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 In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows: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