Problem set Chapter 6

Q37. In E. coli, four Hfr strains donate the following markers, shown in the order donated: Strain 1: M Z X W C Strain 2: L A N C W Strain 3: A L B R U Strain 4: Z M U R B All these Hfr strains are derived from the same F+ strain. What is the order of these markers on the circular chromosome of the original F+? 39. A cross is made between two E. coli strains: Hfr arg+ bio+ leu+ × F− arg− bio− leu− . Interrupted mating studies show that arg+ enters the recipient last, and so arg+ recombinants are selected on a medium containing bio and leu only. These recombinants are tested for the presence of bio+ and leu+. The following numbers of individuals are found for each genotype: arg+ bio+ leu+ 320 arg+ bio− leu+ 0 arg+ bio+ leu− 8 arg+ bio− leu− 48 a. What is the gene order? b. What are the map distances in recombination percentages? Q44. Five Hfr strains A through E are derived from a single F+ strain of E. coli. The following chart shows the entry times of the first five markers into an F− strain when each is used in an interrupted- conjugation experiment: A B C D E mal+ 1 ade+ 13 pro+ 3 pro+ 10 his+ 7 str s 11 his+ 28 met+ 29 gal+ 16 gal+ 17 ser+ 16 gal+ 38 xyl+ 32 his+ 26 pro+ 23 ade+ 36 pro+ 44 mal+ 37 ade+ 41 met+ 49 his+ 51 met+ 70 str s 47 ser+ 61 xyl+ 52 a. Draw a map of the F+ strain, indicating the positions of all genes and their distances apart in minutes. b. Show the insertion point and orientation of the F plasmid in each Hfr strain. c. In the use of each of these Hfr strains, state which allele you would select to obtain the highest proportion of Hfr exconjugants. (*solution manual is incorrect – to get highest proportion of Hfr exconjugates, select for LAST marker on the chromosome) Q53. Deduce the genotypes of the following E. coli strains 1 through 4:

Q 57. Four E. coli strains of genotype a+ b− are labeled 1, 2, 3, and 4. Four strains of genotype a− b+ are labeled 5, 6, 7, and 8. The two genotypes are mixed in all possible combinations and (after incubation) are plated to determine the frequency of a+ b+ recombinants. The following results are obtained, where M = many recombinants, L = low numbers of recombinants, and 0 = no recombinants: 1 2 3 4 5 0 M M 0 6 0 M M 0 7 L 0 0 M 8 0 L L 0 On the basis of these results, assign a sex type (either Hfr, F+, or F−) to each strain. Q 58. An Hfr strain of genotype a+ b+ c+ d− str s is mated with a female strain of genotype a− b−c− d+ str r . At various times, mating pairs are separated by vigorously shaking the culture. The cells are then plated on three types of agar as shown in the accompanying table, where nutrient A allows the growth of a− cells; nutrient B, of b− cells; nutrient C, of c− cells; and nutrient D, of d− cells. (A plus indicates the presence of streptomycin or a nutrient, and a minus indicates its absence.) Agar type Str A B C D 1 + + + – + 2 + – + + + 3 + + – + + a. What donor genes are being selected on each type of agar?