Interviewee: Matt Meselson. Matt Meselson describes the experiment he did with Frank Stahl to prove that DNA is replicated semi-conservatively. (DNAi Location: Code > Copying the code > Players > Matthew Meselson > The experiment)

So we start the experiment by growing the bacteria for a long time, many hours, many generations, in heavy nitrogen medium, so everything's heavy nitrogen. And now we put a little bit of that bacteria into some Dupenol, some detergent to liberate the DNA, we put that in the ultra-centrifuge and we centrifuge it and all the DNA is heavy, it forms a single band. Meanwhile most of the bacteria have been left behind in the culture tube, and we centrifuge them down and re-suspend them in light nitrogen medium. So now any more DNA that's going to be made is light, not heavy, and we see that as time goes on there builds up a new band until at one generation, one full generation after the transfer from heavy to light, all of the DNA is at this new density, which is the density we expect for DNA with one heavy strand and one light strand, namely hybrid DNA. And then as time continues to go on, back in the culture tube where the bacteria are growing in light nitrogen medium, still another band begins to appear, that's fully light DNA. Both strands are nitrogen 14, the light kind of nitrogen. And now as time goes on, the heavy strands that are inherited from way back there, are never destroyed, they're in a sense immortal, but as more and more DNA is made it can be made only out of light DNA, and so the light-light DNA, two strands both light, begins to become more and more predominant in each sample, until we ran this experiment in this case to four generations.

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