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 In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows: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


List all possible bacterial genotypes growing on each medium.

Verified step by step guidance

1. Step 1: Understand the genotypes of the Hfr strain and the recipient strain. The Hfr strain is cys⁺ leu⁺ met⁺ strˢ, meaning it can synthesize cysteine, leucine, and methionine but is sensitive to streptomycin. The recipient strain is cys⁻ leu⁻ met⁻ strᴿ, meaning it cannot synthesize these amino acids but is resistant to streptomycin.
2. Step 2: Recognize the role of the media. Each medium contains specific supplements and streptomycin. Streptomycin ensures that only the recipient strain (strᴿ) or recombinants (which inherit strᴿ from the recipient) can grow. The supplements in each medium allow growth of bacteria that can synthesize the missing amino acids.
3. Step 3: Analyze Medium 1 (minimal medium + leucine, methionine, and streptomycin). Bacteria growing here must be able to synthesize cysteine (cys⁺) since leucine and methionine are provided. The possible genotype of bacteria growing on this medium is cys⁺ leu⁻ met⁻ strᴿ.
4. Step 4: Analyze Medium 2 (minimal medium + cysteine, methionine, and streptomycin). Bacteria growing here must be able to synthesize leucine (leu⁺) since cysteine and methionine are provided. The possible genotype of bacteria growing on this medium is cys⁻ leu⁺ met⁻ strᴿ.
5. Step 5: Analyze Medium 3 (minimal medium + cysteine, leucine, and streptomycin). Bacteria growing here must be able to synthesize methionine (met⁺) since cysteine and leucine are provided. The possible genotype of bacteria growing on this medium is cys⁻ leu⁻ met⁺ strᴿ.

Key Concepts

Conjugation in Bacteria

Conjugation is a process of genetic transfer in bacteria where one bacterium transfers genetic material to another through direct contact. This often involves the transfer of plasmids or chromosomal DNA, allowing for the exchange of traits such as antibiotic resistance or metabolic capabilities. Understanding conjugation is crucial for analyzing the genetic outcomes of mating experiments, as it determines which traits can be passed on to the recipient strain.

Hfr Strains

Hfr (high frequency of recombination) strains are bacterial strains that have a plasmid integrated into their chromosomal DNA, allowing them to transfer chromosomal genes to a recipient during conjugation. The order and timing of gene transfer can be mapped based on the time of withdrawal during an interrupted mating experiment. Recognizing the significance of Hfr strains is essential for predicting which genes may be transferred and expressed in the recipient strain.

Selection Media

Selection media are specialized growth environments that allow only certain bacteria to thrive based on their genetic traits. In this experiment, different media are used to select for bacteria that can synthesize specific amino acids or resist antibiotics. By analyzing which genotypes grow on each medium, one can infer the genetic changes that occurred during conjugation and identify the successful gene transfers from the Hfr strain to the recipient strain.