We know DNA unzips to replicate itself, each new double helix containing half the parent. But how do we know that? An incredible experiment, conducted in the 1950s, proved how DNA replicated itself without anyone taking a peek inside a cell. Find out how you know what you know.
In the late 1950s, the scientific community was electrified by the Watson and Crick double-helix model of DNA, but still very much in the dark as to the details. One mystery, which was so fundamental that it didn't really count as a "detail," was the duplication process. How did such a complicated molecule manage to copy itself? Watson and Crick insisted that it "unzipped," each half-strand serving as a model for the next generation. Others thought it somehow duplicated as a whole. Still others believed that some helper molecule snipped the helix every few nucleotides, replicating it in pieces until whole strands were created.
Matthew Meselson and Franklin Stahl came to the rescue with the now-celebrated Meselson-Stahl experiment. The basis of the experiment was the nitrogen in DNA. Every nucleotide is built with nitrogen, which can be one of two different isotopes; 14N and 15N. Generally, 14N is used.
Meselson and Stahl grew E. coli in a solution rich in 15N. After a few generations, they grabbed some of the bacteria and extracted their DNA, along with the DNA of regular E. coli made with nothing but 14N. They dumped the DNA of each in a tube full of a solution of cesium chloride. If a person were to stick a plain solution of cesium chloride into a centrifuge and let it spin for a while, a density gradient would be established. The denser material would have sunk to the bottom of the tube, and the solution would get less dense as one moved upwards through it. The DNA with 14N is far less dense than DNA with 15N, so the 14N makes a line that's high up in the tube. The 15N DNA makes another line that's far down in the tube.
Next Meselson and Stahl grabbed some of the E. coli that had been growing in the 15N solution and transferred it to a 14N solution. They let it grow for one generation, then extracted the DNA and put it in a centrifuge again. This time they got a line right between the all-14N DNA line and the all-15N DNA line. The new generation of DNA clearly comprised both the lighter and the heavier isotope.
Finally, they let the transplanted E. coli go to a second generation in the 14N solution. They repeated the extraction and centrifuging process and got two different lines. One was right back up at the 14N line, while the other was still half-way between 14N and 15N. There was only one conclusion. The first generation 15N DNA had split in half and each strand of the formerly-double-helix had replicated itself using the new lighter nitrogen. When the DNA split in half once again, to form the second generation, one half the new generation was entirely made of 14N, while one half of the new generation still had the old 15N strands to work with, leading to two different lines. The process of replication, called semi-conservative replication because each new DNA molecule conserves half the parent DNA, was proven correct.
This just goes to show, every scientific fact that we casually learn in junior high school represents a huge mystery, a years-long-fight among competing theories, and a brilliant experiment that finally settled the question. What we take for granted, someone else had to prove.