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 In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows: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

A triple-auxotrophic strain of E. coli having the genotype phe⁻ met⁻ ara⁻ is used as a recipient strain in a transduction experiment. The strain is unable to synthesize its own phenylalanine or methionine, and it carries a mutation that leaves it unable to utilize the sugar arabinose for growth. The recipient is crossed to a prototrophic strain with the genotype phe⁺ met⁺ ara⁺. The table below shows the selected marker and gives cotransduction frequencies for the unselected markers.

Use the cotransduction data to determine the order of these genes.

A triple-auxotrophic strain of E. coli having the genotype phe⁻ met⁻ ara⁻ is used as a recipient strain in a transduction experiment. The strain is unable to synthesize its own phenylalanine or methionine, and it carries a mutation that leaves it unable to utilize the sugar arabinose for growth. The recipient is crossed to a prototrophic strain with the genotype phe⁺ met⁺ ara⁺. The table below shows the selected marker and gives cotransduction frequencies for the unselected markers.

Identify the compounds present in each of the selective media.

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 In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows: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

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.

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.

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

Suppose a fourth selection medium containing leucine and streptomycin is prepared. At what sampling time do you expect the first-growing colonies to appear? Explain your reasoning.