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 In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows: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

The following table shows the number of colonies growing on each selection medium. The sampling time indicates how many minutes have passed since conjugation began.

Sampling Time (minutes)              Number of Colonies
                                            Plate 1         Plate 2         Plate 3
           3                                   0                  0                   0
           6                                   0                  0                   0
           9                                   0                 62                  0
         12                                   0                 87                  0
         15                                  51               124                 0
         18                                  79               210                 62
         21                                 109               250                85
         24                                 144               250               111
         27                                 152               250               122
         30                                 152               250               122

Determine the order of donor genes cys, leu, and met from the interrupted mating data.

Key Concepts

Hfr Strains

Hfr (high frequency of recombination) strains are bacterial strains that have a plasmid integrated into their chromosomal DNA. This allows them to transfer chromosomal genes to a recipient bacterium during conjugation. The order in which genes are transferred can be determined by analyzing the time it takes for specific traits to appear in the recipient cells after mating.

Interrupted Mating Experiment

An interrupted mating experiment is a technique used to study gene transfer during bacterial conjugation. In this method, mating is halted at specific time intervals, allowing researchers to analyze which genes have been transferred based on the traits expressed in the recipient cells. The timing of gene transfer helps establish the relative positions of genes on the chromosome.

Selection Media

Selection media are specialized growth environments that allow only certain bacteria to grow based on their genetic traits. In this experiment, different media are used to select for bacteria that have acquired specific genes (cys, leu, met) from the Hfr strain. The number of colonies that grow on each medium indicates which genes have been successfully transferred and helps determine their order.