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Questions about the Hershey-Chase experiment

  1. Sep 22, 2007 #1
    This Biology textbook I'm reading describes how DNA was finally identified as the material that contains the information of a life-form* or virus. How they did this by marking two batches of E. coli viruses with either a radioactive isotope of sulfur (35)—to tag protein—or a radioactive isotope of phosphorus (32)—to tag DNA—. Then they would identify if the carrier of information is the DNAS or the protein (whichever was present in the infected cells and their offspring).

    The book doesn't go on a lot of detail on the actual process though. How is a cell or virus "marked" with these isotopes? how do you insert radioactive phosphorus atoms into a DNA sequence without destroying or corrupting the virus? ... and wouldn't the unstable atoms destroy such a small structure (the virus and its genetic code)?

    * was this hyphen used correctly? I wonder... :biggrin:
  2. jcsd
  3. Sep 22, 2007 #2


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    Ah, I have a bit of memory of having those types of questions back in my intro biology courses as well...would be nice for textbooks to explain these things rather than making you think they "magically" inserted the labels onto DNA.

    The way this is done, in a nutshell, is to let the bacteria do the work themselves. All you need to do is provide the radioactive precursors in the culture medium they're being grown in, and they use it like any other nutrient. Then you have a bunch of bacteria with both protein and DNA labeled, you wash away all the remaining radioactive "nutrients" you provided, and then watch what the bacteria do on their own.

    The isotopes chosen have a relatively long half life (in terms of bacterial growth rates anyway), and don't do a lot of harm in the time scale that the experiments are performed. It's possible that some cell functions are a bit hindered by incorporation of these isotopes, but for the purpose of these experiments, it's not relevant as long as it isn't lethal.
  4. Sep 22, 2007 #3
    that's really clever. cool thanks :biggrin:
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