Tuesday, May 17, 2011

The "Most Beautiful" Experiment

I'm pretty sure that this will be the last biology post on this blog. You can interpret that as a good thing or a bad thing...I won't tell you which one I think it is.

It's been called the most beautiful experiment in biology. It's just one of those things that it seems like everyone knows about.

I'm referring to, of course, the Meselson-Stahl experiment, showing that DNA replication is semi-conservative.

This experiment's original purpose was to provide data regarding the process of replication of DNA. Watson and Crick's model suggested a way that DNA could replicate, but there wasn't actual information regarding this. So, a few years after Watson and Crick published their model, Meselson and Stahl set out to conclusively establish the method through which DNA replicated.

There were actually several potential models. For example, a DNA molecule could somehow create an entirely new DNA molecule. Or, parts of the DNA could be interspersed with new DNA.

Although Watson and Crick seemed to support the semi-conservative model, where half of the DNA was preserved and the other half was newly created, this model, like all others, was not supported. Meselson and Stahl found a way to provide this support. Here's how:

The key to their experiment was finding a way to tell whether DNA was "old" or newly created, and this was found by labeling the DNA when it was first created. In essence, this was done by growing E. coli bacteria when it was contained in a heavy isotope of nitrogen and another culture in the presence of the ordinary isotope.

After several generations, the bacteria's DNA contained one of the two isotopes of nitrogen. Samples of both cultures--one with the heavy nitrogen, one with the normal--were taken, and the DNA was removed. The DNA from both samples was mixed together, and this solution was mixed with a cesium salt solution which had the same density as DNA.

This final solution of salt and DNA was centrifuged until the cesium ions formed a sediment at the bottom of the tube, causing the solution to be more dense at the bottom than the top. This meant that the DNA formed bands within the tube, with the DNA containing the denser nitrogen closer to the bottom than the DNA with the more common nitrogen.

The process was repeated, but this time, some of the bacteria were allowed to grow in the lighter nitrogen for several generations. This meant that any newly created DNA would have this form of nitrogen included. For every twenty minutes, a sample was taken from this mix of DNA grown in both light and heavy nitrogen. DNA from these samples were centrifuged.

Here were the results: In the first sample (which did not grow in the light nitrogen) all of the DNA contained the "heavy" nitrogen. One generation later, the DNA had an intermediate density between the two. A generation after this, the DNA was half heavy and half light, and as more generations passed, the DNA contained less of the heavy and more of the light.

These results supported semi-conservative reproduction, because they show that part of the DNA is conserved from one generation to the next--otherwise, the DNA synthesized in the lighter nitrogen would only contain the light nitrogen. However, there has to be another part of the DNA that is newly synthesized, because the density of the DNA did not remain at the heavy end of the spectrum.

Now, this experiment is simply wonderfully designed. There was a need--a need to support a model of DNA reproduction, and the hypothesis of semi-conservative reproduction. To test this, there was a variable that was changed--the amount of time that the bacteria were allowed to reproduce within the lighter isotope of nitrogen. Everything else--the speed of the centrifuge, the type of bacteria, the amount of time the bacteria were allowed to grow--was kept constant. There was even a control group--the first group, which did not have any time within the lighter nitrogen.

If you have stayed with me all the way to this point, I want to thank you for putting up with these posts for this long. Although I may have rambled just a few times, and set the world record for worst procrastination, it has been a fun year within this class. 

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