History of the Semiconservative Model |
How it Works |
The semiconservative model was introduced and proven in 1957 through an experiment conducted by Matthew Meselson and Franklin Stahl. Meselson and Stahl utilized 14N and 15N isotopes to differentiate the original DNA template and the new DNA made from that template. They began by growing the bacteria in a medium containing 15N in ammonium chloride for 17 generations. This resulted in nitrogenous bases in DNA labeled with 15N. Their first bacterial sample from the previous process was collected and then placed in a medium containing 14N. It was allowed to replicate once in a twenty minute span followed by the collection of the second sample. The third sample was collected after an hour. Meselson and Stahl isolated DNA from the samples by using cesium chloride density gradient centrifugation which can separate a solution according to the density of each DNA, meaning that the heavier the DNA was, the lower its band would appear on the centrifuge tube. The picture below depicts their results. |
The term semi-conservative pertains to the basis that half of the newly-made DNA is a part of the original template. One strand is used as a template to make a new strand based on complementary base pairing rules. The image to the below illustrates the general concept of the semiconservative model. |
Applying the Semiconservative Model
The semiconservative model begins with a double stranded DNA, such as the one above in blue. The two strands are 3'GCTGCGAGTC5' (left) and 5'CGACGCTCAG3' (right). The DNA then spits and replicates according to the base pairs of the divided DNA. The left and the right side make two new strands 5'CGACGCTCAG3' and 3'GCTGCGAGTC5' respectively shown in red on the right hand side. The first generation of DNA is made up of one strand from the original template and a newly made strand.