Anonymous asked in Science & MathematicsBiology · 1 decade ago

AP Bio question? please help? i'm clueless after winter break!?

An investigator mixed a culture of E. Coli that was drug resistant (i.e., contained the R plasmid) with a culture that was drug sensitive. The resultant mixed culture initially contained 10% drug-resistant cells. After one hour, testing revealed that 60% of the cells in the culture were drug resistant. The investigator then concluded that conjugation had transferred drug-resistance genes to 50% of the cells in the culture. Which of the following was the investigator most likely assuming?

a. The mixed culture initially contained both drug-sensitive and drug-resistant cells.

b. Conjugation cannot occur unless both the R and F plasmids are present.

c. 60% of the cells in the mixed culture were drug resistant after one hour.

d. If cell division occurred after the cultures were mixed, the drug-resistant cells divided at the same rate as the drug-resistant cells.

e. Cell division can occur in drug-resistant cells.

1 Answer

  • Anonymous
    1 decade ago
    Favorite Answer

    No need to assume (e). The investigator (let's call her Jill) actually grew samples of drug resistant cells when she plated and incubated a sample after 1 hour. Ditto for (c) - that's a measurement, not an assumption. Certainly she remembers mixing resistant and sensitive cells herself at the beginning of the experiment, so (a) is not an assumption but an accomplished fact. I don't think anyone's assuming anything about F plasmids in this experiment. However, to make a quantitative determination of what proportion of uninfected cells were transfected, you have to assume something about the relative reproductive rate, and the most obvious assumption is that E. coli reproduce at the same rate with or without the R plasmid.

    Maybe. The trouble is, the arithmetic's all wrong. If 50% of the R- cells were transfected, and 10% of the cells were initially R+, then only 10+45=55% of the cells would be R+ after 1 hour. So, if 60% were R+ in the sample, then R+ cells must reproduce faster than R- cells, yeah? I think your instructor made an error, but call it as you see it.

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