DNA Structure and Replication

ID #2343

One of our homework problem asks "Which one of the following scientists helped to demonstrate that DNA, not protein, was the genetic information that viruses inject into bacteria?", and my answer (Hershey/Chase experiment) was correct. However, when I decided to look at the practice exam there is a question very similar to this one where the correct answer is that Avery, McCarty, and MacLeod made the discovery that "DNA and not protein is genetic material" which is exactly how the question is worded on the homework problem. Is this just really misleading or incorrect?


The experiments performed by Hershey and Chase and Avery, McCarty and MacLeod can be confusing because they are trying to answer the same question: "What is the genetic material?" It also helps to think of the experiments sequentially as they are laid out in your textbook. First, Griffith demonstrated that virulence was transferred from heat-killed virulent bacteria to live nonvirulent bacteria. He named this process transformation, and this demonstrated that genetic material was transferred from the heat-killed bacteria to the nonvirulent bacteria. The identity of the genetic material was not known yet. Next, Avery, McCarty and MacLeod attempted to identify the genetic material that was transferred between the bacteria. They removed all of the protein from the heat-killed bacteria (leaving only DNA) and showed that the virulence could still be "transferred" to the live, nonvirulent bacteria. This showed that the genetic material transferred between bacterial strains was DNA. Hershey and Chase provided additional experimental evidence that DNA is the genetic material by using phage that was grown on radioactive 32P (which will label DNA) or 35S (which will label proteins). Once bacteria were infected with this labeled phage, the 32P was found inside of infected bacteria, demonstrating that DNA was the genetic material that phage injected into infected hosts. (As mentioned in the textbook, the Hershey and Chase experiments were performed because many scientists did not believe the results from Avery, McCarty and MacLeod because they could not fathom that protein was not the genetic material). 
Hence, both the Avery, McCarty and MacLeod experiments and the Hershey/Chase experiments BOTH showed that DNA is the genetic material, but using different methods. Because of this, it is important to pay attention to the wording of questions that ask about these experiments. For the homework problem, the correct answer is Hershey/Chase, because they demonstrated this using phage. The key thing to look for in this question is "viruses inject into bacteria," and we can eliminate Avery, McCarty and MacLeod because they never used phage to identify the genetic material in their experiments. For the Exam II question, one of the listed discoveries is "DNA and not protein is the genetic material," and the other is "DNA is the genetic material of T2 phage." As I explained above, Hershey and Chase showed that DNA is the genetic material of phage. Avery, McCarty and MacLeod showed that DNA and not protein is the genetic material transformed from one bacterial strain to another. It is important to pay attention to additional information that is included with the descriptions of the discoveries. 
The exam question is not meant to be misleading. The Avery, McCarty and MacLeod and Hershey/Chase experiments were attempting to answer the same questions, but they used different means to find the answers. Both experiments were very important into determining that DNA is the genetic material. 

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