Question

The following set of three separate experiments was conducted. Two strains of Escherichia coli are used. Strain 1 needs histidine added to the medium to grow. Strain 2 needs methionine added to the medium to grow. Experiment 1: After strain 1 and strain 2 are mixed in a culture flask containing a minimal medium (containing no added amino acids) and allowed to grow overnight. Results: It is possible isolate new strains that grow on the minimal medium. Experiment 2: Strains 1 and 2 are mixed in a culture flask containing a minimal medium but also containing the enzyme DNAse and grown overnight. Results: It is possible to isolate new strains that grow on the minimal medium. Experiment 3: Media is placed in a U-shaped tube that contains, at the bottom, a membrane filter that prevents cells from crossing between the two chambers of the U-tube, but allows anything small (for example DNA and bacteriophages) to pass between the chambers. The strains are put in different sides of the U-tube. Results: It is possible to isolate new strains that grow on the minimal medium. Conclusions: Genetic exchange is observed in experiments 1,2, and 3 a. Why was Experiment 1 performed? What information does it give us? b. Why was DNase added in Experiment 2? c. What is the purpose of the U-shaped tube in Experiment 3? d. Taking the evidence from all three experiments together, are the scientists observing transformation, conjugation, or transduction?

Answer 1

Q.A

Answer: Experiment-1 was performed to check whether there is a transfer of genetic element (DNA) between the strain. Yes, the new strain of E.coli is obtained by acquiring the foreign DNA.

Q.B

Answer: DNAse was added to check whether the transfer of genetic element (DNA) was through transformation. If transformation would have taken place then the DNAse in the media would have cleaved the DNA and henceforth no new strain would have observed. So the genetic element is not transferred through transformation. The new strain acquired foreign DNA through either conjugation or transduction.

Q.C

Answer: From the experiment-2 it is very clear that the new strain acquired foreign DNA through either conjugation or transduction. Conjugation requires cell-cell interaction, which is prevented in U-shaped tube. From experiment-3 result we observed new strain acquiring foreign DNA. Only possible explanation for that would be through bacteriophages.

Q.D

Answer: Scientists are observing Transduction from these experimental results.