Meselson and Stahl grew bacteria in a growth medium containing ammonium ions (14NH4). N is the common, light form (isotope) of nitrogen. The heavier form (15NH4) has an additional neutron in the nucleus. The source of nitrogen for new DNA produced during cell division was the 14NH4 in the ammonium ions.
Extraction of DNA
Meleson and Stahl extracted DNA in solution from the bacterial cells. They spun the DNA solution in a fast centrifuge.
The DNA accumulated in a band suspended in the solution, because all the DNA molecules were about the same density. All the DNA contained 14N as the only source of nitrogen for the bacterial cells was to make new DNA was the Nitrogen14 in the ammonium ions of the growth medium.
Extraction of DNA in heavy nitrogen
Meselson took another batch of bacterial cells and grew them in a growth medium containing N15. They extracted and spun the DNA as before.
This time the DNA band formed lower down because it had a higher density. Meselson and Stahl used this idea of labelling DNA with heavy nitrogen to try to find out how DNA replicate.
Molecules called nucleotides are made in the cell. Nucleotides join together to make new DNA when DNA molecules replicate. One theory suggested that the original DNA breaks down into fragments, which join up with new nucleotides to form mixed DNA strands. These strands formed of both old and new DNA fragments to form mixed DNA strands. These strands formed of both old and new DNA fragments reassemble into two seperate DNA molecules. The new DNA is inserted among the original DNA.
A second possible theory tested by Meselson and Stahl was that the DNA molecule seperates into two strands. The free nucleotides in the cell join up alongside each strand. Two new 'hybrid' molecules are formed, each made up one original DNA trand and one strand made of new DNA.
A third theory was that the free nucleotides in the cell assemble alongside the original DNA molecule. This produces a second molecule composed entirely of the new DNA.
Testing the theories
Meselson and Stahl took the bacteria which had grown for severl generations in a medium containing 15 NH4. All the DNA in these cells was then heavier than normal. They then transferred bacteria with 'heavy' DNA into a growth medium containing 'light' ammonium ions '14NH4) and allowed them to divide just once. New molecules made could only be made using the nitrogen 14 in the new growth medium.
Testing the theories continued
New nucleotides made by the cells are light (14N) and the original DNA is heavy 15N. Theory 1 new DNA molecules should be a mixture of the original heavy DNA and the light DNA making it medium DNA. Theory 2 would be medium DNA. Theory 3 would be light and heavy DNA. Meselson and Stahl found that it was medium DNA after centriguging the extracted DNA.
Second Division of DNA
New DNA strands formed by dispersive replication (theory 1) would form two new DNA molecules and both molecules would be between medium and light. New DNA strands formed by semi-conservative replication (theory 2) in a solution containing 'light' DNA to from two new DNA molecules. One medium and one light molecule.
Testing the theories (2)
Meselsen and Stahl allowed the bacteria to divide once more. They found light and medium DNA meaning that they could reject theory 1.