LS4_midtermII_w05key

LS4_midtermII_w05key - LS4 Midterm II INSTR: L. Johnson...

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LS4 Midterm II INSTR: L. Johnson Monday, Feb. 28, 2005 NAME:_________________________________ (last) (first) I.D. #__________________________________ INSTRUCTIONS 1. This exam is 6 pages long. Problem Value Score Before starting make sure you have all the pages. 1 13 2. Be sure to write clearly. Only 2 7 legible answers will be graded. 3 14 4 8 5 20 6 21 7 17 Total 100 KEY
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NAME:_________________________________ ID# _______________________ (last) (first) (13 pts) 1. Two different strains of E. coli (A: thr+leu-; B:thr-leu+) are found in two different forms with regard to their ability to transfer genetic markers (1 and 2). All possible crosses are made between the A and B strains, but the only ones that give any prototrophs are shown below. Serial dilutions are performed to determine the frequency of transfer. 0.1 ml of each dilution is plated on either minimal media (MM) or minimal media plus threonine and leucine (MM+thr+leu). The results are shown below. Cross Number of colonies on MM Number of colonies on MM+thr+leu No dilution 10 -2 10 -4 10 -6 No dilution 10 -2 10 -4 10 -6 A1 x B1 56 0 0 0 conf conf conf 367 A2 x B2 conf conf 50 0 conf conf conf 475 a) What fraction of the exconjugants are prototrophs in cross A1 x B1? 56 /(0.1ml) = 1.5 x 10 -7 367/10 -6 (0.1 ml) (+3) b) What fraction of the exconjugants are prototrophs in cross A2 x B2? 50/10 -4 (0.1 ml) = 1.05 x 10 -3 475/10 -6 (0.1 ml) (+3) c) How could you experimentally determine (not using microscopy) which parent is which sex ? (+3) Either use streptomycin to kill one parent and do the cross again. Repeat treating the other parent with streptomycin. This will allow you to determine directionality and hence which parent is the male. Alternatively, mate each strain to a known male or female strain and look for prototrophs. Only male x female will give prototrophs. d)
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LS4_midtermII_w05key - LS4 Midterm II INSTR: L. Johnson...

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