ch18 - Chapter 18 18.1 Which of the following pairs of DNA...

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Chapter 18 18.1 Which of the following pairs of DNA sequences could qualify as the terminal repeats of a bacterial IS element: (a) 5 -GAATCCGCA-3 and 5 -ACGCCTAAG-3 , (b) 5 -GAATCCGCA-3 and 5 -CTTAGGCGT-3 , (c)5 -GAATCCGCA-3 and 5 -GAATCCGCA-3 , (a) 5 -GAATCCGCA-3 and 5 -TGCGGATTC-3 . Explain. ANS: The pair in (d) are inverted repeats and could therefore qualify. FEEDBACK: 18.2 DIFFICULTY: medium 18.2. Which of the following pairs of DNA sequences could qualify as target site duplications at the point of an IS 50 insertion? (a) 5 -AATTCGCGT-3 and 5 -AATTCGCGT-3 , (b) 5 -AATTCGCGT-3 and 5 -TGCGCTTAA-3 , (c) 5 -AATTCGCGT-3 and 5 -TTAAGCGCA-3 , (d) 5 -AATTCGCGT-3 and 5 -ACGCGAATT-3 . Explain. ANS: The pair in (a) are direct repeats and could therefore qualify. FEEDBACK: 18.2 DIFFICULTY: medium 18.3 One strain of E. coli is resistant to the antibiotic streptomycin, and another strain is resistant to the antibiotic ampicillin. The two strains were cultured together and then plated on selective medium containing streptomycin and ampicillin. Several colonies appeared, indicating that cells had acquired resistance to both antibiotics. Suggest a mechanism to explain the acquisition of double resistance. ANS: Resistance for the second antibiotic was acquired by conjugative gene transfer between the two types of cells. FEEDBACK: 18.2 DIFFICULTY: medium
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18.4. What distinguishes IS and Tn 3 elements in bacteria? ANS: Tn3 elements carry a gene that is not essential for transposition. FEEDBACK: 18.2 DIFFICULTY: easy 18.5 The circular order of genes on the E. coli chromosome is * A B C D E F G H *, with the * indicating that the ends of the chromosome are attached to each other. Two copies of an IS element are located in this chromosome, one between genes C and D, and the other between genes D and E . A single copy of this element is also present in the F plasmid. Two Hfr strains were obtained by selecting for integration of the F plasmid into the chromosome. During conjugation, one strain transfers the chromosomal genes in the order D E F G H A B C, whereas the other transfers them in the order D C B A H G F E . Explain the origin of these two Hfr strains. Why do they transfer genes in different orders? Does the order of transfer reveal anything about the orientation of the IS elements in the E. coli chromosome? ANS: In the first strain, the F factor integrated into the chromosome by recombination with the IS element between genes C and D . In the second strain, it integrated by recombination with the IS element between genes D and E . The two strains transfer their genes in different orders because the two chromosomal IS elements are in opposite orientation. FEEDBACK:
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ch18 - Chapter 18 18.1 Which of the following pairs of DNA...

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