2005_Exam2_key - Name: _ 7.06 Cell Biology EXAM #2 March...

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Name: _____________________________ 1 7.06 Cell Biology EXAM #2 March 17, 2005 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. And be sure to put your name on each page in case they become separated! Remember that we will Xerox all of the exams. Good luck! Question 1. 20 pts __________ Question 2. 18 pts __________ Question 3. 22 pts __________ Question 4. 20 pts __________ Question 5. 20 pts __________
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Name: _____________________________ 2 Question 1. (20 points) Many neurons from one side of the body send out projections that cross to the other side of the body. You are interested in the development of these “midline crossings” and decide to study them in the fruit fly D. melanogaster . In particular you want to identify factors that prevent neurons from re-crossing the midline once the neuron has crossed the midline, known as repulsion of axons from the midline. You screen for mutant flies that have an abnormally high or abnormally low number of midline crossings relative to wild-type flies (which have a certain set number of midline crossings). You name your mutants xdf1- , xdf2- , xdf3- , and xdf4 * (where a minus sign denotes a recessive loss of function mutation and a star denotes a mutation that causes a dominant phenotype due to constitutive activity of the gene product). The corresponding wild-type proteins are referred to as XDF1, XDF2, XDF3, and XDF4. The following is a list of the single mutant phenotypes you obtain: (Note that the flies are homozygous mutants for all mutations listed.) xdf1- high # of crossings xdf2- high # of crossings xdf3- low # of crossings xdf4* low # of crossings To order the genes into a pathway you analyze the phenotype of various double mutants. These are the results you obtain: xdf1-/xdf3- low # of crossings xdf1-/xdf4* high # of crossings xdf2-/xdf3- low # of crossings xdf2-/xdf4* low # of crossings xdf3-/xdf4* low # of crossings (a, 8 pts) Write out the linear arrangement of the signaling pathway that leads to midline crossing. Your model should show the order of the genes, and should use positive arrows ( ) or negative arrows ( ----] ) to indicate the relationship between the genes. In your model, include only the following: wild-type XDF1, XDF2, XDF3, XDF4 and the output “midline crossings.” The correct pathway is: XDF2 --> XDF4 --> XDF1 --| XDF3 --> midline crossings
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Name: _____________________________ 3 General rules to interpret the relationship between the XDF genes: Genes that are required midline crossing: loss of function mutant will show low number of midline crossings. Genes that inhibit midline crossing: loss of function mutants will show a high number of midline crossings XDF3: functions downstream of all other XDF genes. Loss of XDF3 function leads to reduced midline crossing, so XDF3 promotes midline crossing XDF1: functions upstream of XDF3, but downstream of XDF2 and XFD4. As the single mutant phenotype is opposite to that of the xdf3 mutant, Xdf1 must inhibit Xdf3 function. So XDF1 inhibits midline crossing.
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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_Exam2_key - Name: _ 7.06 Cell Biology EXAM #2 March...

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