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 In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows: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

Verified step by step guidance

1. Step 1: Understand the experimental setup. In this interrupted mating experiment, an Hfr strain (donor) with genotype cys⁺ leu⁺ met⁺ strˢ is mated with a recipient strain cys⁻ leu⁻ met⁻ strᴿ. The selection media are designed to identify which genes have been transferred based on colony growth. Streptomycin resistance (strᴿ) ensures only recipient cells grow, and the presence of specific amino acids in the media allows identification of transferred genes.
2. Step 2: Analyze the data from the table. Growth on Medium 1 indicates transfer of the leu gene, Medium 2 indicates transfer of the cys gene, and Medium 3 indicates transfer of the met gene. The number of colonies at each time point reflects the order in which genes are transferred during conjugation.
3. Step 3: Identify the first gene transferred. Look at the time points where colonies first appear on each plate. Colonies appear on Plate 2 (Medium 2) at 9 minutes, indicating that the cys gene is transferred first.
4. Step 4: Determine the second gene transferred. Colonies appear on Plate 1 (Medium 1) at 15 minutes, indicating that the leu gene is transferred second.
5. Step 5: Determine the third gene transferred. Colonies appear on Plate 3 (Medium 3) at 18 minutes, indicating that the met gene is transferred last. The order of gene transfer is cys → leu → met.

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 mapped based on the time of transfer. Understanding Hfr strains is crucial for analyzing gene transfer and mapping gene order.

Interrupted Mating Experiment

An interrupted mating experiment is a technique used to study gene transfer in bacteria by stopping the conjugation process at specific time intervals. By withdrawing samples at regular intervals and analyzing the resulting colonies on selective media, researchers can determine the order in which genes are transferred. This method provides insights into the relative positions of genes on the bacterial chromosome.

Selection Media and Gene Function

Selection media are specially formulated growth environments that allow only certain bacteria to thrive based on their genetic capabilities. In this experiment, different media contain specific nutrients that correspond to the genes being studied (cys, leu, met). By observing which colonies grow on which media, researchers can infer the presence or absence of specific genes, aiding in the determination of gene order based on the timing of their transfer.