Practice exam 3 answer key final

Practice exam 3 answer key final - Practice exam 3 Fall 09...

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Practice exam 3 Fall 09 (answer key) 1) The following diagrams describe two seminal experiments performed by Lederberg (experiment on the left) and Davis (on the right). a. Combining the results of these experiments, the major conclusion was that: 1. Conjugation is unidirectional 2. Horizontal gene transfer requires a physical interaction between the bacteria 3. Bacteriophages serve as agents that transfer genetic material between bacteria 4. Spontaneous mutations can transform auxotrophs into prototrophs b. Lederberg pursued his experiment. He took one of the prototroph colonies from plate #2 and mixed it with the B strain (met+, bio+, thr-, leuc-, thi-). After mixing the strains, he washed them with a minimal medium and plated them on a minimal medium plate (MM). NO colonies grew on that plate. Assuming you know that strain A was the donor in the conjugation process that took place in the original experiment (the diagram on the left), it is most likely that strain A is an 1) HFR 2) F+ 3) F- 4) F’ Explain your answer. HFR. A is the donor so it cannot be an F-. If it was an F+ or an F’, it is most likely that the recipient would have become an F+ at the end of the conjugation process. If this was the case, the colony from plate 2 (a prototroph donor) would have formed a pilus and would have transfered its genes to the recipient B strain. This is also consistent with our expectation that only an Hfr strain will transfer three bacterial (3) genes from the donor to the recipient [although there is a slight chance that F’ (but not F+) would carry those specific genes].
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2) Consider the following Hfr x F- cross: Hfr genotype: a+ b+ c+ str S X F- genotype: a- b- c- str R where the order of transfer is: a 6 minutes b 28 minutes c 30 minutes Recombinants are selected by plating on a medium that lacks particular nutrients (a, b, c) and contains Streptomycin. Which of the following are TRUE and which are FALSE:
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Practice exam 3 answer key final - Practice exam 3 Fall 09...

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