Determining the DNA Replication Model
Scientists Watson and Crick came up with the structure model of DNA in 1953 ( Alberts 200). They proposed that the two strands of the double helix unwind, and once it does each strand serves as a template for the synthesis of a complementary daughter strand (Alberts 200). Watson and Crick’s semiconservative model was not accepted when it was proposed, since other scientists did not think that it would be possible for such a big molecule to unwind and serve as templates for the new daughter strands without the whole process becoming a huge mess. Therefore other models were proposed.
Physicist Max Delbruck proposed the dispersive model. He believed that DNA was broken into short segments and that these would be replicated and rejoined again, and the end product then would be DNA strands with a mixture of old and new strands (Alberts 200). There was also a conservative model that was proposed. This model suggested that the parent helix would stay the same and somehow replicate itself and give rise to two new strands of DNA, so the parental DNA strand did not need to unwind.
In order to find out which of these three models was the correct one, Meselson and Stahl used Nitrogen isotopes 14 and 15 for their experiment. The first part of their experiment was to grow bacteria, some in the 14N medium and others in 15N medium because they wanted to have a heavy and a light DNA. Once they grew enough bacteria, the Nitrogen became part of their nucleotide bases and eventually their DNA. After the isotopes became part of the bacteria’s DNA, the scientists opened the cells and placed the DNA into test tubes that had a high concentration of cesium chloride salt. Cesium chloride formed a density gradient and the DNA molecules sunk or floated in the solution until they reached a density where they were equal to the salt (Alberts 201). When this was centrifuged the heavy DNA would be located at the bottom of the test tube and the light on the top. Once the Scientists saw that they could differentiate between heavy and light DNA they went to the next part of their experiment.
In this part they took bacteria that was grown in heavy nitrogen and placed it into a culture that was grown in the light nitrogen. After the first generation of replication of the mixed bacteria the researchers saw that the parental, heavy DNA molecules disappeared and were replaced by a new species of DNA that banded at a density halfway between the 15N and 14N-DNA (Alberts 201). The researchers then concluded that this new band was a hybrid with both isotopes, which clearly showed that the conservative model was not the right model for DNA replication. If that model were true then they would have observed two bands; one at the top and one on the bottom and nothing in between. In order to find out if the model was semiconservative or dispersive the researchers heated the DNA molecules. The hydrogen bonds that held the two strands together broke with the heat and the two strands became separated. Once they centrifuged the single strands they observed that one strand of DNA was heavy and the other one was light. This definitely supported the semiconservative model because if the dispersive model were correct then the strands would have banded in the intermediate density (Alberts 202). Since this was not the case it is now known that the semiconservative model is the one for DNA replication.
Alberts, Bruce. Essential Cell Biology, 3rd ed. New York, New York: Garland Science, Taylor &Francis Group, 1998.