Genetics: Analysis and Principles 5th Edition by Robert Brooker

Exercise 9

With regard to the experiment described in Figure 9.2, answer the following:
a. List several possible reasons why only a small percentage of the type R bacteria was converted to type S
b. Explain why an antibody was used to remove the bacteria that are not transformed. What would the results look like, in all five cases, if the antibody/centrifugation step had not been included in the experimental procedure
C. The DNA extract was treated with DNase, RNase, or protease. Why was this done (In other words, what were the researchers trying to demonstrate )
FIGURE 9.2 Experiments of Avery, MacLeod, and McCarty to identify the transforming principle. Samples of Streptococcus pneumoniae cells were either not exposed to a type S DNA extract (experiment 1, left side) or exposed to a type S DNA extract (experiments 2-5). Experiments 3, 4, and 5 also contained DNase, RNase, or protease, respectively. After incubation, the cells were exposed to antibodies, which are molecules that can specifically recognize the molecular structure of macromolecules. In this experiment, the antibodies recognized the cell surface of type R bacteria and caused them to clump together. The clumped bacteria were removed by a gentle centrifugation step. Only the bacteria that were not recognized by the antibody (namely, the type S bacteria) remained in the supernatant. The cells in the supernatant were plated on solid growth media. After overnight incubation, visible colonies may be observed.