Exam 2_Sp'10_KEY_all_

Exam 2_Sp'10_KEY_all_ - BIOLOGY 25L - SPRING '10 MIDTERM II...

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1 BIOLOGY 25L - SPRING '10 MIDTERM II KEY 1) Drosophila embryogenesis converts a fertilized egg into a larva in just 24 hours. The fly larvae have a predictable pattern of bands of bristles alternating with bands of epidermal tissue without bristles on their belly sides. Many regulatory proteins have been identified in the cells that give rise to the bristle-less tissue. These proteins are expressed in two adjacent rows of cells that carry out inductive interactions with neighboring cells. Their names, cellular locations, and row number are given in the table below. a) Fill in the final column of the Table 1 predicting the type of protein for each regulatory protein. (6 pts.) TABLE 1: Expression data showing where each molecule is located in the embryo (row 1 or row 2) and where the protein winds up in relation to the cell that expressed it (nucleus, cell surface or extracellular). Protein name Location in embryo Location in cell Type of Protein Arm Row 2 cells Nucleus Transcription factor Ci Row 1 cells Nucleus Transcription factor Fz Row 2 cells Cell surface Receptor Hh Row 2 cells Extracellular Signal Ptc Row 1 cells Cell surface Receptor Wg Row 1 cells Extracellular Signal 1 pt. for each correct answer TABLE 2: Perturbation Data experiments manipulating the presence of these proteins were carried out, and the resulting changes in the expression of other components were observed as shown below. Protein name Perturbation Observed component Observed result Wg Absent Hh Expression decreased Wg Absent Arm No change in expression Hh Absent Wg Expression decreased b) Use the results of the experiments summarized in Tables 1 and 2 to deduce relationships and interactions among these proteins. Create a gene regulatory network model representing the interactions between Row 1 cells and Row 2 cells. (10 pts.) Question 1 key continued next page. ..
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2 Credit for part b was awarded as follows: Depicting the parts of the cell in a logical and consistent manner - 2 pts. Drawing the correct connections between the labeled components withou t extra, unnecessary step s - 6 pts. Drawing a diagram that could also correctly explain the perturbation data - 2 pts. (see below) Explanation of diagram and connections between components: Based on perturbation 1, we know that Wg is a positive regulator of Hh . Based on perturbation 2, we know that Wg does not affect the expression of ARM . Some other transcription factor must be controlling ARM . However, once ARM is made, Wg ’s signal causes ARM to turn on Hh via Fz . Based on perturbation 3, we know that Hh is a positive regulator of Wg, which combined with perturbation 1 data tells us that it must be a cycle. Some answers received less than full credit for having a transcription factor turn itself on before it regulated the gene. For example, showing Ci turning itself on, and then turning on Wg . This extra step is illogical as Ci could just turn on Wg directly. Other misconceptions that resulted in partial credit included:
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Exam 2_Sp'10_KEY_all_ - BIOLOGY 25L - SPRING '10 MIDTERM II...

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