If lac+ Hfr bacteria mated with a lac- F- bacteria in an interrupted mating experiment, the F- bacteria will only gain the ability to metabolize lactose if a copy of the lac+ gene recombines with the F- bacteria’s chromosome. Why is recombination into the bacterial chromosome necessary? Recombination is required in order to convert the F- bacteria into Hfr bacteria capable of expressing the genes to metabolize lactose. Although the incoming DNA from the Hfr cell forms a circle, the lac+ locus is not active on a plasmid. Linear DNA is unstable in bacterial cells and unless the lac+ gene is recombined into the bacterial chromosome, it will be destroyed. Recombination is necessary to remove the lac- gene from the bacterial chromosome as this would interfere with the function of the lac+ gene from the Hfr cell. Recombination is required in order to convert the F- bacteria into F+ bacteria capable of expressing the genes to metabolize lactose.