Question

6. A bacterial geneticist mixes 109 cells of E. coli strain A which requires lysine and is ampicillin sensitive

with 109 cells of strain B which requires phenylalanine and tyrosine and is ampicillin resistant. In order to detect whether genetic recombination had occurred between the two strains it would be best to grow the mixture of cells in

A) minimal glucose medium plus ampicillin


B) minimal glucose medium with no supplements


C) minimal glucose medium plus lysine and phenylalanine


D) minimal glucose medium plus lysine and tyrosine


E) minimal glucose medium plus phenylalanine, tyrosine and ampicillin

7. In order to obtain a lysine revertant from strain A (above in #6) the culture should be plated on

A) minimal glucose medium plus lysine and phenylalanine


B) minimal glucose medium plus ampicillin


C) minimal glucose medium with no supplements

D) minimal glucose medium plus phenylalanine and tyrosine

E) minimal glucose medium plus lysine and ampicillin

8. Which of the following would be true about the lac operon in the absence of glucose with lactose present?

A) The LacI repressor would be bound to the operator.

B) cAMP would not be bound to CRP.


C) The LacI repressor would be inactivated.


D) Both b and c are true.

Answer 1

6. A) In this question,to detect whether genetic recombination had occured between two strains the  mixture of cells should be grown in minimal glucose medium plus ampicillin.Because while growing in medium containing ampicillin all the strain A cells will be disappeared as they are ampicillin sensitive and only the ampicillin resistant strain B cells will remain in the medium.So if the medium has only strain B cells remaining ,it shows that genetic recombination has occured.

7.E) To obtain a lysine revertant from strain A ,the culture should be plated on minimal glucose medium plus lysine and ampicillin.I think lysine revertant from strain A means the strain A that is ampicillin resistant and requires lysine .So while growing in this medium, all the strains that are ampicillin sensitive will disappear and only the strains that are ampicillin resistant and requires lysine will remain in the medium.By this way we can separate the lysine revertant from strain A.

8.C) is the answer.In lac operon ,if only lactose is present ,an isomer of lactose -allolactose will also be present.This allolactose binds to the Lac repressor and makes it inactivate.So the lacrepressor can no longer bind to the operator and RNA polymerize can easily transcribe the genes.