What was the purpose of the experiments of Avery, MacLeod, and McCarty? O to determine the nature of the transforming principle and thus the nature of the genetic material O to determine the structure of DNA Oto show that DNA was a double-helix O to refute the work of Griffith that they did not believe
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Frederick Griffith demonstrated that the persistent genetic identity of Streptococcus pneumoniae cells could be transformed by exposing them to dead cells of a different type. In the early 1940s, Oswald Avery, Colin MacLeod, and Maclyn McCarty conducted an experiment (published in 1944) to identify the molecule(s) responsible for that process, because they felt it would reveal the physical molecules responsible for storing genetic information. In brief, their experiment consisted of several treatments of type R S. pneumoniae cell cultures to which were added prepared compounds. Those compounds were derived from live cultures of type S cells, which were killed and from which saline-soluble components were extracted. Then the proteins were precipitated from the extract with chloroform, and the bacterial polysaccharide capsules were hydrolized with an enzyme. Finally, long molecules were precipitated from the solution with alcohol and retained as a starting point for preparing compounds to add to the live R cells. After (possibly) mixing the live R cells with additional compounds (see the treatments below) and allowing them to grow, antibodies to the live R cells were added, and the cell-antibody complexes were removed from the solution with centrifugation. The remaining supernatant was plated on agar plates and colonies allowed to form. The specific experimental treatments were as follows: 1. A colony of type R cells otherwise untreated, 2. A colony of type R cells exposed to the compounds isolated as described above, 3. A colony of type R cells exposed to the compounds isolated as described above that had also been exposed to a crude preparation of deoxyribonuclease, 4. A colony of type R cells exposed to the compounds isolated as described above that had also been exposed to a preparation of ribonuclease, and finally, 5. A colony of type R cells exposed to the compounds isolated as described above that had also been exposed to a preparation of protease. 1. First, draw a flowchart of the experiment performed by Avery et al; in chemistry this might be referred to as a separation chart. Although it should include the live R cells being plated at the end, the bulk of it should focus on how four of the different treatments were derived from live S cells. Thus, it should begin with live S cells, include lysing them to yield the complex cellular components, and end with the four different sets of molecules used.
Sri K.
The Transforming Principle Case Study Bio 449 Name: The properties of the rough strain are its phenotype. In the Griffith experiment you read about, the phenotype of the living rough strain changed from nonpathogenic to pathogenic. Scientists proposed that it changed because the genotype changed. The next set of experiments started with the hypothesis that the genotype change was due to some extra genetic material being added to the rough bacteria to change their phenotype to a smooth phenotype. Avery, MacLeod, and McCarty had three tools to use in their experiments (in addition to the smooth and rough strains and mice): 1. Proteases 2. DNase 3. RNase All three of these tools are enzymes, as you can guess because their names end in "ase." Questions 1. Given the names of these three enzymes, what reaction do you expect each one to catalyze? Write your answers in the space to the right of the name above. 2. If you performed the following experiment what results would you expect if DNA were, in fact, "the transforming principle"? Explain your conclusion. Heat-killed smooth bacteria Treated with: RNase Protease DNase Added to live Rough bacteria Would the mice live or die? 3. Using a well-constructed essay paragraph (on the back of this sheet), describe the experiments by Avery, MacLeod, and McCarty that determined the role for DNA as the genetic material (include the logic behind the experiment).
Adi S.
Griffith, in his 1928 experiments, demonstrated that bacterial strains could be genetically transformed. The evidence that DNA was the transforming principle responsible for this phenomenon came later. What was the key experiment that Avery, MacCleod, and McCarty performed to prove that DNA was responsible for the genetic change from rough cells into smooth cells?
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