In a bacterial culture in which all cells are unable to synthesize leucine ( ),
ID: 29137 • Letter: I
Question
In a bacterial culture in which all cells are unable to synthesize leucine ( ), a potent mutagen is added, and the cells are allowed to undergo one round of replication. At that point, samples are taken, a series of dilutions is made, and the cells are plated on either minimal medium or minimal medium containing leucine. The first culture condition (minimal medium) allows the growth of only cells, while the second culture condition (minimal medium with leucine added) allows growth of all cells. The results of the experiment are shown.
What is the rate of mutation at the locus associated with leucine biosynthesis? Express your answer using two significant figures.
Explanation / Answer
You do not need to wear gloves while pouring the agar plates, since no mutagenic chemicals or bacteria are involved in this procedure. 1. Use the work area with a 2.5-3 foot long piece of absorbent lab counter paper. Label the petri dishes you will use with your initials and your lab time (e.g. Tues. Am). I recommend you put your initials on the bottom, along the perimeter. 2. Look in the water bath on your table for a flask labelled DMA. This is Davis Minimal Agar that has been autoclaved to make it sterile, and is being kept at 47 C to keep it liquified. 3. Think about these important points in pouring a petri plate before doing it: a) You must work quickly, because once the container of minimal agar is removed from the water bath, it will start to harden within 2-3 minutes. b) When pouring into the petri dish, pour just enough to fill the dish about half way; this should fill the dish about half full (or half empty for the pessimists). c) Although you must work fairly quickly, pour the agar gently to minimize the number of bubbles (bubbles look amazingly similar to colonies when the agar hardens). 4. When you are ready to pour: a) Pull out the container of DMA, remove the cap. b) Open the cover of the petri dish halfway and pour in the agar to just cover the bottom of the dish. Try to minimize the introduction of bubbles. c) Repeat for all the dishes. d) Immediately rinse the flask with warm water to facilitate washing the flask. 5. Let the plate harden 15 minutes before moving it. These plates will be stored "upside down" until next week's lab.