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The Meselson-Stahl experiment has been called "the most beautiful experiment in biology," and it's certainly one of the most important. It showed that DNA replication is semi-conservative; that is to say that when a cell divides, the two strands of a double-stranded DNA molecule separate and serve as a template for a new copy of that strand to be made. The parent cell and the progeny cell each get one double-stranded DNA molecule with one old strand and one newly synthesized strand.

Semi-conservative replication of DNA
legend: # = old strand, X = newly synthesized strand

   ##             #       #           #X       X#
   ##             #       #           #X       X#
   ##             #       #           #X       X#
   ##  ------->   #   +   #  ------>  #X   +   X#
   ##             #       #           #X       X#
   ##             #       #           #X       X#
   ##             #       #           #X       X#

Semi-conservative replication had been predicted by James Watson and Francis Crick after their discovery of the double-helix structure of DNA in 1953. It wasn't until 1958, however, that this hypothesis would be experimentally verified by Matthew Meselson and Frank Stahl using the technique of analytical ultracentrifugation

An analytical ultracentrifuge is an instrument that can spin samples at great speeds -- many tens of thousands of RPM. At these speeds, the G-forces become great enough to induce a density gradient in certain solutions, such as cesium chloride(CsCl). These gradients can seperate molecules based on their isotopic composition. If you centrifuge a mixture of DNA molecules, some of which incorporate only14N and some of which incorporate only 15N, they will separate into distinct bands in the test tube.

In their experiment, Meselson and Stahl first grew bacteria on growth media that contained only 15N. Because of this, the DNA molecules in the bacteria only incorporated 15N.

After several generations, they switched the bacteria to growing on media that only contained 14N. They then took samples at various times, extracted the DNA, and centrifuged it.

They found that DNA taken from the first generation grown on 14N formed a band in between where 15N and 14N DNA normally band. This showed that the DNA molecules in the offspring generations contained a mix of the original DNA plus newly synthesized DNA (as opposed to the parents keeping all their original DNA and the offspring getting only a newly synthesized molecule).

The first generation result showed this:


   ##           ##       XX
   ##           ##       XX
   ##           ##       XX
   ##  -------> ##   +   XX
   ##           ##       XX
   ##           ##       XX
   ##           ##       XX

did not happen. If it did, you would expect to see two distinct bands.

In DNA taken from the second generation, they got one band at the "middle" position, and one at the 14N position. This showed that the DNA strands were being kept intact as they were copied.

Second generation result showed that this:


  #X         #X     XX
  #X         #X     XX
  #X         #X     XX
  #X  -----> #X  +  XX
  #X         #X     XX
  #X         #X     XX
  #X         #X     XX

happened instead of this:


  #X         X#     #X
  #X         #X     XX
  #X         XX     XX
  #X  -----> XX  +  X#
  #X         ##     XX
  #X         XX     #X
  #X         XX     XX

(if the second thing happened, you would expect to just see one band slowly migrating towards the 14-N band, but never reaching it, through successive generations.)

Despite the monumental contribution of this experiment to our understanding of the basic mechanisms of biology, this work has yet to be recognized with a Nobel Prize.

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