Replica plating has been used to address profoundly important questions in bacterial genetics For example, in the 1940,'s there was much debate regarding the issue of whether or not mutants pre-exist in a population of bacteria. Researchers observed that when they inoculated wild type(pen )bacteria onto growth medium containing penicillin, and thus selected for bacteria that had mutated to become penicillin resistant, a small fraction(-10-6)of cells would always grow. Thus, pen k colonies had arisen from a pens population. There were two models for this Model A: Directed Mutations"One group of researchers argued that these mutants originated as a result of the selective pressure. Their line of reasoning was that the bacteria can sense the need to grow on penicillin and that a small fraction of them successfully mutate in a directed manner so that they become pen Model B: Pre-existing Mutations"A second group of researchers argued that pen k mutants pre-existed within the wild type population before ever coming into contact with penicillin thus,( they argued) penicillin doesn t direct mutations, it simply reveals mutants Replica plating provided a rapid means for testing these two hypotheses. The following is a simplified version of the experiment. Plate 1 contains a"lawn"of cells(a solid layer of cells packed together), all of which are the offspring of a single wild type cell. About 5 X 106 cells were spread on a plate, and after a day of growth, they formed a lawn containing about 109 cells. Plate 1 was used as the master plate that was replicated onto plates 2, 3, 4, and 5 lawn colony plate 1 plate 2 plate 3 plate 4 plate 5 The distribution of colonies on plates 3, 4, and 5 is identical a)Which of the two hypotheses(directed mutations or pre-existing mutations )does this result more strongly support. Explain your reasoning These data support Model B, the pre-existing mutations model. Each pen colony on plate 3 is also represented on plates 4 and 5, suggesting that these penk colonies already existed on plate 1. If the mutations were arising anew upon exposure to penicillin, you would expect different patterns of pen r colonies on plates 3, 4, and 5 012Fal20037.012 Fall 2003 Question 3 Replica plating has been used to address profoundly important questions in bacterial genetics. For example, in the 1940's there was much debate regarding the issue of whether or not mutants pre-exist in a population of bacteria. Researchers observed that when they inoculated wild type (penS) bacteria onto growth medium containing penicillin, and thus selected for bacteria that had mutated to become penicillin resistant, a small fraction (~10-6) of cells would always grow. Thus, penR colonies had arisen from a penS population. There were two models for this: Model A: "Directed Mutations" One group of researchers argued that these mutants originated as a result of the selective pressure. Their line of reasoning was that the bacteria can sense the need to grow on penicillin and that a small fraction of them successfully mutate in a directed manner so that they become penR. Model B: "Pre-existing Mutations" A second group of researchers argued that penR mutants pre-existed within the wild type population before ever coming into contact with penicillin; thus, (they argued) penicillin doesn't direct mutations, it simply reveals mutants. Replica plating provided a rapid means for testing these two hypotheses. The following is a simplified version of the experiment. Plate 1 contains a "lawn" of cells (a solid layer of cells packed together), all of which are the offspring of a single, wild type cell. About 5 X 106 cells were spread on a plate, and after a day of growth, they formed a lawn containing about 109 cells. Plate 1 was used as the master plate that was replicated onto plates 2, 3, 4, and 5. The distribution of colonies on plates 3, 4, and 5 is identical. a) Which of the two hypotheses (directed mutations or pre-existing mutations) does this result more strongly support. Explain your reasoning. These data support Model B, the pre-existing mutations model. Each penR colony on plate 3 is also represented on plates 4 and 5, suggesting that these penR colonies already existed on plate 1. If the mutations were arising anew upon exposure to penicillin, you would expect different patterns of penR colonies on plates 3, 4, and 5