An Hfr strain with the genotype cys⁺ lue⁺ met⁺ strˢ is mated with an  strain carrying the genotype cys⁻ lue⁻ met⁻ strᴿ. In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows:

Medium 1: Minimal medium plus leucine, methionine, and streptomycin
Medium 2: Minimal medium plus cysteine, methionine, and streptomycin
Medium 3: Minimal medium plus cysteine, leucine, and streptomycin

What donor gene is the selected marker in each medium?

Key Concepts

Hfr Strains and Conjugation

Hfr (high frequency of recombination) strains are bacterial strains that have integrated a plasmid into their chromosomal DNA, allowing them to transfer chromosomal genes to a recipient cell during conjugation. This process involves direct contact between two bacteria, where the Hfr strain donates genetic material, which can be selected based on the recipient's genotype and the selective media used.

Interrupted Mating Experiment

An interrupted mating experiment is a technique used to study gene transfer during bacterial conjugation. By withdrawing samples at regular intervals, researchers can determine the order of gene transfer based on the time it takes for specific genes to be incorporated into the recipient strain. This method helps identify which genes are transferred first and their relative positions on the chromosome.

Selection Media

Selection media are specially formulated growth environments that allow only certain bacteria to thrive based on their genetic traits. In this experiment, different media are used to select for specific donor genes from the Hfr strain. The presence of specific nutrients (like leucine or cysteine) in the media allows for the identification of which genes have been successfully transferred to the recipient strain.