leu- bacteria are mixed in a flask with leu+ bacteria, and soon all bacteria are leu+. However, if the leu– cells are on one side of a U-tube and the leu+ cells are on the other, the leu– cells do not become prototrophic. This suggests which mechanism of gene transfer?

Question

leu- bacteria are mixed in a flask with leu+ bacteria, and soon all bacteria are leu+. However, if the leu– cells are on one side of a U-tube and the leu+ cells are on the other, the leu– cells do not become prototrophic. This suggests which mechanism of gene transfer?

BlackTom AI · Accepted Answer

In this scenario, the leu- and leu+ bacteria are able to mix and all become leu+ when together, suggesting that genetic material is being transferred between cells. However, when the two populations are separated by a U-tube so that physical contact is not possible, the leu- cells do not become prototrophic. This pattern points to a mechanism that requires direct cell-to-cell contact for gene transfer.

Option 1: generalized transduction. This involves transfer of bacterial genes by a bacteriophage, which can occur without the donor and recipient cells needing to touch. If transduction were responsible, separating the populations with a barrier would not necessarily prevent the transfer, so this option is unlikely given the observed contact dependence.

Option 2: specialized transduction. Like generalized transduction, specialized transduction uses a phage to move specific donor genes, but it still does not require direct mating between donor and recipient cells. The contact barrier would not reliably block phage-mediated transfer in all cases, so this does not fit the described result.

Option 3: conjugation. Conjugation is a cell-to-cell transfer process that requires physical contact between donor and recipient cells, often via a pilus. The classic U-tube experiment demonstrating that separated compartments do not exchange genetic material aligns with conjugation as the mechanism, because the barrier prevents the mating contact required for transfer.

Option 4: transformation. Transformation involves uptake of free DNA from the environment by competent cells. While some forms could operate without direct contact, the described setup explicitly tests for contact-dependent transfer, and the absence of transfer across a barrier argues against transformation being the primary mechanism in this case.

Overall, the results are most consistent with conjugation, since only a mechanism requiring direct cell contact can explain why mixing yields prototrophy while separation prevents it. The other options rely on processes (phage-mediated transduction or DNA uptake from the medium) that do not necessitate direct contact, making them inconsistent with the observed outcome.

leu- bacteria are mixed in a flask with leu+ bacteria, and soon all bacteria are leu+. However, if the leu– cells are on one side of a U-tube and the leu+ cells are on the other, the leu– cells do not become prototrophic. This suggests which mechanism of gene transfer?单项选择题

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