An Hfr strain is used to map three genes in an interrupted mating experiment. The cross is Hfr/a⁺b⁺c⁺ rif x F⁻/a⁻b⁻c⁻ rif^T (No map order is implied in the listing of the alleles; rif^T is resistance to the antibiotic rifampicin.) The a⁺ gene is required for the biosynthesis of nutrient A, the b⁺ gene for nutrient B, and c⁺ for nutrient C. The minus alleles are auxotrophs for these nutrients. The cross is initiated at time = 0 and at various times, the mating mixture is plated on three types of medium. Each plate contains minimal medium (MM) plus rifampicin plus specific supplements that are indicated in the following table. (The results for each time interval are shown as the number of colonies growing on each plate.) Time of Interruption _ 5 min 10 min 15 min 20 min Nutrients A and B 0 0 4 21 Nutrients B and C 0 5 23 40 Nutrients A and C 4 25 60 82 Based on these data, determine the approximate location on the chromosome of the a, b, and c genes relative to one another and to the F factor.

Key Concepts

Hfr Strains and Gene Mapping

Hfr (high frequency of recombination) strains are bacterial strains that facilitate the transfer of chromosomal genes during conjugation. In interrupted mating experiments, the timing of gene transfer is crucial for mapping gene locations on the chromosome. By analyzing the order and timing of gene transfer, researchers can infer the relative positions of genes based on the number of colonies that grow on selective media.

Auxotrophy and Supplementation

Auxotrophy refers to the inability of an organism to synthesize a particular compound required for its growth, necessitating supplementation in the growth medium. In this experiment, the a⁻, b⁻, and c⁻ alleles are auxotrophic for nutrients A, B, and C, respectively. The presence of these nutrients in the media allows for the selection of successful gene transfer events, as only bacteria that have received the corresponding wild-type alleles can grow.

Interrupted Mating Technique

The interrupted mating technique involves stopping the conjugation process at specific time intervals to analyze which genes have been transferred. By plating the mating mixture on selective media at various time points, researchers can determine the sequence of gene transfer. The number of colonies that grow on each type of medium provides insight into the order of gene acquisition, allowing for the construction of a genetic map.