I was shadowing a researcher at a lab recently, and I saw her prepare a solution for genotyping a mouse. At one point, she centrifuged a solution of chemicals and the mouse's tail to create the supernatant. Then she extracted the supernatant to use for PCR. I was wondering, isn't the DNA of the mouse in the dense material on the bottom of the micro tube, instead of in the supernatant? According to Hershey and Chase experiment, I thought you were supposed to throw away the liquid, as it contained protein and other unnecessary material for PCR.

As a general point Yujin, when we watch a person perform a lab technique with which we are unfamiliar we should ask questions at the time (or subsequently) to make sure we understand what is going on (e.g., “what is in that solution?”). Your question can probably be answered by a quick search on-line, even the Wikipedia entry on 'DNA extraction' will help you. Briefly, genomic DNA is soluble in most DNA genotyping-extraction buffers and you need to precipitate it out of solution, which subsequently requires centrifugation to isolate the DNA (and perhaps other material that can be reduced/removed if required).

The classical Hershey-Chase experiment is something a little different, involving incorporation of radiolabelled ‘DNA’ by bacteriophage into bacteria (which were pelleted by centrifugation - note that bacteria will settle to the bottom of a test tube slowly even without centrifugation)

see - Hershey AD and Chase M (1952). Independent functions of viral protein and nucleic acid in growth of bacteriophage. J Gen Physiol 36: 39-56 (this can be downloaded - not many scientific articles from this era can be!).

Last edited by Steve Lolait (13th Jun 2016 08:53:01)