A Tricky Lac-Operon Group Quiz
(When it gets dark the eternal stars come out.)

If you can answer these, you understand "all about" the lac-operon and how to work with it in the lab using E.coli and various types of growth media.

  1. An E.coli strain that is met- must be fed methionine in order to grow. ---True False

  2. An E.coli strain that is lacZ- must be fed lactose in order to grow. ---True False

  3. You swab some E.coli lacZ- onto two plates. One plate contains MacConkey agar with glucose, and the other contains MacConkey agar with lactose. On which plate(s) does this mutant grow.

  4. You inoculate some glucose broth with equal numbers of both E.coli lac+ (wild-type) and E.coli lacOc. After two hours, which strain predominates in numbers? Why?

  5. You irradiate a wild-type E.coli with UV light. After a lot of work you finally have isolated two clones of lac-operon mutants. You then determine that both mutations are in the lacR cistron. So you subject them to several tests:

    Strain MacConkey + Glc MacConkey + Lac ONPG on Glc culture
    #a red pale not yellow
    #b red red yellow

    How can this be?! (Hint: draw and explain diagrams!)

  6. To help in interpretting the real data to be derived using "wild-type" E.coli, you are to create two graphs as you speculate what might or might not occur.

    1. At time=0, you add lac-operon inducer, and then follow the amount of ß-gal'ase per cell as time goes by. What does the graph look like? (Remember that you will do this in lab tomorrow by watching how much ONPG is converted to yellow ONP. So it is intensity of yellow over time. What does the line look like?

    2. You have three cultures and, at time=0, you add different amounts of inducer. You speculate that the more inducer is added the faster the gene is turned on and the more "on" it becomes.