformatted text preview: Name: ______________key_________________ 1 7.06 Cell Biology EXAM #2 KEY March 21, 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 TEN pages including this cover sheet. Question 1. 20 pts __________ Question 2. 24 pts __________ Question 3. 26 pts __________ Question 4. 30 pts __________ TOTAL 100 pts __________ Name: ______________key_________________ 2 Question 1. (20 pts) You are studying the Wingless/Wnt pathway in mammalian cells. Your cells contain a reporter gene construct that consists of the open reading frame of GFP driven by the promoter for hedgehog. The type of cells you are using do indeed regulate expression from this reporter gene construct in response to extracellular Wnt as expected. (a, 6 pts) When would your reporter gene construct be expressed if the following mutant versions of Wnt pathway components were ectopically expressed in wild-type cells? YOUR CHOICES ARE: never, only + Wnt, only – Wnt, OR both + and – Wnt. (i) A version of beta-catenin in which all of its phosphorylation sites have been changed to alanine. Both + and – Wnt. The reporter gene will be constitutively expressed, because beta- catenin that cannot be phosphorylated cannot be degraded, so it will always be active. Beta-catenin activates transcription of hedgehog, so constitutively active beta-catenin will turn on hedgehog transcription constitutively. Regardless of whether or not wild- type beta-catenin is present, the mutant form will cause a constitutive phenotype. (ii) A version of GSK3 that cannot interact with Dishevelled. Never. If GSK3 can never interact with (and thus be inhibited by) Dishevelled, GSK3 will always be active. Thus GSK3 will always phosphorylate beta-catenin, and so beta- catenin will always be degraded. Therefore hedgehog transcription will never be activated. This will occur whether or not there is wild-type GSK3, because the mutant form of GSK3 is constitutively active as a kinase, and thus will cause a dominant phenotype. (iii) A mutant form of Frizzled that does not localize to the membrane. Only + wnt. Frizzled is the receptor for Wnt ligand. If Frizzled is not at the membrane, it cannot bind ligand, so it cannot activate the pathway, and thus cannot lead to hedgehog activation. However this would result in a standard loss of function phenotype of uninducible expression of the hegehog reporter, a defect that is easily fixed by expressing wild-type Frizzled in the cells....
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