subject: Taking a Look at the Three DNA Replication Models [print this page] Taking a Look at the Three DNA Replication Models
In earlier years, one of the secrets of DNA replication was how two daughter strands are created from the parent cell. Scientists have found that there are three possible models which accurately demonstrate the creation of daughter chains. These models are known as the semi-conservative replication model, the conservative replication model, and the dispersive replication model.
According to the model for semi-conservative replication, after one replication we should have 2 hybrid molecules for each original strand of DNA. These hybrid molecules contain one strand from the original DNA, and one entirely new strand.
The dispersive replication model would also create 2 distinct replicas of the parent DNA. Each copy would contain specific areas of DNA composed of parts of the original strands as well as parts that were from the daughter molecules. In essence, parts of both of the parent chains could be found dispersed throughout both of the daughter strands along with new strands.
The third and final model was the conservative replication model. According to this model, the original DNA molecule would be left intact, and a second, completely new molecule would also be created. Nothing from the parent strands would be found in the second sister strand. This DNA model was disproved when it was found that half of the daughter strands came from the parent, and half were composed of newly polymerized nucleotides.
This left scientists looking at the semi-conservative replication model and the dispersive replication model to find how new DNA was created. Both of them suggested that the sister strands would be made of both the parent strand and new material, so both were seen as possible answers. In order to find which replication model was correct, scientists left DNA within test tubes to replicate once again, and looked at the results once it had.
In the end, scientists were able to determine that the correct DNA replication model is actually the first one, the semi-conservative DNA replication model. This was finally proven in the experiment of Meselson - Stahl. After leaving DNA to replicate again, scientists noticed that 2 hybrid molecules and 2 N14 (nitrogen base which was originally placed in the test tube along with the DNA) molecules were produced. If the dispersive DNA model was correct, after 2 divisions in the DNA the scientists should have observed a single line in the test tube further up because abundant N14 causes DNA molecules to become less dense. Since two lines were observed, the replication model that was correct was the semi-conservative theory.
