Bio-366-Test4-Fall-2009

Bio-366-Test4-Fall-2009 - Bio-366 Test-4 October 20, 2009...

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1 Bio-366 Test-4 October 20, 2009 Allowed time: 11.00 am to 12.10pm (I hr and 10 min) Name: UTEID: 1 . Please be brief and to the point. 2 . If you think that a simple diagram will help clarify your answer, please d r a w i t . I . A . When a DNA marker segregates normally during meiosis (without genetic exchange), what is the expected ratio of segregation for a heterozygous marker M/m in ( i ) Saccharomyces yeast (four spores) and in ( ii ) Ascobolus (eight spores)? How do the segregation ratios change from the norm when genetic exchange occurs at this locus? The four spores of Saccharomyces yeast; M : m = 2 : 2. The eight spores of Ascobolus; M : m = 4 : 4. Aberrant modes of segregation: 4 : 4 aberrant; 5 : 3; 6 : 2. (4 : 4 aberrant is the same as 2 : 2 aberrant in Saccharomyces yeast). B . What mode of aberrant segregation is most easily explained by the Holliday model? Is the heteroduplex generated by this model symmetric or asymmetric? How many pairs of identical and non-identical spores would you see in Ascobolus as a result of this type of aberrant segregation? Holliday model most easily explains 4:4 aberrant mode of segregation. The heteroduplex generated is symmetric. In Ascobolus, this mode of aberrant segregation will be seen as two pairs of identical (M,M; m,m) and two pairs of non-identical (M,m; M,m) sister spores. [In yeast, the
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2 4:4 aberrant segregation will be seen as two spores giving rise to half- sectored colonies (Mm), the other two normal colonies, one of ‘M’ phenotype(MM) and the other of ‘m’ phenotype (mm).] Symmetric heteroduplex C . What is the nature of the heteroduplex generated by the Meselson-Radding model? What sort of aberrant segregation does it readily explain? In Saccharomyces yeast, how many half- sectored colonies will result from the four spores of a meiotic division when the markers M/m follow this pattern of segregation? The heteroduplex generated in the Meselson-Radding model is asymmetric. It most easily explains the 5:3 aberrant segregation pattern. There will be one half-sectored colony and three normal colonies: MM; MM; Mm; mm. Asymmetric heteroduplex D . Which mode of segregation is most easily explained by the double strand break repair model? Why is this model for recombination also good at explaining all three classes of aberrant segregation?
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3 The double strand break repair model most easily explains 6:2 aberrant segregation. When the double strand break repair is completed, a double Holliday junction is formed. Branch migration of these junctions can create symmetric heteroduplex. Furthermore, the repair region adjacent to each junction has asymmetric heteroduplex, red-green DNA on one duplex, red-red DNA on the other. If the marker of interest lies in the asymmetric duplex region, it will segregate 5:3. If it happens to fall within the symmetric heteroduplex region, it will segregate as 4:4 aberrant. Thus the double strand gap repair model has features that can explain all three types of aberrant segregation: 4:4
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This note was uploaded on 02/21/2010 for the course BIO 45 taught by Professor Sadasivan during the Spring '08 term at University of Texas.

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Bio-366-Test4-Fall-2009 - Bio-366 Test-4 October 20, 2009...

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