Question

3. (10 pts) A friend sets up the following cross between an Hfr strain and an F- strain; Hfr: rif lac str pheA X F:rif lac str pheA In the Hfr, the F factor is integrated at 9 minutes on the E. coli genetic map and transfers the genome in a counter clockwise direction, similar to CSHI19 Gene function Position on genetic map Gene Name RNA polymerase (target of rifamycin) lactose utilization phenylalanine biosynthesis ribosomal protein (target of streptomycin) rpoB 90 minutes 8 lac phe A 57 73 str Your friend plates the cells on minimal medium with glucose, streptomycin and rifamycin. Since the rif gene is transferred early, she is surprised to find very few recombinant colonies growing on her plates. Furthermore, she analyzes her recombinants for lactose utilization and finds that though the lac gene transfers very early, all of the recombinants examined are Lac" even are there so many Lac" colonies? How could she have tested whether the cells were Lac+ Why or Lac-? One short sentence, each question al b. Draw out a diagram of the cross below. Show all genes on both chromosomes and don't forget to include a scale and the location and direction of the F factor. Show crossovers Why did she get very few recombinant colonies? One sentence. c.

Answer 1

Ans a: colonies are Lac⁺ because they have Lac operon, utilized lactose and grow.

Ans c: recombinant cannot grow in minimal media