Conservative Replication – One completely new double helix was
made from the old one.
Semi–Conservative Replication – Each new molecule would
contain one new strand and one
old one.
Dispersive Replication – Each new molecule would be made from
old and new bits in random places
through the molecule.
Semi – Conservative replication seemed to be the most likely and Meselsohn and Stahl carried out experiments to prove it. This is what they did:
The experiment started with bacteria grown in 15N, a heavy isotope of nitrogen. As the bacterial culture grew and divided, the 15N was placed into each of the nitrogen bases of DNA, Adenine, Thymine, Cytosine and Guanine.
Into the First test tube was the sample of the bacteria grown in 15N. The rest of that was put into another culture media containing normal nitrogen, 14N. Culture samples were taken from the 14N each time the bacterial culture doubled. They were put into test tube 2 and 3. A sample was then taken from a culture only containing 14N.
The Nitrogen make up of the bacterial DNA was examined. DNA from the bacterial cultures was then isolated and put into a strong salt solution. The DNA solution was then spun at a high speed and the centrifugal force set a gradient, pulling more salt molecules to the bottom and less at the top. The DNA molecules of the Heavy 15N isotope would sink further toward the bottom of the salt gradient than the 14N. This is what was found:
The important samples were those from test tubes 1 and 2. In sample number 2 taken after the bacterial population the band was midway between the 15N and 14N bands. In sample 3 after the next doubling time there were two bands, one lined up with the 14N band and the other was midway between the 14N and 15N bands.
These results were identical to those predicted. The heavy DNA molecules were found in sample 1 where both strands had heavy 15N. The lightest DNA molecules were found at test tube 4, where the bacteria were only grown in 14N.
Sample 2 shows that the heavy DNA was used as templates for new DNA strands. Thus, all the DNA had one light strand and one heavy strand.
Sample 3 went through 2 rounds of replication. In this case each molecule was separated into one heavy and one light strand. The heavy parent strand acted as a template for making a new light strand, resulting in another hybrid molecule. The light strand also acts as a template, but this results in a molecule with two light strands.
These results supported Watson & Cricks predictions about semi – conservative replication and show that DNA was replicated by semi – conservative replication.
