Genetics: Analysis and Principles 5th Edition by Robert Brooker

Exercise 17

Acridine orange is a chemical that inhibits the replication of F factor DNA but does not affect the replication of chromosomal DNA, even if the chromosomal DNA contains an Hfr. Let's suppose that you have an E. coli strain that is unable to metabolize lactose and has an F factor that carries a streptomycin-resistant gene. You also have an F strain of E. coli that is sensitive to streptomycin and has the genes that allow the bacterium to metabolize lactose. This second strain can grow on a lactose-containing medium. How would you generate an Hfr strain that is resistant to streptomycin and can metabolize lactose (Hint: F factors occasionally integrate into the chromosome to become Hfr strains, and occasionally Hfr strains excise their DNA from the chromosome to become F + strains that carry an F factor.)