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Additional recommended knowledge
The deciphering of the structure of DNA by Watson and Crick in 1953 suggested that each strand of the double helix would serve as a template for synthesis of a new strand. However, there was no way of guessing how the newly synthesized strands might combine with the template strands to form two double helical DNA molecules. The semiconservative model seemed most reasonable since it would allow each daughter strand to remain associated with its template strand. The semiconservative model was verified by the Meselson-Stahl experiment and other even more revealing experiments that allowed for autoradiographic visualization of the distribution of old and new strands within replicated chromosomes.
Testing the semi-conservative theory
The semi-conservative theory can be proven making use of the fact that DNA is made up of nitrogen bases. Nitrogen has an isotope N15 (N14 is the most common isotope) called heavy nitrogen. The experiment that proves the semi-conservative theory makes use of this isotope and runs as follows:
1.- Bacterial (E coli) DNA is placed in a media containing heavy nitogen, which binds to the DNA, making it identifiable. 2.- This DNA is then placed in a media with the presence of N14 and left to replicate only once. The new bases will contain nitrogen 14 while the originals will contain N15 3.- The DNA is placed in test tubes containing caesium chloride (heavy compound) and centrifuged at 40,000 revolutions per hour. 4.- The caesium chloride molecules sink to the bottom of the test tubes creating a density gradient. The DNA molecules will position at their corresponding level of density (taking into account that N15 is more dense than N14) 5.- These test tubes are observed under ultraviolet rays. DNA appears as a fine layer in the test tubes at different heights according to their density.
Up to this point, either the semi-conservative or the dispersive theories could be truthful, as experimental evidence proved that only one line appeared after one replication. In order to conclude between those two, DNA had to be left to replicate again, still in a media containing N14.
Experimental evidence proved that two lines were observed therefore proving the semi-conservative theory.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Semiconservative_replication". A list of authors is available in Wikipedia.|