Question

Explain the observations that led Zinder and Lederberg to conclude that the prototrophs recovered in their transduction experiments were not the result of \(\mathrm{F}^{+}\) mediated conjugation.

Short answer

Answer: Zinder and Lederberg concluded that prototrophs were not a result of F+ mediated conjugation based on the following observations: 1) Genetic transfer occurred without direct contact between donor and recipient bacteria, as they were separated by a U-tube; 2) Different genetic markers could be transferred selectively, indicating specific gene transfer, not the whole F+ factor; 3) The transfer of genetic material occurred at a low frequency, consistent with phage transduction though not with F+ mediated conjugation's high frequencies.

Step-by-step solution

1. Background on Zinder and Lederberg Experiment

Zinder and Lederberg conducted an experiment in which they were able to transfer genetic material from one strain of bacteria to another, using bacteriophages as the vector for transferring the genetic material. In this instance, the donor bacterium was an auxotrophic strain (unable to synthesize essential nutrients) while the recipient bacterium was a prototrophic strain (able to synthesize all essential nutrients for growth). The experiment aimed to find out whether the genetic transfer was occurring through transduction mediated by bacteriophages or through F+ mediated conjugation.

2. F+ Mediated Conjugation

F+ mediated conjugation is a process in which genetic material is exchanged between bacterial cells through direct contact of the bacterial cells via a connecting tube called a pilus. In this process, one bacterial cell, known as the donor, transfers part or all of its genetic material to a recipient cell. The donor cell contains F+ factor (fertility factor), which allows the formation of the pilus and the transfer of genetic material.

3. Observations that Supported Transduction

Zinder and Lederberg observed some critical points in their experiment that led them to conclude that the recovered prototrophs were a result of transduction mediated by bacteriophages, rather than F+ mediated conjugation. These observations include: - The genetic transfer occurred without direct contact, as they used a U-tube to separate the donor and recipient bacteria. Bacteriophages were observed to pass through the filter, but bacterial cells could not. - They observed that different genetic markers could be transferred between bacteria selectively, which indicated that the bacteriophage could transfer only specific genes rather than the whole F+ factor. - They were also able to demonstrate that the transfer of genetic material occurred at low frequency, which was consistent with the characteristics of phage transduction, but not with F+ mediated conjugation, which typically occurs at high frequencies.

4. Conclusion

Based on these observations, Zinder and Lederberg concluded that the prototrophs recovered in their experiments were a result of phage-mediated transduction, and not the F+ mediated conjugation. The experiment was essential as it provided early evidence for the presence of a bacterial virus capable of transferring genetic information between bacteria, a process called transduction. This finding has been crucial for understanding bacterial evolution and genetics.