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 In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows: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


What donor gene is the selected marker in each medium?

Verified step by step guidance

1. Step 1: Understand the genotypes of the donor (Hfr strain) and recipient strain. The donor strain is cys⁺ lue⁺ met⁺ strˢ, meaning it can synthesize cysteine, leucine, and methionine, and is sensitive to streptomycin. The recipient strain is cys⁻ lue⁻ met⁻ strᴿ, meaning it cannot synthesize these amino acids and is resistant to streptomycin.
2. Step 2: Analyze the selection media. Each medium contains specific supplements and streptomycin. Streptomycin selects for recipient cells (strᴿ), ensuring only recipient cells that have received donor genes through conjugation can grow. The supplements in each medium allow growth of cells that lack the corresponding amino acid synthesis gene.
3. Step 3: Determine the donor gene selected in Medium 1. Medium 1 contains leucine, methionine, and streptomycin. Cells that grow on this medium must have received the cys⁺ gene from the donor, as cysteine is not supplemented in the medium.
4. Step 4: Determine the donor gene selected in Medium 2. Medium 2 contains cysteine, methionine, and streptomycin. Cells that grow on this medium must have received the lue⁺ gene from the donor, as leucine is not supplemented in the medium.
5. Step 5: Determine the donor gene selected in Medium 3. Medium 3 contains cysteine, leucine, and streptomycin. Cells that grow on this medium must have received the met⁺ gene from the donor, as methionine is not supplemented in the 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 genetic material to a recipient strain during conjugation. This process involves direct contact between two bacteria, where the Hfr strain donates part of its chromosome to the recipient. Understanding Hfr strains is crucial for analyzing gene transfer and mapping bacterial genes based on the order of gene transfer.

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, researchers can determine the order in which genes are transferred from the donor to the recipient strain. This method helps in mapping the relative positions of genes on the bacterial chromosome based on the time of transfer.

Selection Media and Genetic Markers

Selection media are specially formulated growth environments that allow only certain bacteria to thrive based on their genetic traits. In this experiment, the media contain specific amino acids and antibiotics that select for bacteria with particular donor genes. By analyzing which genes allow growth on each medium, researchers can identify the donor genes responsible for the observed phenotypes, such as cys⁺, lue⁺, and met⁺.